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U. S. DEPARTMENT OF AGRICULTURE, 
 
 BUREAU OF SOILS— BULLETIN No. 32. 
 MILTON \A/-HITNEY, Chief. 
 
 THE ABSORPTION OF PHOSPHATES AND 
 POTASSIUM BY SOILS. 
 
 
 OSWALD SCHREINER and GEORGE H. FAILYER. 
 
 WASHINGTON: 
 
 GOVERNMENT PRINTING OFFICE, 
 
 lAonfl' 
 

 LETTER OF TRANSMITTAL. 
 
 U. S. Department of Agriculture, 
 
 Bureau of Soils, 
 Was/imgfon, D. C, February 23, 1906. 
 Sir: I have the the honor to transmit herewith the manuscript of a 
 technical paper entitled "The Absorption of Phosphates and Potas- 
 sium by Soils," and to recommend that it be published as Bulletin No. 
 32 of the Bureau of Soils. 
 
 Respectfully, Milton Whitney, 
 
 CJtlef of Bureau. 
 Hon. James Wilson, 
 
 Secretary of Agriculture. 
 2 
 
 ^^■^ ^1/SOfi 
 
PREFACE. 
 
 It has long- been known that soils possess the power of selectively 
 absorbing to varying extents the several constituents from a mixture 
 in solution, and that even strong salts may in this way be decomposed. 
 For instance, potassium is generally absorbed more than chlorine when 
 a soil is treated with a solution of potassium chloride. This has been 
 explained in the past as resulting from a simple metathetical reaction 
 between the potassium salt and other salts already present in the soil. 
 This explanation is now known to be unsatisfactory, for, while other 
 bases can be found in the resulting solution, they are not present in 
 quantities equivalent to the potassium removed and the solution is 
 generally acid. Moreover, precisely similar results can be obtained 
 with other absorbent media which do not contain any bases to replace 
 the potassium. The further fact has been established that an absorb- 
 ing medium has a limited capacity for taking up any particular sub- 
 stance or constituent, and this capacity, or saturation limit, is generally 
 independent of any simple molecular ratio between the absorbed sub- 
 stance and any component or constituent of the absorbing medium, 
 organic substances, such as dyes, being absorbed in precisely the same 
 way as mineral solutes. 
 
 The absorption phenomena resemble in many respects well-known 
 solution phenomena, and there is a distribution of the substance which 
 is being absorbed between the solid absorbing medium and the liquid 
 medium, the resulting concentration of the latter depending upon the 
 amount of the absorbed material in the solid. Whether or not there 
 is a definite law governing the distribution is a matter of controversy, 
 experimental evidence for and against being recorded in the literature. 
 The reverse process of the leaching of a soil or solid containing an 
 absorbed substance shows similar distribution phenomena between the 
 solid and liquid media. 
 
 From these observations it follows that absorption is not merely the 
 result of simple metathesis, although such a reaction may be involved 
 in any given case. So may be involved adsorption or surface con- 
 densation, the formation of solid solutions, or the formation of new 
 molecular species, it being often difficult if not impossible definitely 
 to determine the cause or causes in any given case. But however 
 
 3 
 
4 PREFACE. 
 
 complex the ultimate causes of absorption, the law ^overnin^ the 
 effect itself is one of the utmost simplicity, as the following pages will 
 show, and the reverse process of the removal of absorbed constituents 
 by leaching appears to follow a law similar to the law for absorption. 
 The differential equation describing the law is identical in form with 
 the equation describing rate of solution. 
 
 An important observation indicated by this study is that when a 
 soil containing phosphates or potassium is leached, after a certain 
 small amount of leaching has taken place, the subsequent leachings are of 
 practically constant small concentration, no matter how readily soluble 
 the phosphatic or potassium compounds in the soil may be in them- 
 selves. This suggests a very close analogy to the case of a slightly 
 soluble solid in contact with a saturated solution of itself. It is much 
 more probable, however, that there is actually a diminution of the 
 concentration in successive leachings, although a diminution which 
 would not be appreciated over any range that would ever be realized 
 either in the laboratory or in nature. 
 
 That the solubility of the absorbed substance is not a primary factor 
 in the phenomena is shown by the action of soils toward certain dyes 
 which can be readily leached out with alcohol but not with water, 
 although as soluble or even more soluble in the latter as they are in 
 the former. A very interesting case has been cited by Walker and 
 Appleyard and by van Bemmelen, where picric acid is absorbed by 
 silk more readily from an aqueous solution than from an alcoholic 
 solution and not at all from a benzene solution, although picric acid is 
 more soluble in alcohol than in either water or benzene. 
 
 It is thus made manifest that the absorptive power of the soil is to 
 a much larger extent than any other the controlling factor in regu- 
 lating the concentration of the soil solution and that, in the absence of 
 some exceptional disturbing cause, the concentration will remain prac- 
 tically constant. It further appears that different soils will yield 
 solutions of the same order of concentration. This absorption study 
 confirms in a positive way conclusions regarding the concentration of 
 the soil moisture, which have alread}'^ been advanced in former bulle- 
 tins of this Bureau, but* which were reached by entirely different lines 
 of reasoning and evidence. These conclusions, of obvious funda- 
 mental importance, furnish a new point of view for the study of the 
 relations of plant growth to the soil and the development of a rational 
 system of fertilizer usage which may reasonably be expected to yield 
 results of great practical importance in soil management. 
 
 Frank K. Cameron. 
 
CONTENTS, 
 
 Page. 
 
 Introduction 7 
 
 Description of the soils 8 
 
 Description of the apparatus 11 
 
 Eemoval of phosphate by water 13 
 
 Absorption of phosphate from a solution of monocalcium phosphate 15 
 
 Removal of absorbed phosphate by water 18 
 
 Graphical representation and discussion of the results obtained with mono- 
 calcium phosphate 21 
 
 Absorption of phosphate from a solution of disodium phosphate and removal 
 
 of the absorbed phosphate by water 24 
 
 Graphical representation and discussion of the results obtained with sodium 
 
 phosphate 28 
 
 Absorption of potassium from a solution of potassium chloride 31 
 
 Eemoval of absorbed potassium by water 34 
 
 Graphical representation and discussion of the results obtained with potas- 
 sium chloride 35 
 
 Summary 38 
 
 5 
 
ILLUSTRATIONS. 
 
 Page. 
 Fig. 1. Apparatus used in the absorption experiments 12 
 
 2. Solution curves. Absorption of phosphate by soils from a solution of 
 
 monocalcium phosphate and the removal of the absorbed phosphate 
 
 by water 22 
 
 3. Soil curves. Absorption of phosphate by soils from a solution of 
 
 monocalcium phosphate and the removal of the absorbed phosphate 
 
 by water - 22 
 
 4. Soil curves. Absorption of phosphate by soils from a solution of 
 
 disodium phosphate and the removal of the absorbed phosphate by 
 water 29 
 
 5. Solution curves. Absorption of potassium by soils from a solution of 
 
 potassium chloride and the removal of the absorbed potassium by 
 water 36 
 
 6. Soil curves. Absorption of potassium by soils from a solution of 
 
 potassium chloride and the removal of the absorbed potassium by 
 water 36 
 
 6 
 
THE ABSORPTION OF PHOSPHATES AND 
 POTASSIUM BY SOILS. 
 
 INTRODUCTION. 
 
 In soils there is present the mineral debris of rock degradation and 
 decomposition, ^his mineral matter forms b}'^ far the greater part 
 of the soil, the quantity of organic material being comparatively 
 small, although agriculturally of great significance. In the formation 
 of soils b}^ rock degradation numerous small fragments of the original 
 rock persist to a greater or less extent in the soil. This has been 
 frequently shown, notably by the recent work of Delage and Lagatu, 
 and has been more fully discussed in Bulletin No. 30 of this Bureau. 
 These investigations have shown that practically all the minerals 
 originally present in rocks are found as such in arable soils. Besides 
 these minerals from the original rocks, the soil naturally contains 
 the products resulting from metamorphism and epigenesis, as well as 
 the products resulting from synthetical processes within the soil itself. 
 These minerals are all soluble to a slight degree in water, or are acted 
 upon by water, i. e., they are hydrolized and the more soluble products 
 of h5^drolysis are found in the soil moisture. 
 
 The minerals present in the soil determine the character and, to a 
 certain extent, the quantity of the mineral constituents in the soil solu- 
 tion. The quantity of the mineral constituents in the free soil mois- 
 ture is, however, largely dependent on the absorptive power of the soil, 
 as will be shown in the following pages. It has long been known that 
 soils have the power to absorb mineral constituents in considerable 
 quantities from solution, and it is highly probable that synthetic as 
 well as destructive processes are taking place in the mineral species of 
 the soil. The well-known experiments of Way, Liebig, Heiden, Knop, 
 Rautenberg, Peters, Yoelcker, King, and others have established the 
 general fact that the arable soils show a selective absorption toward dif- 
 ferent mineral constituents. In view of the importance of this subject 
 to a proper understanding of the chemistry of the soil and of soil solu- 
 tions, a systematic study of the behavior of several soil types toward 
 phosphate and potassium has been made. The absorption of phosphate 
 (POJ and of potassnum (K) has been specially studied in the light of 
 
 7 
 
8 ABSORPTION OF PHOSPHATES AND POTASSIUM. 
 
 its influence in maintaining the concentration of these two important 
 plant-food constituents in the soil moisture. The rock-forming phos- 
 phatic and potassium minerals of the soil yield and apparently continue 
 to yield a solution whose concentration approaches equilibrium between 
 the solution and the solid. As actual equilibrium is probably never 
 realized under such conditions, the concentration is influenced by the 
 area of surface exposed to the action of the soil moisture during a lim- 
 ited period of time. This concentration appears to be constant for any 
 given soil and is dependent on the nature of the minerals in the soil, 
 whether they are readily acted upon by water, carbon dioxide, or other 
 substances dissolved in the soil moisture, and on the absorptive capac- 
 ity of the soil. The magnitude of the absorption and the slow rate of 
 removal of the absorbed material from the soil make it highly proba- 
 ble that the concentration of the constituents of the soil moisture is 
 largely controlled b}' the absorptive power of the soil. If, for instance, 
 the concentration of the soil solution in phosphate be reduced through 
 any cause, such as removal of phosphate by plants or influx of rain 
 water, the tendency will be to restore the original concentration by 
 more of the absorbed phosphate of the soil entering into the free soil 
 moisture. If, on the other hand, the phosphate content of the soil 
 moisture be increased above the natural concentration for that soil — 
 as, for instance, by the application of a soluble phosphatic fertilizer or 
 the evaporation of soil moisture — the concentration would be reduced 
 by absorption to the original strength. This is shown in the follow- 
 ing experiments on the absorption of phosphate, the removal of the 
 absorbed phosphate, and the removal of the phosphate originally in 
 the soil. The concentration of phosphate in the solution is maintained 
 with much persistence, although only a fractional part of the absorbed 
 phosphate has been removed, thus indicating that while the absorbed 
 phosphate is apparently rendered insoluble, it is nevertheless slowly 
 but constantly going into the soil moisture. The results with potas- 
 sium, though not so complete, appear to be similar in all respects to 
 those obtained with the phosphate. 
 
 DESCRIPTION OF THE SOILS. 
 
 A preliminary series of absorption experiments was made with four- 
 teen soils. They were so selected as to include types having the tex- 
 ture of a clay, a clay loam, a loam, a sandy loam, and a sand. The soils 
 were treated with twice their weight of a sodium phosphate (NagHPOJ 
 solution containing 100 parts PO4 per million, this being equivalent to 
 an application of 200 parts PO^ per million parts of the soil. After 
 standing several days with occasional shaking, the supernatant liquid 
 was filtered through a Pasteur-Chamberland filter and the concentra- 
 tion of phosphate in the filtrate determined colorimetrically. The 
 results are given in Table I. 
 
DESCRlPTIOl^ OF SOILS. 
 
 Table I. — Absorption of phosphate by different soils from a solution of disodium phos- 
 phate containing 100 parts PO^ per million. 
 
 Soil. 
 
 Cecil clay 
 
 Susquehanna clay. . . 
 
 Brightwood clay 
 
 Penn loam 
 
 Hagerstown loam . . . 
 
 Elkton clay 
 
 Sea island cotton soil 
 
 Quantity 
 PO4 in fil- 
 trate. 
 
 Paris per 
 
 million. 
 
 5 
 
 9 
 
 10 
 5 
 7 
 10 
 16 
 
 Quantity 
 PO4 ab- 
 sorbed by 
 soil. 
 
 Parts per 
 million. 
 190 
 182 
 180 
 190 
 186 
 180 
 168 
 
 Soil. 
 
 Leonardtown loam 
 
 Memphis silt loam 
 
 Sassafras sandy loam . . . 
 
 Cecil sandy loam 
 
 Podunk fine sandy loam 
 
 Norfolk fine sand 
 
 Sandhill 
 
 Quantity 
 PO4 in fil- 
 trate. 
 
 Parts per 
 million. 
 28 
 13 
 15 
 35 
 25 
 30 
 60 
 
 Quantity 
 PO4 ab- 
 sorbed by 
 soil. 
 
 Parts per 
 million. 
 \4A 
 174 
 170 
 130 
 150 
 140 
 
 It is at once apparent that all these soils have a marked power to 
 absorb the phosphate from the solution, reducing the concentration 
 from 100 to as low a concentration as 5 parts per million. On the 
 whole a second treatment gave results that were much the same as in 
 the jfirst, and it became obvious that the absorptive effect of the soil 
 was not reduced by the phosphate already absorbed, in such quanti- 
 ties as resulted from the first treatment. On the other hand, the 
 different soils did show considerable variations in respect to the con- 
 centration of the phosphate remaining in solution. That the absorp- 
 tion is also quite rapid is shown by the following experiment, in which 
 100-gram portions of the soils were treated with 500 c. c. of a mono- 
 calcium phosphate (CaH^(P0j3) solution containing 100 parts per 
 million PO^. This is equivalent to an application of 500 parts per 
 million PO^ to the soil. At the end of the respective periods of time 
 given in Table II the solutions were filtered as rapidly as possible. 
 The clay mixtures required some minutes to filter, and this soil was 
 therefore in contact with the solution longer than the periods indicated. 
 It will be noticed that in the case of the clay soil the absorption was 
 very rapid, 400 parts per million being absorbed by the soil in the 
 three-minute period out of the total absorption of 445 parts per mil- 
 lion in the twenty-four hour period. In the case of the fine sandy soil 
 the absorption was not so rapid, only 235 parts per million being 
 absorbed in the shorter period out of the 370 parts per million 
 absorbed in the longer period. 
 
 Table II. — Absorption of phosphate from a solution of monocalcium phosphate containing 
 
 100 parts PO^ per Tnillion. 
 
 
 Clay soil. 
 
 Fine sandy soil. 
 
 Time. 
 
 Quantity 
 P04in 
 
 filtrate. 
 
 Quantity 
 PO4 ab- 
 sorbed 
 by soil. 
 
 Quantity 
 POiin 
 filtrate. 
 
 Quantity 
 PO4 ab- 
 sorbed 
 by soil. 
 
 
 Paris per 
 million. 
 20 
 18 
 17 
 13 
 12 
 11 
 
 Parts per 
 million. 
 400 
 410 
 415 
 435 
 440 
 445 
 
 Parts per 
 million. 
 53 
 49 
 48 
 37 
 33 
 26 
 
 Parts per 
 million. 
 235 
 
 
 255 
 
 
 260 
 
 
 315 
 
 4 hours . . . . 
 
 335 
 
 
 370 
 
 
 
 21719— No. 32—06- 
 
10 
 
 ABSOEPTION OF PHOSPHATES AND POTASSIUM. 
 
 Four of the above soils were taken for a more thorough study, the 
 selection being based on texture as well as on differences in absorp- 
 tive power, as shown in the above experiment. The soils chosen were 
 the Cecil clay, the Penn loam, the Podunk fine sandy loam, and the 
 Norfolk fine sand. 
 
 The following tables show the results of the mechanical and chem- 
 ical analyses of the samples of these soils used: 
 
 Table III. — Mechanical analyses of the soils used in the absorption of phosphates. 
 
 Soil. 
 
 Fine grav- 
 el, coarse 
 sand, me- 
 dium sand, 
 2.0 to 0.25 
 mm. 
 
 Fine sand, 
 very fine 
 
 sand, 0.25 to 
 0.05 mm. 
 
 Silt, 0.05 to 
 0.005 mm. 
 
 Clay, 0.005 
 to mm. 
 
 Cecil clay 
 
 Per cent. 
 20.3 
 11.0 
 2.9 
 10.8 
 
 Per cent. 
 26.3 
 12.8 
 66.5 
 69.0 
 
 Per cent. 
 22.8 
 44.6 
 23.1 
 13.2 
 
 Per cent. 
 30 5 
 
 
 31 4 
 
 Podunk fine sandy loam 
 
 6 8 
 
 
 6 7 
 
 
 
 Table IV. — Chemical analyses of the soils used in the absorption of phosphates, by digestion 
 ivith hydrochloric acid of sp. gr. 1.115. 
 
 Soil. 
 
 Cecil clay 
 
 Penn loam 
 
 Podunk fine sandy loam 
 Norfolk fine sand 
 
 K.O. 
 
 CaO. 
 
 MgO. 
 
 ALOj. 
 
 Fe^Os. 
 
 Per cent. 
 
 Per cent. 
 
 Per cent. 
 
 Per cent. 
 
 Per cent. 
 
 0.19 
 
 0.34 
 
 0.29 
 
 18.61 
 
 3.28 
 
 .61 
 
 .32 
 
 1.16 
 
 11.56 
 
 2.34 
 
 .21 
 
 1.37 
 
 .85 
 
 5.32 
 
 1.21 
 
 .22 
 
 .28 
 
 .23 
 
 2.51 
 
 1.19 
 
 P205. 
 
 Per cent. 
 
 0.13 
 
 .11 
 
 .47 
 
 The Cecil clay came from States ville, N. C, and is described'* as a 
 heavy red soil from 2 to 7 inches in depth, with an average depth of 5 
 inches, resting on a subsoil of stiff, tenacious red clay. It is a residual 
 soil formed h^ the long-continued process of decomposition from a 
 number of rock types distinct in their physical and mineralogical char- 
 acters. The soil has been derived from nearly all the rocks occurring 
 in this section, mainly hornblende gneiss, micaceous schists, coarse- 
 grained granites, and to a less extent soapstone or steatite. Many of 
 these rocks differ quite widely, yet so completely have they been 
 weathered that they give rise to the same distinctive red clays. The 
 change from the soil to the unweathered rock is nearly always gradual, 
 and frequently it is difficult to mark the dividing line between the 
 two. Often the rocks have decayed so thoroughly that it is dijBficult 
 to determine their original composition. Again the structure of the 
 rocks may be preserved, but on digging into what seems to be com- 
 paratively sound rock the material crumbles between the fingers, show- 
 ing how completely decomposition has taken place. 
 
 « Soil Survey of the Statesville Area, North Carohna. Field Operations of the 
 Bureau of Soils, 1901. 
 
DESCRIPTION OF THE APPARATUS. 11 
 
 The Penn loam came from Leesburg, Va., and is described^* as con- 
 sisting of from 8 to 12 inches of dark, Indian-red loam, underlain by 
 a heavier loam of the same color. It is residual in origin, being 
 derived from the weathering of the Triassic red sandstone. This rock 
 IS composed of grains of sand coated with films of ferruginous clay. 
 In the vicinity of Leesburg, Va., and northward, some wedges, or 
 lenses, of limestone conglomerate are intercalated into the formation, 
 thinning out very graduallj'^ into a fine point. Where this limestone 
 conglomerate occurs in considerable quantities with the red sandstone 
 it gives rise to the type of soil known as Penn clay. Owing to the 
 very irregular and peculiar contact of the two formations the bounda- 
 ries between the Penn clay and the Penn loam are not sharply marked, 
 but consists of a zone of slow gradation of one type into the other. 
 
 The sample of soil used in these experiments is shown by its mechan- 
 ical analysis to be one of these gradations, having rather the texture 
 of a clay loam. 
 
 The Podunk fine sandy loam came from South Windsor, Conn., and 
 is described^ as consisting of 12 inches of friable, dark-brown fine 
 sandy loam, underlain with a yellow or brownish fine sandj^ loam. 
 The material composing this soil originated b}^ deposition in deeper 
 lake waters, but it has been largely rewashed and redeposited by later 
 stream action, and much of it lies within the flood plain of the rivers 
 along which it occurs. 
 
 The Norfolk fine sand came from Upper Marlboro, Md., and is 
 described^ as a light-brown or yellow loamy fine sand having a depth 
 of about 8 inches. 
 
 These four soils — the Cecil claj'^, the Penn loam, the Podunk fine 
 sandy loam, and the Norfolk fine sand — were subjected to a thorough 
 stud}^, both as regards the absorption of phosphate and the removal of 
 the absorbed phosphate by water. This was accomplished by passing 
 the solution, or water, continuously through the soil contained in a 
 filter especially devised for this purpose. The rate of flow of the 
 liquid through the soil was entirely under control. 
 
 DESCRIPTION OF THE APPARATUS. 
 
 The apparatus used is shown in figure 1. The short filter tube A is 
 made by cutting off the lower part of a PasteurrChamberland filter 
 tube and plugging the open end with a rubber stopper. The object in 
 
 « Soil Survey of Leesburg Area, Virginia. Field Operations of the Bureau of Soils, 
 1903. 
 
 &Soil Survey of the Connecticut Valley. Field Operations of the Bureau of Soils, 
 1899. 
 
 cSoil Survey of Prince George County, Md. Field Operations of the Bureau of 
 Soils, 1901. 
 
12 
 
 ABSOEPTION OF PHOSPHATES AND POTASSIUM. 
 
 cutting down the filter tube is twofold, first, to decrease the filtering 
 surface so as to make it possible thoroughl}^ to control the flow by the 
 device to be described presently, and second, to have the filtering sur- 
 face as nearlj^ as practicable at the bottom of the chamber so that the 
 solution must pass through the entire soil column. 
 This might be more effectually accomplished by the 
 use of filtering disks, but no filtering material could 
 be found which was suitable for this work. The filter 
 tube was cut so as to leave about 2 cm. of the tube 
 projecting above the rubber stopper, which holds it 
 firmly in position in the metallic tube B serving as the 
 receptacle for the soil. The metallic tube is closed at 
 the upper end with a two-hole rubber stopper provided 
 with two glass tubes. One of these tubes is about 6 
 feet in length, the end being connected with a funnel. 
 If a greater pressure than 6 feet is necessary for 
 forcing the liquid through a soil, a piece of tubing is 
 readil}'^ added and any desired pressure obtained. A 
 height of more than 6 feet was, however, necessary in 
 onl}^ a single case in the course of these experiments. 
 The shorter tube provided with a rubber tube and 
 pinchcock serves to let the entrapped air out of the 
 metal tube when starting the apparatus. 
 
 The rate of flow of the liquid through the soil is con- 
 trolled by means of the glass spiral siphon C^ con- 
 structed of very thin walled capillary tubing, the 
 tubing being of such a length that the friction of the 
 liquid in passing through it gives the desired rate of 
 flow. The spiral was suspended in the solution to be 
 used, contained in a beaker of such large diameter that the level of 
 the liquid was not materially changed overnight by the flow through 
 the siphon. It was found that the rate of flow was not sensibly 
 affected by slight changes in the height of the liquid in the reservoir. 
 Devices like the Mariotte constant-level bottle were also tried, but 
 the rate required was so slow that no satisfactory flow of liquid could 
 be maintained. The length of the spiral was adjusted so that about 
 50 c. c. passed in twenty-four hours. 
 
 The apparatus may be charged by putting about 50 c. c. of water or of 
 the solution to be used into the percolating tube and then adding the 
 finely powdered soil in such a way as to prevent trapping of air, or 
 the soil may be made into a thin paste with the solution and this 
 poured into the tube. The latter is then set in a vertical position in a 
 stand and connected with the rest of the apparatus by inserting the 
 
 Fig. 1. — Appara- 
 tus used in the ab- 
 sorption e X p 6 r i - 
 ments. 
 
REMOVAL OF PHOSPHATE BY WATER. 13 
 
 rubber stopper. The pinchcock is opened and some of the liquid 
 poured into the funnel until the metal tube is full. The pinchcock is 
 then closed and the glass tube filled with liquid by pouring into the 
 funnel. The constant-dropping spiral is then put in position, as shown 
 in the figure, and the funnel covered to prevent evaporation. The 
 pressure of the column of liquid in the tube forces the solution through 
 the soil and through the filter, A, and if the rate of filtration is greater 
 than the rate with which the liquid is supplied by the constant-drop- 
 ping spiral the height of liquid in the glass tube will decrease until the 
 pressure of the column is sufiicient to force the liquid through the 
 filter at the same rate as it is supplied by the dropping siphon. As 
 the column of liquid becomes shorter, the pressure is diminished and 
 the filtration is less rapid, until finally the rate is exactly that of the 
 dropping spiral. If the filtration should tend to become slower than 
 this rate, the column of liquid in the tube will rise and the consequent 
 greater pressure will be sufiicient to keep the rate of filtration constant 
 and identical with that of the dropping spiral. The apparatus becomes, 
 therefore, perfectly automatic as far as the rate of flow of the liquid 
 is concerned. It is of course necessary that the filter be sufficiently 
 close grained, a condition easily obtained with the filters mentioned, 
 and that the filtering surface be sufficientl}^ small so as not to filter 
 faster than the desired rate without the application of some pressure. 
 
 REMOVAL OF PHOSPHATE BY WATER. 
 
 A study of the absorption by soils of phosphate from aqueous solu- 
 tions and the removal of the absorbed phosphate by water requires a 
 definite knowledge as to the behavior of the phosphate originally in 
 the soils toward the solvent action of water under the identical condi- 
 tions used in the absorption experiments. To ascertain this the soils 
 selected for the investigation were treated with distilled water, and 
 determinations made of the phosphate in the various fractions of 
 percolate. 
 
 For this purpose the apparatus already described was used and the 
 tubes were charged with 100 grams of soil. As already mentioned, 
 the soils studied were a clay, a clay loam, a fine sandy loam, and a fine 
 sand. The soils had been collected several months previously and were 
 air dry. Pure distilled water was allowed to flow through the soil at 
 the rate of 50 c. c. in twent^^-four hours. The percolate was collected 
 in fractions and the phosphate concentrations determined by means of 
 the colorimetric method described in a former bulletin.^ The results 
 obtained with the clay soil are given in Table V. 
 
 «Bul. No. 31, Bureau of Soils, U. S. Dept. of Agr., 1905, p. 42. 
 
14 
 
 ABSORPTION OF PHOSPHATES AND POTASSIUM. 
 
 Table V. — Removal of phosphate from a clay soil by water. 
 
 
 
 Total 
 
 
 
 Total 
 
 
 
 Total 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 of perco- 
 
 of PO4 in 
 
 of PO4 ex- 
 
 of perco- 
 
 of PO4 in 
 
 of PO4 ex- 
 
 of perco- 
 
 of P04in 
 
 of PO4 ex- 
 
 late. 
 
 solution. 
 
 tracted 
 from soil. 
 
 late. 
 
 solution. 
 
 tracted 
 from soil. 
 
 late. 
 
 solution. 
 
 tracted 
 from soil. 
 
 Cubic cen- 
 
 Parts per 
 
 Parts -per 
 
 Cubic cen- 
 
 Parts per 
 
 Parts per 
 
 Cubic cen- 
 
 Parts per 
 
 Parts per 
 
 timeters. 
 
 million. 
 
 million. 
 
 timeters. 
 
 million. 
 
 million. 
 
 timeters. 
 
 million. 
 
 million. 
 
 40 
 
 26 
 
 10 
 
 360 
 
 7 
 
 35 
 
 690 
 
 7 
 
 55 
 
 130 
 
 8 
 
 18 
 
 540 
 
 6 
 
 45 
 
 740 
 
 5 
 
 57 
 
 170 
 
 8 
 
 21 
 
 590 
 
 6 
 
 48 
 
 790 
 
 7 
 
 61 
 
 260 
 
 8 
 
 28 
 
 640 
 
 6 
 
 61 
 
 850 
 
 7 
 
 64 
 
 In the first column is given the total number of cubic centimeters 
 of the solution which have passed through the soil, the concentrations 
 of the separate fractions in phosphate being given in the second col- 
 umn. In the third column are the figures for the total quantity- 
 removed by the water, the results being expressed in terms of parts 
 PO4 per million parts of the soil. It will be noticed that after the first 
 portion, which is considerably stronger than the others, has passed 
 the solution has practically a constant concentration in phosphate. 
 
 The results for the clay loam are given in Table VI. In this case 
 the concentration is likewise greatest in the first portion, diminish- 
 ing with succeeding portions, although the drop is not so abrupt as 
 with the clay soil, until again a constant concentration of solution is 
 obtained. 
 
 Table VI. — Removal of phosphate from a day loam hy water. 
 
 
 
 Total 
 
 
 
 Total 
 
 
 
 Total 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 of perco- 
 
 of PO4 in 
 
 of PO4 ex- 
 
 of perco- 
 
 of PO4 in 
 
 of PO4 ex- 
 
 of perco- 
 
 of PO4 in 
 
 of PO4 ex- 
 
 late. 
 
 solution. 
 
 tracted 
 from soil. 
 
 late. 
 
 solution. 
 
 tracted 
 from soil. 
 
 late. 
 
 solution. 
 
 tracted 
 from soil. 
 
 Cubic cen- 
 
 Parts per 
 
 Parts per 
 
 Cubic cen- 
 
 Paris per 
 
 Parts per 
 
 Cubic cen- 
 
 Parts per 
 
 Paris per 
 
 timeters. 
 
 million. 
 
 million. 
 
 timeters. 
 
 million. 
 
 million. 
 
 timeters. 
 
 million. 
 
 million. 
 
 30 
 
 19 
 
 6 
 
 250 
 
 6 
 
 28 
 
 510 
 
 6 
 
 45 
 
 80 
 
 17 
 
 15 
 
 360 
 
 6 
 
 35 
 
 550 
 
 6 
 
 47 
 
 140 
 
 10 
 
 20 
 
 410 
 
 6 
 
 38 
 
 
 
 
 190 
 
 10 
 
 25 
 
 460 
 
 7 
 
 41 
 
 
 
 
 In Table VII are given the results obtained with the fine sandy loam. 
 Here the first result is lower than the succeeding ones, but likewise 
 there is the same tendency to yield a solution of constant concentra- 
 tion in phosphate, although this is considerably higher than in the 
 solutions obtained from the other soils. This is perhaps due to the 
 greater rate of solubility of the phosphatic mineral constituents" of 
 the Podunk fine sandy loam used in this experiment as well as to the 
 lower absorptive capacity of this soil. The other minerals of this soil 
 are likewise quite soluble, and a solution quite high in total salts, com- 
 paratively speaking, is obtained. 
 
 «Cameron and Hurst [Jour. Amer. Chem. Soc, 26, 885 (1904)] have shown that 
 for short-time intervals the rate of solubiUty of phosphoric acid from slightly soluble 
 phosphates is conditioned by the ratio between the solid and the water. 
 
SOLUTION" OP MONOCALCIUM PHOSPHATE. 
 Table VII. — Removal of phosphate from a fine sandy loam by water. 
 
 15 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 of PO4 in 
 solution. 
 
 Total 
 quantity 
 of PO4 ex- 
 tracted 
 from soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 of PO4 in 
 solution. 
 
 Total 
 quantity 
 of PO4 ex- 
 tracted 
 from soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 of PO4 in 
 solution. 
 
 Total 
 quantity 
 of PO4 ex- 
 tracted 
 from soil. 
 
 Cubic cen- 
 timeters. 
 50 
 100 
 150 
 
 Parts per 
 
 million. 
 
 17 
 
 25 
 
 25 
 
 Parts per 
 
 million. 
 
 8 
 
 22 
 
 34 
 
 Cubic cen- 
 timeters. 
 200 
 300 
 350 
 
 Parts per 
 
 million. 
 
 23 
 
 21 
 
 18 
 
 Parts per 
 
 million. 
 
 45 
 
 55 
 
 64 
 
 Cubic cen- 
 timeters. 
 400 
 460 
 510 
 
 Parts per 
 
 million. 
 
 19 
 
 19 
 
 21 
 
 Parts per 
 
 million. 
 
 75 
 
 85 
 
 96 
 
 The results obtained with the fine sandy soil are given in Table VIII 
 and are similar to those already given. 
 
 Table YIII. — Removal of phosphate from a fine sandy soil by water. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 of PO4 in 
 solution. 
 
 Total 
 quantity 
 of PO4 ex- 
 tracted 
 from soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 of POiin 
 solution. 
 
 Total 
 quantity 
 of PO4 ex- 
 tracted 
 from soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 of P04in 
 solution. 
 
 Total 
 quantity 
 of PO4 ex- 
 tracted 
 from soil. 
 
 Cc. 
 
 60 
 
 110 
 
 180 
 
 230 
 
 P. p.m. 
 16 
 12 
 
 7 
 
 7 
 
 P. p.m. 
 9 
 
 36 
 21 
 25 
 
 Cc. 
 380 
 430 
 480 
 530 
 
 P. p. m. 
 5 
 7 
 6 
 5 
 
 P. p.m. 
 32 
 36 
 39 
 42 
 
 Cc. 
 580 
 640 
 
 P. p.m. 
 6 
 6 
 
 P. p.m. 
 45 
 
 48 
 
 The general tendency of the soils is to yield more concentrated 
 fractions at the start, but soon to give a solution of nearly constant 
 concentration in phosphate for each soil examined. The higher 
 concentration obtained at the start is interesting, especially in the 
 light of some of the absorption phenomena to be described further on 
 in this bulletin. To ascribe this to the existence in the soil of more 
 readily soluble phosphates which are quickly leached out seems not 
 to be tenable in view of the great absorptive power of these soils for 
 phosphate, even when introduced in exceedingly soluble forms, as 
 has already been shown. The greater concentration in the first por- 
 tions seems rather to be connected with the air-dry condition of the 
 soils. In earlier bulletins from this Bureau '^ it has been demonstrated 
 that oven and air dried soils yield a greater quantity of soluble salt to 
 water than do the same soils in the moist condition. This greater 
 concentration in phosphate in the case of the dried soils may be, at 
 least in part, due to the lower absorptive power of the soil when used 
 in the dry form, and will be considered later. 
 
 ABSORPTION OF PHOSPHATE FROM A SOLUTION OF MONOCALCIUM 
 
 PHOSPHATE. 
 
 The phosphate used in these experiments was the monocalcium phos- 
 phate, CaH^(P0j2, as this is the most soluble of the phosphates of 
 
 « Bureau of Soils, U. S. Dept. of Agr., BuL 22, 1903, p. 42; Bui. 26, 1905, p. 55. 
 
16 
 
 ABSORPTION OF PHOSPHATES AISTD POTASSIUM. 
 
 calcium and also the one occurring in superphosphate fertilizers. A 
 solution containing 200 parts of PO^ per million of the solution was 
 prepared by diluting a stronger solution which had been standardized 
 by gravimetric analysis. 
 
 In these experiments the same samples of soils used in the previous 
 experiment with the water percolation were studied. The tubes of soil 
 were allowed to drain thoroughly. The apparatus was then filled with 
 this solution as already described under the water percolation. The 
 volume of water or rather soil solution remaining in the apparatus 
 must necessarily have been less than 50 e.c. and this volume of perco- 
 late was discarded at the beginning of the flow of solution through it. 
 The flow was maintained at a rate of about 50 e.c. in twenty-four 
 hours. Fractional percolates were collected and the phosphate deter- 
 mined colorimetrically. The results obtained with the clay soil are 
 given in Table IX. In the first column is given the total volume of 
 the phosphate solution which has passed through the soil, the second 
 column giving the concentration of phosphate in the separate portions. 
 The third column gives the amount of phosphate absorbed by the soil, 
 expressed in parts per million. In the fourth column are given the 
 results calculated by means of a formula derived from certain theoret- 
 ical considerations which will be developed later. 
 
 It will be seen from the table that the phosphate is almost completely 
 absorbed from the first ■iOO e.c. passed, the concentration having been 
 reduced from 200 parts per million to 6 and 8 parts per million, practi- 
 call}^ the concentration juelded by the percolation of pure water through 
 the soil. As more solution flows through the soil the concentration 
 of phosphate in the percolate increases, rapidly at first, and then more 
 slowly. The absorption is still going on to a considerable extent even 
 after the passage of nearly 6 liters of the solution and a total absorp- 
 tion by the soil of nearl}^ 5,000 parts per million of phosphate. The 
 quantity absorbed by the soil as shown in the third column increases 
 rapidly at first and then more slowly as the total amount already 
 absorbed increases. The fourth column will be discussed presently. 
 
 Table IX. — Absorption of phosphate by a clay soil from a solution of monocalcium phos- 
 phate, CaH^{P0^2i containing 200 jiarts pex million PO^. 
 
 
 
 Total quantity 
 
 
 
 Total quantity 
 
 
 
 Total quantity 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 
 
 
 
 
 
 
 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 e.c. 
 
 P. p.m. 
 
 P. p.m. 
 
 P. p.m. 
 
 e.c. 
 
 P. p.m. 
 
 P. p.m. 
 
 P. p. m. 
 
 e.c. 
 
 P. p.m. 
 
 P. p.m. 
 
 P. p.m. 
 
 250 
 
 6 
 
 480 
 
 480 
 
 1,660 
 
 101 
 
 2,470 
 
 2,480 
 
 3,890 
 
 150 
 
 4,100 
 
 4,160 
 
 410 
 
 8 
 
 790 
 
 740 
 
 2,050 
 
 115 
 
 2,800 
 
 2,880 
 
 4,290 
 
 152 
 
 4,300 
 
 4,330 
 
 570 
 
 12 
 
 1,090 
 
 1,020 
 
 2, ,510 
 
 108 
 
 3,220 
 
 3,270 
 
 4,640 
 
 156 
 
 4,460 
 
 4,480 
 
 780 
 
 31 
 
 1,4.50 
 
 1,3.50 
 
 2,820 
 
 111 
 
 3.440 
 
 3, 510 
 
 5,000 
 
 162 
 
 4, .590 
 
 4,600 
 
 1,030 
 
 58 
 
 1,800 
 
 1,720 
 
 3,260 
 
 127 
 
 3,760 
 
 3,810 
 
 5,370 
 
 158 
 
 4,740 
 
 4,720 
 
 1,330 
 
 84 
 
 2, 1.50 
 
 2, 100 
 
 3,640 
 
 142 
 
 3,980 
 
 4,030 
 
 5,740 
 
 169 
 
 4,860 
 
 4,820 
 
SOLUTION OF MONOCALCIUM PHOSPHATE. 
 
 17 
 
 In Table X are found the results obtained with the clay loam. It 
 will be noticed that here again the concentration of the first portions 
 is very low, approximately that obtained by the action of water on 
 the soil, and that the concentration of the percolate rises as more solu- 
 tion is passed through the soil, but much more rapidly than in the 
 case of the clay soil. The concentration when about 4 liters of solu- 
 tion have passed through the clay loam is appreciably^ greater than 
 when 6 liters of solution have passed through the clay soil. The 
 quantity absorbed hy the clay loam is considerably less than by the 
 clay soil when any given volume of solution has passed through the 
 soil. The column giving the quantities absorbed by the clay loam is 
 not materially different from that in the table for the clsiy soil, except 
 in the quantities absorbed. This will be discussed later with the results 
 in the last column. 
 
 Table X. — Absorption of phosphate by a day loam from a solution of monocalcium 
 phosphate, CaH^{P0^).2, containing SOO parts per million PO^. 
 
 
 
 Total quantit}^ 
 
 
 
 Total quantity 
 
 
 
 Total quantity 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 
 
 
 
 
 
 
 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 C.c. 
 
 P. p. m. 
 
 P. p. m. 
 
 P. p. m. 
 
 C.c. 
 
 P. p. m. 
 
 P. p. m. 
 
 P. p. m. 
 
 C.e. 
 
 P. p. m. 
 
 P. p. m. 
 
 P. p. m. 
 
 110 
 
 6 
 
 220 
 
 200 
 
 1,420 
 
 138 
 
 1,830 
 
 1,810 
 
 3,410 
 
 167 
 
 2,720 
 
 2, 720 
 
 220 
 
 6 
 
 420 
 
 390 
 
 1,690 
 
 137 
 
 2,000 
 
 2,000 
 
 3,700 
 
 176 
 
 2,790 
 
 2,780 
 
 360 
 
 38 
 
 650 
 
 610 
 
 1,890 
 
 159 
 
 2,080 
 
 2,130 
 
 4,050 
 
 187 
 
 2,880 
 
 2,850 
 
 590 
 
 60 
 
 990 
 
 940 : 
 
 2,170 
 
 155 
 
 2,210 
 
 2, 280 
 
 4,340 
 
 189 
 
 2,860 
 
 2,900 
 
 730 
 
 47 
 
 1,200 
 
 1,120 
 
 2,470 
 
 150 
 
 2,360 
 
 2,420 
 
 4,570 
 
 178 
 
 2,920 
 
 2,910 
 
 940 
 
 78 
 
 1,460 
 
 1,370 
 
 2, 800 
 
 148 
 
 2,530 
 
 2, 550 
 
 
 
 
 
 1,110 
 
 95 
 
 1,640 
 
 1,550 
 
 3,150 
 
 170 
 
 2,630 
 
 2, 650 
 
 
 
 
 
 In Table XI will be found the results obtained with the fine sandy 
 loam. These are very similar in character to those for the soils 
 already given, the concentration of the first fraction being again 
 approximately that of the solution obtained from the soil by percola- 
 tion with pure water. The total amount absorbed by the soil is, 
 moreover, less throughout than that absorbed by either the clay loam 
 or clay soil. 
 
 Table XI. — Absorp)tion of phosphate by a fine sandy loam from a solution of monocalcium 
 phosphate, Call^^POi)^, containing 200 parts per million PO^. 
 
 
 
 Total quantity 1 
 
 
 
 Total quantity 
 
 
 
 Total quantitv 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 ume of 
 
 tity POi 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 
 
 
 
 
 
 
 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calo<i- 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 
 P. p.m.' 
 
 C.c. 
 
 P. p.m. 
 
 P. p. m,. 
 
 P. p.m. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 
 P. p.m. 
 
 150 
 
 22 
 
 260 
 
 290 
 
 1,270 
 
 127 
 
 1,660 
 
 1,650 
 
 3,000 
 
 156 
 
 2, 460 
 
 2,450'. 
 
 280 
 
 27 
 
 500 
 
 500 1 
 
 1,450 
 
 146 
 
 1,7.50 
 
 1,780 
 
 3, 300 
 
 185 
 
 2,500 
 
 2,510 
 
 430 
 
 31 
 
 740 
 
 740 ■ 
 
 1,660 
 
 135 
 
 1,890 
 
 1,920 
 
 3,540 
 
 190 
 
 2,530 
 
 2,560 
 
 640 
 
 55 
 
 1,040 
 
 1,010 ' 
 
 1,970 
 
 157 
 
 2,020 
 
 2,080 
 
 3,860 
 
 177 
 
 2,600 
 
 2,610 
 
 840 
 
 64 
 
 1,320 
 
 1,2.50 
 
 2, 340 
 
 167 
 
 2,140 
 
 2,250 
 
 4,150 
 
 185 
 
 2, 640 
 
 2,640 
 
 1,040 
 
 115 
 
 1,490 
 
 1,450 
 
 2,710 
 
 149 
 
 2,330 
 
 2,370 
 
 4,390 
 
 182 
 
 2,690 
 
 2, 670 
 
 21719— No. 32—06- 
 
18 
 
 ABSORPTION OF PHOSPHATES AND POTASSIUM. 
 
 In Table XII are given the results obtained with the tine sandy soil. 
 The absorptive power of this soil is considerably less than that of the 
 soils alread}^ considered, as is shown by the third column of figures 
 and also by the relatively higher concentrations in the phosphate solu- 
 tions throughout. It will be noticed also that this lower absorptive 
 power of the soil is shown by the fact that the concentration of the 
 first fractions is higher than the concentration obtained by percolating 
 water through the soil, while with the other soils they were approxi- 
 mately the same. 
 
 Table XII. — Absorption of phosphate by a fine sandy soil from a solution of monocnlcium 
 phosphate, CaH^{POi).,, cordaining 200 jyarts per million PO^. 
 
 
 
 Total quantity 
 
 
 
 Total quantity 
 
 
 
 Total quantity 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 Vol- 
 
 Quan- 
 
 PO4 absorbed 
 
 vim e of 
 
 tity FO4 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity PO4 
 in solu- 
 
 by soil. 
 
 
 
 
 
 
 
 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 
 
 seryed. 
 
 lated. 
 
 
 
 ?e;yed. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 C.c. 
 
 P. p. m. 
 
 P. p. m. 
 
 P. p. in. 
 
 C.c. 
 
 P. p. m. 
 
 P. p m. 
 
 P. p. m. 
 
 a c. 
 
 P. p. m. 
 
 P. p. m. 
 
 P. p. in. 
 
 140 
 
 20 
 
 250 
 
 210 
 
 1,150 
 
 152 
 
 1,210 
 
 1,260 
 
 2,750 
 
 159 
 
 2, 050 
 
 2,060 
 
 270 
 
 19 
 
 480 
 
 400 
 
 1,340 
 
 141 
 
 1,320 
 
 1,400 
 
 3,090 
 
 148 
 
 2,230 
 
 2,150 
 
 380 
 
 39 
 
 660 
 
 530 
 
 1,560 
 
 142 
 
 1,440 
 
 1,540 
 
 3,330 
 
 174 
 
 2,290 
 
 2,210 
 
 540 
 
 82 
 
 850 
 
 700 
 
 1,800 
 
 159 
 
 1,540 
 
 1,660 
 
 3, 620 
 
 191 
 
 2,320 
 
 2, 270 
 
 670 
 
 121 
 
 950 
 
 850 
 
 1,980 
 
 122 
 
 1,690 
 
 1,760 
 
 3,850 
 
 192 
 
 2,340 
 
 2,310 
 
 810 
 
 129 
 
 1,050 
 
 980 
 
 2,210 
 
 143 
 
 1,820 1 1,870 
 
 4,040 
 
 191 
 
 2,350 
 
 2,340 
 
 950 
 
 156 
 
 1,120 
 
 1,100 
 
 2,460 
 
 155 
 
 1,930 1,960 
 
 
 1 
 
 
 REMOVAL OF ABSORBED PHOSPHATE BY WATER. 
 
 At the conclusion of the absorption experiments described in the 
 preceding section the tubes with the soils were allowed to drain 
 thoroughly, and then the apparatus was filled with distilled water in 
 the manner already described. The flow of water was again at the 
 rate of about 50 c. c. in twenty-four hours, the percolate being col- 
 lected in fractions and the concentration of phosphate determined 
 colorimetrically as before. The results obtained with the ck}" soil are 
 given in Table XIII. The first column gives the volume of solution 
 which has passed through the soil and the second the concentrations of 
 the separate fractions as before. At the head of the third column is 
 given the total quantity of phosphate absorbed by the soil in the pre- 
 vious experiment, and following this are the amounts of absorbed 
 phosphate not washed out. 
 
 Table XIII. — Removal of absorbed phosphate from a clay soil by tvater. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 4,860 
 4,730 
 4,630 
 4,510 
 4,450 
 
 C.c. 
 730 
 910 
 1,140 
 1,330 
 1,750 
 
 P. p.m. 
 36 
 32 
 27 
 ■ 21 
 21 
 
 P. p.m. 
 4,410 
 4,350 
 4,290 
 4,250 
 4,160 
 
 C.c. 
 2,280 
 
 2, 590 
 2,970 
 3,370 
 3,870 
 
 P.p.m. 
 20 
 19 
 16 
 16 
 11 
 
 P.p.m. 
 4,050 
 3,990 
 3,930 
 3, 870 
 3,810 
 
 C.c. 
 
 4,450 
 
 5,030 
 
 5,450 
 
 5,870 
 
 6,300 
 
 P.p.m. 
 8 
 
 7 
 8 
 7 
 7 
 
 P.p. m. 
 
 3,760 
 
 3,720 
 
 3,690 
 
 3,660 
 
 3,630 
 
 100 
 230 
 440 
 580 
 
 127 
 85 
 55 
 41 
 
EEMOVAL OF ABSORBED PHOSPHATE BY WATEE. 
 
 19 
 
 It* will be noticed that the concentration of the first fractions is 
 very hig'h. The concentration in phosphate decreases rapidly at first 
 and then very slowly, until at about 4 liters the concentration of the 
 solution has become constant and is practically the same as that found 
 at the beginning of the absorption and that of the percolate from the 
 original soil, although the quantity of absorbed phosphate still remain- 
 ing in the soil is nearl}^ 3,800 parts per million, or approximatel}^ 75 
 per cent of the total phosphate absorbed. The results with the clay 
 loam, given in Table XIV, show a similar tendency. The high con- 
 centration in phosphate in the first portions gradually gives place to 
 lower concentrations, until at about 5 liters the concentration reached 
 is that of the solution obtained at the start of the absorption or by 
 percolating water through the original soil. The amount of absorbed 
 phosphate still in the soil is, however, considerable at this point, 
 being about 1,900 parts per million, or approximate!}^ 65 per cent of 
 the total phosphate absorbed. 
 
 Table XIV. — Removal of absorbed phosphate from a clay loam bi/ water. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity PO4 
 in solu- 
 tion. 
 
 Total i 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity PO4 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity PO4 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity PO4 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 2,920 
 2,840 
 2,760 : 
 2,670 ; 
 2,690 
 
 ■ 2,540 ' 
 
 C.c. 
 
 1,030 
 
 1,250 
 
 1,460 
 
 1,700 
 
 1,900 
 
 2,200 
 
 P.p. m. 
 23 
 
 17 
 17 
 16 
 15 
 14 
 
 P.p. m. 
 2,500 
 2,460 
 2,420 
 2,390 
 2, 360 
 2, 310 
 
 C.c. 
 
 2,460 
 
 2,770 
 
 3,080 
 
 3,400 
 
 3,730 
 
 3,970 
 
 P. p.m. 
 17 
 17 
 13 
 
 16 
 15 
 14 
 
 P. p.m. 
 2,270 
 2,210 
 2,180 
 2,120 
 2,070 
 2,040 
 
 ac. 
 
 4, 530 
 5,100 
 5,690 
 6,270 
 6,830 
 7,340 
 
 P. p.m. 
 11 
 8 
 7 
 7 
 6 
 6 
 
 P. p.m. 
 1,980 
 1,940 
 1,900 
 1,860 
 1,820 
 1,790 
 
 90 
 180 
 400 
 630 
 830 
 
 89 
 77 
 41 
 36 
 25 
 
 The results obtained with the fine sandy loam are given in Table 
 XY. A concentration of phosphate in the solution approximately 
 equal to that of the first fraction in the absorption experiments, or to 
 that of the solution obtained by percolating water through the original 
 soil, is found to be reached when about 3 liters of percolate have been 
 obtained. The absorbed phosphate still remaining in the soil at this 
 point is about 1,600 parts per million, or about 60 per cent of the total 
 quantity absorbed. The concentration of the solution in phosphate 
 decreases, however, until it falls considerably below that of the solu- 
 tions obtained by percolating water through the original soil, although 
 there is present a much larger amount of phosphate in the soil at this 
 stage than there was in the original soil. 
 
20 
 
 ABSORPTION OF PHOSPHATES AND POTASSIITM. 
 
 Table XV. — Removal of absorbed phosphate from a fine sandy loam by imter. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 PO4 in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 2,690 
 2.540 
 2,430 
 2,330 
 2,200 
 2,110 
 
 C.c. 
 
 1,030 
 
 1,240 
 
 1,470 
 
 1,690 
 
 1,890 
 
 2,120 
 
 P. p.m. 
 34 
 30 
 28 
 26 
 23 
 22 
 
 P. p.m. 
 2,040 
 1,980 
 1,910 
 1,860 
 1,810 
 1,760 
 
 C.c. 
 
 2,320 
 
 2,820 
 
 3,140 
 
 3,490 
 
 3,890 
 
 4,290 
 
 P. p. III. 
 21 
 23 
 23 
 22 
 15 
 13 
 
 P. p. m. 
 1,720 
 1,600 
 1, 530 
 1,450 
 1,390 
 1,340 
 
 ac. 
 
 4,600 
 5,180 
 5,720 
 6,140 
 6,570 
 
 P. p. m. 
 12 
 11 
 10 
 11 
 10 
 
 P. p.m. 
 
 1,300 
 
 1,240 
 
 1,190 
 
 1,140 
 
 1,100 
 
 i25 
 
 250 
 3S0 
 610 
 820 
 
 124 
 87 
 76 
 56 
 42 
 
 It has been pointed out alread}- that the Podunk fine sandy loam 
 used in these experiments contained minerals which were acted upon 
 by water to a very considerable extent. The percolation of over 40 
 times its weight of the calcium phosphate solution, containing- 42 parts 
 per million Ca in addition to the 200 parts per million PO^, together 
 with the subsequent washing with over 60 times its weight of water, 
 has produced chemical changes in the soil itself, so that we have no 
 longer the identical soil started with. In fact, this must be the con- 
 clusion reached from the figures given above, so far as the removal of 
 the phosphate from the soil is concerned. In Table XVI are found 
 the results for the fine sandy soil. 
 
 Table XVI. — Removal of absorbed pihosphate from a fine sandy soil by water. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 PO4 in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol. 
 umeof 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 P04 re- 
 main- 
 ing in 
 soil. 
 
 C.c. 
 
 P.p. m. 
 
 P. p. m. 
 2, .3.50 
 2,19C 
 2,120 
 2,060 
 2,020 
 1,970 
 
 C.e. 
 1,060 
 1,270 
 1,490 
 1,740 
 1,950 
 2,180 
 
 P. p. m. 
 19 
 17 
 13 
 13 
 12 
 11 
 
 P.p. m. 
 1,930 
 1,890 
 1,860 
 1,830 
 1,810 
 1,780 
 
 Cc. 
 2,400 
 2,680 
 2,940 
 3, 200 
 3,540 
 4,100 
 
 P. p. m. 
 10 
 9 
 8 
 
 9 ■ 
 7 
 8 
 
 P.p. m. 
 1,760 
 1,740 
 1,710 
 1,690 
 1,670 
 1, 620 
 
 a c. 
 4,610 
 5,090 
 5,670 
 6,230 
 6,660 
 7,080 
 
 P. p. m. 
 7 
 7 
 5 
 6 
 6 
 5 
 
 P. p. m. 
 1,590 
 1,550 
 1,530 
 1,490 
 1,470 
 1,450 
 
 110 
 240 
 430 
 620 
 840 
 
 150 
 51 
 31 
 24 
 21 
 
 The concentration of the phosphate in the fractions of percolate 
 decreases rapidly at first and then very slowly, approaching a uniform 
 concentration toward the end, which is considerably lower than that of 
 the first fraction obtained in the absorption experiment. It is approxi- 
 matel}^ the concentration obtained from the original soil hj passing- 
 pure water through it, although the amount of phosphate present in 
 the soil is considerabh" greater. 
 
RESULTS WITH MONOCALCIUM PHOSPHATE. 21 
 
 GRAPHICAL REPRESENTATION AND DISCUSSION OF THE RESULTS OBTAINED 
 WITH MONOCALCIUM PHOSPHATE. 
 
 The results obtained in the foregoing experiments on the absorption 
 of phosphate from a solution of monocalcium phosphate, together with 
 those obtained in removing the absorbed phosphate, are shown graphic- 
 ally in the following figures. In figure 2 are shown the results 
 expressed in terms of the solutions. The abscissas represent the liters 
 of solution or of water which have been passed through the 100 grams 
 of soil in the percolating tube. The ordinates give the concentration of 
 the percolate in phosphate. The upper boundar}^ line of the figure 
 represents the strength of the solution before passing through the 
 soil; the break in each of the curves gives the point where the 200 
 parts per million solution was replaced by distilled water and the 
 washing out of the absorbed phosphate was begun. The intersection 
 of each of the curves on the axis of ordinates shows the concentration 
 of phosphate solution obtained by percolating water through the origi- 
 nal soil. The curves in this figure are smoothed curves; the experi- 
 mental points are not given in the diagram, since the large number 
 and intermingling of the points would be confusing, as can be seen by 
 examining the tables. The curves, however, represent very well the 
 general facts brought out by the figures in the tables. It will be noticed 
 that the absorption of phosphate from the solution by the soil is consider- 
 able for approximately the first 300 c. c. of the solution passed, and 
 that the concentration of the solution is practically reduced to that of 
 the soil extract obtained from the original soil. After this the curves 
 rise with increasing volume of solution passed, rapidly in the case of 
 the sandy soil and much more slowly in the case of the clay soil. All 
 the curves approach the line representing the concentration of the 
 original solution, namel}", 200 parts per million, apparently as an 
 asymptote. The curves for the removal of the absorbed phosphate 
 show a very decided drop at the start and then rapidly tend to become 
 horizontal when concentrations have been reached approximating those 
 of the water extracts of the original soils or those of the first few 
 fractions of percolate in the absorption experiments. This is shown 
 by comparing the extremes of the curves. It is, moreover, significant 
 that the soils, after this absorption treatment, yield solutions which 
 are much closer together in concentration of phosphate than did the 
 original soils, although the quantities of phosphate in the soils are in 
 all cases greater than those originally present. The concentrations at 
 the ends of the curves are practically identical in the case of the fine 
 sandy soil, clay loam, and clay soil studied. In the case of the fine 
 sandy loam the concentration of the aqueous extract of the original 
 soil is somewhat greater than the concentration of the last percolate, 
 
22 
 
 ABSORPTION OF PHOSPHATES AND POTASSIUM. 
 
 although the soil actually contains more phosphates than it did 
 originall3\ 
 
 In figure 3 are shown the results on the basis of the soil itself, the 
 abscissas being the liters of solution passed as before and the ordi nates 
 the parts of phosphate per million parts of soil. The experimental 
 points as found in the tables are indicated in the figure by specific 
 signs. It is at once apparent that the absorption curves of the differ- 
 
 FiG. 2. — Solution curves. Absorption of phosphate by soils from a solutionof monocalcinm phosphate 
 and the removal of the absorbed phosphate'by water. . 
 
 CLftY^SOIL 
 
 PKt. 3. — Soil curves. Absorption of phosphate by soils from a solution of monocalcium phosphate and 
 the removal of the absorbed phosphate by water. 
 
 ent soils are quite similar in character, although these soils diflfer in the 
 amount of the phqsphates absorbed at any given abscissa. The curve 
 for the sand}^ «oil indicates a rapid approach to a horizontal position, 
 that is, a condition of saturation for phosphate, at about 2,400 parts 
 per million, whereas the curve for the clay soil at about twice the height 
 is still showing a decided tendency to rise. The curves for the removal 
 of the absorbed phosphate drop rapidly at first and then run smoothly 
 and graduall}^ downward. The removal curve for the sandy loam has 
 a steeper slope than the others, which fact is of course also shown by 
 the higher concentration of the phosphate solution, as appears from 
 figure 2. A comparison of the two figures shows also very strikingl}^ 
 that with all the soils studied a solution of low phosphate content, 
 approximately of the strength of the extract obtained from the origi- 
 nal soil, is obtained when only a fractional part of the absorbed phos- 
 phate has been removed. 
 
RESULTS WITH MONOCALCIUM PHOSPHATE. 23 
 
 These graphical representations of the absorption results indicate, 
 as has already been pointed out, that the soils are approaching a satu- 
 ration for phosphate under the conditions of the experiment, as is 
 shown by the fact that each curve is evidentl}^ approaching a horizon- 
 tal asymptote. It has been found that these absorption phenomena 
 are quite accuratel}" represented b}^ an expression of the same form as 
 monomolecular reaction velocities, rate of solution, and other analogous 
 processes. Lagergren" has already shown that the rate of adsorption 
 of several organic acids by charcoal, in respect to time, behaves like 
 a monomolecular reaction. 
 
 If we represent the maximum quantity that the soil can take up from 
 the 200 parts per million solution, that is, the ordinate of the asymptote 
 just mentioned, and let y equal the parts per million PO^ it has taken 
 up when the volume v has passed through the soil, the simplest assump- 
 tion that can be made is that the quantity absorbed from a unit volume 
 of solution as it passes through the soil is proportional to the amount 
 the soil can still take up. This is represented by the following differ- 
 ential equation: 
 
 Integrating between limits 
 
 we get 
 
 log (^4 — «'/)— log A^—Kv, 
 or 
 
 log {A—y) =log A~Kv. 
 
 Since log A and K are constants to be found from the observations, 
 Briggsian instead of Naperian logarithms may be used in the calcula- 
 tions. This will merely change the values of A^'and log A in the same 
 ratio. 
 
 The simplest way to test the applicability of the equation appears 
 to be (1) to determine A by inspection of the curve obtained b}" plot- 
 ting the experimental data; (2) to make a table of log {A—y)\ (3) to 
 make a plot using v as abscissa and log {A~y) as ordinate. It will 
 
 «Bihang till K. Sv. Vet.-Akad. Handl., 24, Afd. II, No. 5 (1898). 
 
24 
 
 ABSOKPTTOlSr OF PHOSPHATES AND POTASSIUM. 
 
 be n&ticed that the above logarithmic equation is that of a straight line. 
 The plot will therefore be a straight line if the correct value for J_ 
 has been chosen. The intercept on the axis of ordinates is log A and 
 the slope of the line gives the constant Ii. 
 
 The absorption results for all of the soils studied have been tested 
 in this way, and it has been found that a fair agreement with the 
 formula is obtained when the following constants are used: 
 
 Soil. 
 
 A. 
 
 log^. 
 
 K. 
 
 
 5, 500 
 3,100 
 2,800 
 2,600 
 
 3.740 
 3.491 
 3.447 
 3.413 
 
 0. 000158 
 . 000267 
 . 000299 
 . 000247 
 
 
 
 Fine sandy soil 
 
 
 The results obtained by using these values in calculating the parts per 
 million PO^ absorbed b}^ the soil for any given volume of the solution 
 which has percolated through it are given in the fourth columns of 
 the tables for the respective soils. In figure 3 the absorption curves 
 are drawn through the calculated points, and the experimental points 
 are indicated by the specific signs. The plots show, therefore, how 
 well the calculated and. the experimental results agree. These are 
 given in the third and fourth columns, respectively, of the absorption 
 tables. 
 
 Apparently the removal curves are likewise represented by a similar 
 formula when the first few points are left out of consideration. This 
 disagreement for the first few points may be due to the fact that a part 
 of the strong phosphate solution remained in the soil when the wash- 
 ing process was begun. The equation for the removal of phosphates 
 from the soil may be written as follows: 
 
 i=^^'0/- 
 
 i?) 
 
 where Jj is the ordinate corresponding to the asymptote of the removal 
 curve — that is, the amount of phosphate which has apparently become 
 permanently insoluble so far as any reasonable amount of solvent pass- 
 ing through it is concerned. The calculated results obtained with this 
 equation bring out no information not given b}^ the plotted curves, 
 and are therefore omitted. 
 
 ABSORPTION OF PHOSPHATE FROM A SOLUTION OF DISODIUM PHOSPHATE 
 AND REMOVAL OF THE ABSORBED PHOSPHATE BY WATER. 
 
 In the following experiments a solution of sodium phosphate was 
 used instead of the monocalcium phosphate of the previous experi- 
 ments. The solution used contained 200 parts of PO^ per million of 
 the solution and was prepared by diluting a strong solution which had 
 
DISODIUM PHOSPHATE SOLUTION'. 
 
 25 
 
 been standardized by gravimetric analysis. The same clay, clay loam, 
 fine sandy loam, and fine sand were used as in the experiments with 
 calcium phosphate. The apparatus was charged by putting some of 
 the solution into the percolating tube and then adding the dry soil. 
 The tube was then completely filled with solution in the manner 
 already described, and all other operations were exactly the same as in 
 the preceding experiments. The absorption was not carried so far as 
 in the experiments with the calcium phosphate. 
 
 Table XVII. — Absorption of phosphate by a day soil from a solution of sodium phosphate, 
 Na^HPO^, containing 200 parts per million PO^. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 PO4 in so- 
 lution. 
 
 Total quantity PO4 
 absorbed by soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 PO^ in so- 
 lution. 
 
 Total quantity PO4 
 absorbed by soil. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 C.c. 
 190 
 220 
 320 
 410 
 520 
 
 P. p. m. 
 154 
 23 
 
 8 
 5 
 8 
 
 P. p. m. 
 90 
 150 
 330 
 510 
 710 
 
 P. p. m. 
 
 C.c. 
 610 
 740 
 850 
 900 
 970 
 
 P. p. m. 
 12 
 24 
 
 42 
 48 
 61 
 
 P. p. m. 
 890 
 1,110 
 1,280 
 1,360 
 1,470 
 
 P. p. m. 
 900 
 1,110 
 1,280 
 1,370 
 1,480 
 
 150 
 330 
 510 
 710 
 
 In Table XVII are given the absorption results with the clay soil. 
 As before, the fourth column gives the amount calculated by a for- 
 mula to be discussed presently. It is at once apparent from the 
 second column, giving the concentration in phosphate of the separate 
 percolates, that the results differ from those where the wet soil was 
 treated with the solution of monocalcium phosphate. The concentra- 
 tion in phosphate, instead of being low at the very start and then 
 gradually rising, is in this case quite high at first, rapidly drops down 
 to a very low concentration, and then rises gradually. This high con- 
 centration at the start may be due in part to the more rapid movement 
 of the liquid through the soil in the beginning, while the rate of flow 
 is becoming adjusted, but the less absorptive power of the soil when 
 brought in the dry form into contact with the solution is probably a 
 much more potent cause. 
 
 When 1,470 parts PO^ per million had been absorbed by the soil, 
 the drained soil was treated with water in the manner previously 
 described. The results are given in Table XVIII. 
 
 Table XVIII. — Removal of absorbed phosphate from a clay soil by ivater. 
 
 
 
 Total 
 
 
 
 Total 
 
 
 
 Total 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 of perco- 
 
 PO4 in so- 
 
 PO4 re- 
 
 of perco- 
 
 PO4 in so- 
 
 PO4 re- 
 
 of perco- 
 
 PO4 in so- 
 
 PO4 re- 
 
 late. 
 
 lution. 
 
 maining 
 
 late. 
 
 lution. 
 
 maining 
 
 late. 
 
 lution. 
 
 maining 
 
 
 
 in soil. 
 
 
 
 in soil. 
 
 
 
 in soil. 
 
 C. c. 
 
 P. p. m. 
 
 P. p. m. 
 
 C. c. 
 
 P. p. m. 
 
 P. p. m. 
 
 C.c. 
 
 P. p. m. 
 
 P. p. m. 
 
 
 
 1,470 
 1,420 
 
 500 
 60'' 
 
 14 
 9 
 
 1,340 
 1,330 
 
 1,140 
 1,330 
 
 8 
 11 
 
 1,290 
 1,270 
 
 100 
 
 47 
 
 •220 
 
 31 
 
 1,380 
 
 720 
 
 7 
 
 1, 320 
 
 1,500 
 
 9 
 
 1,250 
 
 320 
 
 15 
 
 1,360 
 
 870 
 
 7 
 
 1,310 
 
 1,650 
 
 11 
 
 1,240 
 
 410 
 
 15 
 
 1, 350 
 
 990 
 
 9 
 
 1,300 
 
 
 
 
26 
 
 ABSORPTIOTSr OF PHOSPHATES AND POTASSIUM. 
 
 The second column in the table indicates very strikingly the simi- 
 larity in the removal of the absorbed phosphate from this soil and 
 from the same soil in the preceding series, in that the solutions rapidly . 
 run down to a practically constant concentration which is comparable 
 with that obtained from the original soil by percolation with water. 
 As in the preceding series, the amount of absorbed phosphate remain- 
 ing in the soil when this constant concentration of the soil solution is 
 reached is still large. 
 
 In Table XIX are given the absorption results obtained with the 
 clay loam. 
 
 Table XIX. — Absorption of phosphate by a clay loam from a solution of sodium phos- 
 phate, Na^HPOi, containing 200 parts per million PO^. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity PO4 
 in solu- 
 tion. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity PO4 
 in solu- 
 tion. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity PO4 
 in solu- 
 tion. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 C.c. 
 60 
 100 
 150 
 260 
 
 P. p. m. 
 
 127 
 
 19 
 
 8 
 
 P.p.m.. 
 
 40 
 
 120 
 
 220 
 
 430 
 
 P. p. m. 
 
 '"lib" 
 220 
 
 430 
 
 C.c. 
 360 
 460 
 550 
 660 
 
 P.p.m. 
 5 
 19 
 32 
 
 77 
 
 P.p.m. 
 
 620 
 
 760 
 
 910 
 1,050 
 
 P.p.m. 
 640 
 760 
 920 
 
 1,050 
 
 C.c. 
 770 
 810 
 
 P.p.m. 
 
 71 
 86 
 
 P.p.m. 
 
 1,180 
 
 1,240 
 
 P. p. m. 
 1,170 
 1,230 
 
 These results are seen to be very similar to those obtained with the 
 clay, the concentration of the solution rapidly decreasing to a very 
 low value, and then graduall}^ rising. The passage of the solution was 
 exceedingly slow in this case and a considerable length of tubing had 
 to be added to the apparatus in order to secure the necessary pressure 
 for forcing the solution through the soil. For this reason the experi- 
 ment was stopped when only about 1,200 parts per million PO^ had 
 been absorbed by the soil. The percolate was far from showing a 
 saturated condition for the soil. An attempt was nevertheless made to 
 study the removal of the absorbed phosphate by water, but when 
 approximately 600 c. c. had passed the pressure necessary for filtration 
 became so great that the experiment could not be continued in the 
 apparatus used. The few results obtained are given in Table XX. 
 
 Table XX. — Removal of absorbed phosphate from a clay loam by uuter. 
 
 Volume 
 of perco- 
 late. 
 
 Qnantitv 
 
 PO4 in 
 solution. 
 
 Total 
 quantity 
 
 PO4 re- 
 maining 
 
 in soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 
 P04in 
 
 solution. 
 
 Total 
 quantity 
 
 PO4 re- 
 maining 
 
 in soil. 
 
 C. c. 
 
 P. p. m. 
 
 P.p.m. 
 1,240 
 1,170 
 1,090 
 1,010 
 
 C.c. 
 380 
 490 
 580 
 
 P.p.m. 
 54 
 48 
 46 
 
 P.p.m. 
 940 
 910 
 870 
 
 90 
 200 
 300 
 
 77 
 75 
 78 
 
DI80DIUM PHOSPHATE SOLUTION". 
 
 27 
 
 The results show the same general tendency as in the experiments 
 already described. It will also be noticed that these results, together 
 with those of the previous experiment, show conclusively that the 
 washing-out process of the absorbed phosphate is essentiall}^ the same 
 in kind, whether the soil has absorbed a large or a small amount of 
 phosphate. 
 
 The absorption results obtained with the fine sandy loam are given 
 in Table XXI. 
 
 Table XXI. — Absorption of phosphate by a fine sandy loam from a solution of sodium 
 phosphate, Na^HPO^, containing 200 parts per million PO^. 
 
 Vol- 
 ume 
 of per- 
 colate. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Vol- 
 ume 
 of per- 
 colate. 
 
 Quan- 
 tity 
 P04in 
 solu- 
 tion. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Vol- 
 ume 
 of per- 
 colate. 
 
 Quan- 
 tity 
 
 PO4 in 
 solu- 
 tion. 
 
 Total quantity 
 PO4 absorbed 
 by soil. 
 
 Ob- 
 served. 
 
 •Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 C.c. 
 50 
 280 
 330 
 380 
 470 
 
 P. p.m. 
 175 
 130 
 48 
 62 
 72 
 
 P.p. in. 
 10 
 170 
 250 
 310 
 440 
 
 P. p.m. 
 
 '""176"" 
 .240 
 310 
 410 
 
 C.c. 
 580 
 680 
 780 
 940 
 1,090 
 
 P. p.m. 
 101 
 105 
 143 
 152 
 167 
 
 P. p. m. 
 540 
 640 
 690 
 
 770 
 810 
 
 P. p.m. 
 520 
 600 
 680 
 770 
 840 
 
 C.c. 
 1,320 
 1,480 
 1,700 
 1,860 
 1,960 
 1 
 
 P. p.m. 
 165 
 174 
 174 
 176 
 195 
 
 P. p.m. 
 890 
 940 
 1,000 
 1,030 
 1,040 
 
 P. p.m.. 
 920 
 960 
 1,000 
 1,020 
 1,030 
 
 These results are similar to those obtained with the other soils. The 
 last results in the second column indicate that the soil is approaching 
 a saturated condition, although only about 1,000 parts per million are 
 absorbed. This is a much lower absorption than in the case of the 
 monocalcium phosphate with the same soil. 
 
 Table XXII gives the results for the removal of the absorbed 
 phosphate from this soil. 
 
 Table XXII. — Removal of absorbed phosphate from a fine sandy loam by ivater. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 
 POiin 
 solution. 
 
 Total 
 quantity 
 
 PO4 re- 
 maining 
 
 in soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 
 P04in 
 solution. 
 
 Total 
 quantity 
 
 PO4 re- 
 maining 
 
 in soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 
 PO4 in 
 
 solution. 
 
 Total 
 quantity 
 
 PO4 re- 
 maining 
 
 in soil. 
 
 C.c. 
 
 P. p. m. 
 
 P. p. m. 
 1,040 
 820 
 650 
 610 
 580 
 
 C. c. 
 
 620 
 
 750 
 
 920 
 
 1,010 
 
 1, 110 
 
 P. p. m. 
 23 
 20 
 16 
 14 
 13 
 
 P. p. m. 
 560 
 530 
 510 
 490 
 480 
 
 C. c. 
 1,240 
 1,340 
 1,470 
 
 P. p. m. 
 13 
 15 
 15 
 
 P. p. m. 
 460 
 450 
 430 
 
 130 
 290 
 400 
 500 
 
 166 
 
 111 
 
 32 
 
 26 
 
 Here again the concentration of the solution runs down rapidly, 
 until when about 900 c. c. have passed the concentration becomes 
 practically constant and comparable with that obtained when mono- 
 calcium phosphate was used, although the absolute concentration is 
 slightly higher. This shows in the case of this soil that the results 
 with sodium phosphate differ from those with monocalcium phosphate, 
 both as regards absorption and removal. 
 
28 
 
 ABSORPTION OF PHOSPHATES AND POTASSIUM. 
 
 The absorption and removal of phosphate for the fine sandy soil are 
 given in Tables XXIII and XXIV, respectively. 
 
 Table XXIII. — Absorption of phosphate by a fine sandy soil from a solution of sodium 
 phosphate, Na^HPOi, containing 200 parts per million PO^. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 PO. 
 
 in so- 
 lution. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 PO4 
 
 in so- 
 lution. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Vol- 
 ume Of 
 perco- 
 late. 
 
 Quan- 
 tity 
 PO4 
 
 in so- 
 lution. 
 
 Total quantity 
 
 PO4 absorbed 
 
 by soil. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 C.c. 
 200 
 240 
 3U0 
 350 
 400 
 
 P. p.m. 
 145 
 22 
 9 
 33 
 88 
 
 P. p.m. 
 110 
 190 
 300 
 380 
 440 
 
 P. p.m. 
 
 '"'igo' 
 
 280 
 350 
 410 
 
 C.c. 
 460 
 610 
 690 
 800 
 970 
 
 P. p.m. 
 97 
 90 
 106 
 159 
 158 
 
 P. p.m. 
 490 
 660 
 730 
 770 
 850 
 
 P. p.m. 
 480 
 630 
 700 
 780 
 870 
 
 C.c. 
 
 1,120 
 
 1,360 
 
 1,550 
 
 1,670 
 
 P. p.m. 
 145 
 155 
 190 
 168 
 
 P. p.m. 
 
 930 
 
 1,040 
 
 1,050 
 
 1,090 
 
 P. p.m. 
 
 940 
 1,020 
 1,070 
 1,090 
 
 Table XXIV". — Removal of absorbed phosphate from a fine sandy soil by water. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04"in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity 
 
 P04in 
 solu- 
 tion. 
 
 Total 
 quan- 
 tity 
 PO4 re- 
 main- 
 ing in 
 soil. 
 
 C. c. 
 
 P. p.m. 
 
 P. p.m. 
 
 1,090 
 
 960 
 
 890 
 
 830 
 
 C.c. 
 400 
 520 
 700 
 900 
 
 P. p.m.. 
 25 
 22 
 12 
 14 
 
 P. p.m. 
 800 
 780 
 760 
 730 
 
 C.c. 
 
 1,040 
 
 1,130 
 
 1,220 
 
 1,390 
 
 P. p. m. 
 
 10 
 
 10 
 
 9 
 
 8 
 
 P. p.m. 
 720 
 710 
 700 
 680 
 
 C.c. 
 1,490 
 1,590 
 1,670 
 
 P. p.m. 
 
 8 
 7 
 7 
 
 P. p.m. 
 680 
 670 
 660 
 
 90 
 170 
 290 
 
 145 
 91 
 52 
 
 The second column of the absorption table indicates that this soil, 
 like the preceding one, is approaching saturation at a much lower 
 phosphate content than in the case with the monocalcium phosphate. 
 The removal of the absorbed phosphate is the same as in the other 
 soils, a practically constant concentration being reached when about 
 1,200 c. c. have passed through the soil. This concentration is prac- 
 tically the same as that in the case of the monocalcium phosphate. 
 
 GRAPHICAL REPRESENTATION AND DISCUSSION OF THE RESULTS 
 OBTAINED WITH SODIUM PHOSPHATE. 
 
 In figure 4 the results of the preceding experiments on the absorp- 
 tion and removal of phosphate are plotted for the soil. As before, 
 the ordinates represent the quantity of phosphate absorbed by the soil, 
 and the abscissas the volume of solution or of water which has been 
 passed through the soil. The break in the curves gives the point 
 where the phosphate solution was replaced by distilled water and the 
 washing out of the absorbed phosphate begun. 
 
 It is at once apparent that the sandy soil and sandy loam are 
 approaching a condition of saturation, whereas the curves for the clay 
 loam and clay soil are still rising rapidly and the soils are obviously 
 far from saturation for phosphate. The greater absorption of the clay 
 
EESULT8 OBTAINED WITH DISODIUM PHOSPHATE. 29 
 
 loam is, moreover, only apparent, being due entirely to the different 
 starting points of the curves. When the starting points of the curves 
 are made to coincide, the clay loam curve lies throughout below that of 
 the clay soil curve, as it did in the case of the monocalcium phosphate. 
 This will be more strikingly brought out in the application of the for- 
 mula to these curves, when it will be shown that they are described 
 by the same formula and have practically the same constants as those 
 found to apply to the case of the monocalcium phosphate. There is, 
 however, an actual inversion of the curves in the case of the sandy 
 loam and fine sand, the former showing the less absorptive power in 
 this case, whereas with the monocalcium phosphate it showed the 
 
 1000 
 
 CLAySO/L 
 
 clay/loam 
 
 
 
 ^ 
 
 
 S^NDYSOIL 
 
 
 5 
 
 
 .^,J-r— \ S/tND) 
 
 'LO/IM 
 
 ■J 1000 
 
 
 ^^^-— -V^^''^ 
 
 
 vl 
 
 
 ^ — K 
 
 
 
 
 '^^ X 
 
 
 ^ 
 
 
 ^^ 
 
 \ 
 
 tt: 
 
 
 
 — A~___^ 
 
 Uj 
 
 
 
 \7 " — * .^__^^ 
 
 0, 
 
 
 
 ^\ """* — ' "-——.-_ 
 
 [0 
 
 
 
 ^"^..__^^ 
 
 ^500 
 
 
 
 ^~^"~~~*~~~*— ~— .^^ 
 
 5 
 
 
 
 
 Fig. 4.— Soil curves. Absorption of phosphate by soils from a solution of disodium phosphate and the 
 removal of the absorbed phosphate by vs^ater. 
 
 higher absorptive power. Here again the difference between the two 
 curves is much less than it appears to be, for if the starting points of 
 the curves were made to coincide the curves would lie much closer 
 together. The fact remains, however, that these two soils show a rela- 
 tively different absorptive power in the case of the sodium phosphate 
 from that shown where monocalcium phosphate was used. That the 
 specific absorptive capacity of both these soils is markedly different in 
 the present series than in the former series will be shown presently. 
 
 The removal curves are in general the same as those in the monocal- 
 cium series already given. With the almost saturated soils the curves 
 drop rapidly at first, but soon descend at a slow and practically uniform 
 rate. With the less saturated soils the drop is less marked, but the 
 results, nevertheless, clearl}^ show that distilled water is slowly but 
 constantly removing the absorbed phosphate, whether the soil is 
 nearly saturated for phosphate or whether it is only partially satu- 
 rated. This latter result was, of course, to be expected, since the 
 
30 
 
 ABSOKPTIOJSr OF PHOSPHATES AND POTASSIUM. 
 
 phosphate originally in the soils was found to be removed by distilled 
 water at a practically constant rate. 
 
 It has been found that the same differential equation describes the 
 absorption curves in the case of the sodium phosphate as in the case of 
 the monocalcium phosphate. It will be observed from the figure that 
 the curves do not go to the origin, as they did in the case of the mono- 
 calcium phosphate. This is due to the lower absorption at the very 
 start of the experiment, either because of the lower absorptive power 
 of the dry soil used in the present series as compared with the wet 
 soil used in the previous series or because of the too rapid flow of the 
 strong phosphate solution at the start before the apparatus was thor- 
 oughl}^ regulated. Whatever may have been the cause, it is for this 
 reason obviously necessary to modify slightly the constant log A in 
 the equation 
 
 log {A—y) — \og A—Kv 
 
 by substituting log {A—y^^ where y^ is the ordinate of the point taken 
 as the first reading, and also substituting the corresponding value of 
 {v — Vq) for V, where % is the volume corresponding to the y^ taken as 
 the first reading. The equation then becomes: 
 
 log {A-y)^\og{A-y,)-K{v-v,). 
 
 It is, of course, impossible to get any idea of the value of the spe- 
 cific absorptive capacity of the clay soil and the clay loam from the 
 two curves given in the figure, as these are far from showing any 
 tendency to approach a horizontal asymptote. A careful examination 
 of the figures, however, shows that when allowance is made for the 
 different starting points of the curves the results fall almost exactly 
 on the curve for the results obtained with these same soils by using 
 monocalcium phosphate. In other words, the clay soil and the clay 
 loam are quite accurately described by using the same value for A as 
 in the case of the monocalcium phosphate, together with the other 
 constants given below: 
 
 Soil. 
 
 Clay soil... 
 Clay loam . 
 
 5,500 
 3,100 
 
 log 
 
 3.728 
 3.474 
 
 K. 
 
 0. 000162 
 . 000290 
 
 The results for the sandy soil and the sandy loam are likewise quite 
 accurately described by the above equation, using the following values: 
 
 Soil. 
 
 log 
 
 Sandy loam 
 Sandy soil . . 
 
 1,100 
 1,200 
 
 2.968 
 3.004 
 
 0. C00680 
 . 000669 
 
POTASSTUM FROM SOLUTIOT^ OF POTASSIUM CHLORIDE. 
 
 31 
 
 The results obtained by using these values are given in the fourth 
 column of the tables for the respective soils. In the figure the absorp- 
 tion curve is drawn through the calculated points, the experimental 
 points being represented by the specific signs. The figure shows, 
 therefore, what agreement exists between the results given in the 
 third and fourth columns, respectively, of the absorption tables. 
 
 In the following table is given a comparison of the specific absorptive 
 capacities for phosphate found for the four soils when sodium phos- 
 phate was used and when monocalcium phosphate Avas used, together 
 with the constant K. The values of log A and log {A—y^) may be 
 omitted in the comparison, as they depend on the above specific absorp- 
 tive capacity and the starting point of the curves. 
 
 Soil. 
 
 Monocalcium 
 phosphate. 
 
 A. 
 
 Sodium phosphate. 
 
 Clay soil . . . 
 Clay loam . . 
 Sandy loam 
 Sandy soil . 
 
 5,500 
 3,100 
 2,800 
 2,600 
 
 0. 000158 
 . 000267 
 . 000299 
 . 000247 
 
 5,500 
 3,100 
 1,100 
 1,200 
 
 0. 000162 
 . 000290 
 . 000680 
 . 000669 
 
 It is at once apparent that the absorption of phosphate by the clay 
 soil and cla}'^ loam studied is the same for the calcium and the sodium 
 phosphate, as is shown by the equal values for the specific absorptive 
 capacities for phosphate, A^ and the close agreement for the value of K 
 in both cases. It is possible that the ferruginous nature of these par- 
 ticular soils is responsible for this similarity in the absorption of the 
 phosphate in the two cases. With the other two soils, however, the 
 result is quite different. The use of the sodium phosphate has changed 
 the specific absorptive capacity of both soils markedly, and also the 
 value of K. It seems possible, therefore, that the sodium and calcium 
 played an important part in the changes which have taken place in 
 these soils. 
 
 ABSORPTION OF POTASSIUM FROM A SOLUTION OF POTASSIUM CHLORIDE. 
 
 For studying the absorption of potassium by soils a solution of 
 potassium chloride containing 200 parts per million of potassium was 
 used. Three of the soils studied were the same as those used in the 
 phosphate experiments, namely, the clay soil, the clay loam, and the 
 fine sandy soil. In addition to these, two other soils were studied — a 
 loam and a sand. The loam was a sample of the Leonardtown loam, 
 from Leonardtown, Md., and the sand a sample of Sandhill, from Dar- 
 lington, S. C. 
 
 The apparatus and general manipulations were the same as in the 
 experiments already described. The soil was not previously washed 
 with water, but was used in the air-dry form in which it had been 
 kept for some weeks in the laboratory. The entire procedure is there- 
 
32 
 
 ABSOEPTION OF PHOSPHATES AND POTASSIUM. 
 
 fore comparable with that used for the sodium phosphate, where the 
 dry soil was wet directly with the solution instead of previously by 
 passing- water through it. The apparatus was then filled with the 
 solution as already described and regulated so as to have a flow of 
 about 50 c. c. in twent^^-four hours. The percolates were then ana- 
 lyzed for potassium by the colorimetric method described in a former 
 bulletin." In Table XXV are given the results obtained with the clay 
 soil. 
 
 Table XXV. — Absorption of potassium by a clay soil from a solution of jjotassium 
 chloride containing 200 parts per million K. 
 
 
 
 Total quantity 
 
 
 
 Total quantity 
 
 
 
 Total quantity 
 
 Vol- 
 
 Quan- 
 
 K absorbed 
 
 Vol- 
 
 Quan- 
 
 K absorbed 
 
 Vol- 
 
 Quan- 
 
 K absorbed 
 
 ume of 
 
 tity K 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity K 
 in solu- 
 
 by soil. 
 
 ume of 
 
 tity K 
 ineolu- 
 
 by soil. 
 
 
 
 
 
 
 
 
 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 late. 
 
 tion. 
 
 Ob- 
 
 Calcu- 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 
 
 served. 
 
 lated. 
 
 C.c. 
 
 P.%).m. 
 
 P. p.m. 
 
 P. p.m. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 
 P. p.m. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 
 P. p.m. 
 
 50 
 
 62 
 
 70 
 
 50 
 
 410 
 
 104 
 
 520 
 
 530 
 
 750 
 
 156 
 
 760 
 
 760 
 
 160 
 
 67 
 
 230 
 
 230 
 
 500 
 
 117 
 
 600 
 
 610 
 
 890 
 
 165 
 
 810 
 
 820 
 
 210 
 
 57 
 
 300 
 
 300 
 
 550 
 
 117 
 
 640 
 
 640 
 
 970 
 
 164 
 
 860 
 
 850 
 
 260 
 
 60 
 
 370 
 
 370 
 
 610 
 
 133 
 
 690 
 
 690 
 
 1,140 
 
 173 
 
 890 
 
 890 
 
 320 
 
 78 
 
 440 
 
 440 
 
 690 
 
 141 
 
 740 
 
 730 
 
 
 
 
 
 In the first column appears the total number of cubic centimeters of 
 solution passed through the soil, in the second column the concentra- 
 tion in potassium of the successive fractions, and in the third column 
 the total quantity of potassium absorbed by the soil. The fourth col- 
 umn gives the calculated quantity of absorbed potassium obtained by 
 a formula which will be described later. It will be noticed from the 
 results in the second column that the first few hundred cubic centi- 
 meters of the 200 parts per million solution in passing through the 
 soil was reduced to a concentration of approximately 60 parts per mil- 
 lion and that in succeeding fractions this concentration gradually rises, 
 showing a tendency to reach the original concentration of the solution, 
 until when about 1,100 c. c. have passed it is 173 parts per million. 
 At this point the soil has absorbed nearly 900 parts per million of 
 potassium. The absorption obtained with the clay loam is given in 
 Table XXVI. 
 
 Table XXVI. — Absorption of potassium by a clay loam from a solution of potassium 
 chloride containing 200 parts per million K. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total quantity 
 
 K absorbed 
 
 by soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total quantity 
 
 K absorbed 
 
 by soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total quantity 
 
 K absorbed 
 
 by soil. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 Ob- 
 served. 
 
 Calcu- 
 lated. 
 
 C.c. 
 50 
 100 
 350 
 
 P. p.m. 
 101 
 99 
 125 
 
 P. p.m. 
 
 50 
 
 100 
 
 290 
 
 P. p.m. 
 
 60 
 
 100 
 
 270 
 
 C.c. 
 500 
 
 680 
 880 
 
 P. p.m. 
 167 
 164 
 167 
 
 P. p.m. 
 340 
 400 
 470 
 
 P. p.m. 
 340 
 410 
 470 
 
 C.c. 
 1,060 
 1,240 
 1,390 
 
 P. p.m. 
 
 178 
 175 
 180 
 
 P. p.m. 
 510 
 550 
 570 
 
 P. p. m. 
 510 
 540 
 560 
 
 «Bul. No. 31, Bureau of Soils, U. S. Dept. of Agr., 1906, p. 31. 
 
POTASSIUM FEOM SOLUTION OF POTASSIUM CHLOEIDE. 
 
 33 
 
 The absorption of potassium by this soil is not so great as in the 
 case of the clay soil, nor is it so great as the absorption of phosphate 
 bj^ this same soil; nevertheless the second column shows that the con- 
 centration of the first fractions of the 200 parts per million solution 
 is reduced to approximately one-half in passing through the soil. 
 As more solution passes the concentration rises and slowly approaches 
 the original value, having risen to 180 parts per million when about 
 1,400 c. c. of the solution have passed through the soil, which at this 
 point has absorbed nearly 600 parts per million of potassium. The 
 results obtained with the loam are given in Table XXVII. 
 
 Table XXVII. — Absorption of potassium by a loam from a solution of potassium chloride 
 containing 200 parts per million K. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 K in so- 
 lution. 
 
 Total 
 quantity 
 K ab- 
 sorbed 
 by soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 K in so- 
 lution. 
 
 Total 
 quantity 
 K ab- 
 sorbed 
 by soil. 
 
 Volume 
 of perco- 
 late. 
 
 Quantity 
 K in so- 
 lution. 
 
 Total 
 quantity 
 K ab- 
 sorbed 
 by soil. 
 
 C.c. 
 80 
 110 
 150 
 
 P. p.m. 
 100 
 72 
 69 
 
 P. p.m. 
 
 80 
 
 120 
 
 170 
 
 C.c. 
 210 
 280 
 320 
 
 P. p.m. 
 82 
 90 
 106 
 
 P. p. m. 
 240 
 320 
 350 
 
 C.c. 
 390 
 460 
 690 
 
 P. p.m. 
 120 
 128 
 150 
 
 P. p.m. 
 410 
 460 
 580 
 
 With this soil the absorption is even more marked than with the 
 clay loam, though less than with the clay. The first fraction is higher 
 than the four succeeding ones. This is doubtless due to the same 
 causes that produced the higher concentrations in the first fractions of 
 the percolate obtained in the sodium phosphate experiments. This 
 higher concentration of the first fraction is noticeable also with the 
 two soils already described, although not so marked, and occurs like- 
 wise in the case of the succeeding soils, as will be seen presently. 
 After reducing the solution to approximately TO parts per million the 
 concentration again rises until when about 700 c. c. had passed through 
 the soil it was 150 parts per million. The experiment was not con- 
 tinued beyond this point. In Table XXVIII are found the results 
 for the sandy loam. 
 
 Table XXVIII. — Absorption of potassium by a sandy loam from a solution of potassium 
 chloride containing 200 parts per million K. 
 
 
 
 Total 
 
 
 
 Total 
 
 
 
 Total 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 Volume 
 
 Quantity 
 
 quantity 
 
 of perco- 
 
 K in so- 
 
 K ab- 
 
 of perco- 
 
 K in so- 
 
 K ab- 
 
 of perco- 
 
 K in so- 
 
 K ab- 
 
 late. 
 
 lution. 
 
 sorbed 
 by soil. 
 
 late. 
 
 lution. 
 
 sorbed 
 by soil. 
 
 late. 
 
 lution. 
 
 sorbed 
 by soil. 
 
 C.c. ■ 
 
 P. p. m. 
 
 P. p. m. 
 
 C.c. 
 
 P. p. TO. 
 
 P. p. m. 
 
 C.c. 
 
 P. p. TO. 
 
 P. p. m. 
 
 50 
 
 104 
 
 50 
 
 220 
 
 141 
 
 180 
 
 740 
 
 172 
 
 290 
 
 100 
 
 96 
 
 100 
 
 300 
 
 171 
 
 200 
 
 910 
 
 180 
 
 330 
 
 150 
 
 126 
 
 140 
 
 520 
 
 185 
 
 230 
 
 
 
 
 The absorption in the case of the sandy loam is much less than in 
 the soils so far described, but it is nevertheless quite marked in the 
 
34 
 
 ABSORPTION OF PHOSPHATES AND POTASSIUM. 
 
 first fractions of the solution passing through the soil. The absorp- 
 tion by the fine sand, given in Table XXIX, is still less, but definitely 
 traceable in its effects on the various portions of percolate. 
 
 Table XXIX. — Absorption of potassium by a fine sandy soil from a solution of potas- 
 sium chloride containing 200 parts per million K. 
 
 Volume 
 of perco- 
 late. 
 
 C.c. 
 60 
 110 
 150 
 
 Quantity 
 K in so- 
 lution. 
 
 P. p.m. 
 150 
 141 
 
 160 
 
 Total 
 quantity 
 K ab- 
 sorbed 
 by soil. 
 
 P. p. in. 
 30 
 60 
 70 
 
 Volume 
 of perco- 
 late. 
 
 C.c. 
 300 
 540 
 770 
 
 Quantity 
 K in so- 
 lution. 
 
 P. p.m. 
 166 
 168 
 
 174 
 
 Total 
 quantity 
 
 K ab- 
 sorbed 
 by soil. 
 
 P. p.m. 
 120 
 200 
 260 
 
 Volume 
 of perco- 
 late. 
 
 C.c. 
 
 1,130 
 
 1,390 
 
 Quantity 
 K in so- 
 lution. 
 
 P. p.m. 
 183 
 185 
 
 Total 
 quan tity 
 K ab- 
 sorbed 
 by soil. 
 
 P. p.m. 
 320 
 360 
 
 REMOVAL, OF ABSORBED POTASSIUM BY WATER. 
 
 The removal of the absorbed potassium from two soils was also 
 studied, the soils used being the clay and the clay loam of the previous 
 experiment. For this purpose the well-drained soils, containing 890 
 and 570 parts per million of absorbed potassium, respectively, were 
 washed by filling the apparatus with distilled water and continuing the 
 percolation at the same slow and constant rate used in passing the 
 potassium solution. The percolate, collected in fractions, was ana- 
 lyzed for potassium as before. The results for the clay soil are given 
 in Table XXX. 
 
 Table XXX. — Removal of absorbed potassium from a clay soil by water. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 
 soil. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 890 
 820 
 790 
 770 
 740 
 
 C.c. 
 220 
 310 
 450 
 520 
 720 
 
 P. p.m. 
 45 
 29 
 21 
 19 
 20 
 
 P. p.m. 
 720 
 700 
 670 
 650 
 610 
 
 C.c. 
 780 
 840 
 1,150 
 1,260 
 1,370 
 
 P. p.m. 
 19 
 19 
 22 
 20 
 19 
 
 P. p.m. 
 600 
 590 
 540 
 520 
 500 
 
 C.c. 
 
 1,510 
 
 1,640 
 
 1,760 
 
 1,870 
 
 2,110 
 
 P.p.m. 
 21 
 19 
 19 
 
 17 
 19 
 
 P.p.m. 
 470 
 440 
 420 
 400 
 360 
 
 40 
 80 
 130 
 170 
 
 171 
 
 78 
 56 
 47 
 
 The rather high concentration of the small fraction collected at the 
 start is due to the stronger solution contained in the soil. On passing 
 more water the percolate rapidly becomes weaker in potassium, until 
 when about 450 c. c. have passed the concentration shows a practically 
 constant composition of about 20 parts per million, although the per- 
 colation was continued until over 2,000 c. c. had been passed and the 
 quantity of absorbed potassium in the soil reduced from nearly 9(»0 to 
 about 350 parts per million. 
 
 The removal of the absorbed potassium from the clay loam is shown 
 in Table XXXI. 
 
RESULTS OBTAINED WITH POTASSIUM CHLOEIDE. 
 
 35 
 
 Table XXXI. — Removal of absorbed potassium from a clay loam by water. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 
 soil. 
 
 Vol- 
 ume of 
 perco- 
 late. 
 
 Quan- 
 tity K 
 in solu- 
 tion. 
 
 Total 
 quan- 
 tity K 
 
 re- 
 main- 
 ing in 
 
 soil. 
 
 C.c. 
 
 P. p.m. 
 
 P. p.m. 
 570 
 510 
 470 
 
 C.c. 
 200 
 330 
 470 
 
 P. p.m. 
 43 
 31 
 30 
 
 P. p.m. 
 440 
 400 
 360 
 
 C.c. 
 660 
 720 
 
 780 
 
 P. p.m. 
 30 
 
 28 
 30 
 
 P. p.m. 
 310 
 290 
 270 
 
 C.c. 
 830 
 880 
 
 P. p.m. 
 26 
 25 
 
 P. p.m. 
 260 
 250 
 
 70 
 140 
 
 89 
 59 
 
 The results in this case are very similar to those of the clay soil, 
 although the washing with water was not continued so long. The 
 concentration of the solution is higher, showing the more ready 
 removal of the potassium from this soil. When 800 c. c. of water had 
 passed through the soil the concentration was 26 parts per million and 
 the quantity of absorbed potassium had been reduced from 570 to about 
 250, whereas in the case of the claj^ the passage of a similar volume of 
 water had reduced the soil from 890 to only about 600, and the con- 
 centration of the percolate had already dropped down to the constant 
 concentration of approximately 20 parts per million. 
 
 GRAPHICAL REPRESENTATION AND DISCUSSION OF THE RESULTS OBTAINED 
 WITH POTASSIUM CHLORIDE. 
 
 In figures 5 and 6 are given the curves for the results obtained in 
 the experiments on the absorption of potassium, as well as those 
 obtained in the removal of the absorbed potassium by water. In fig- 
 ure 5 are shown the results based upon the concentration of solution. 
 The abscissas represent the volume of the solution or of water which 
 has passed through the soil and the ordinates the concentration of 
 potassium in the percolate. The upper boundary line of the figure 
 gives the strength of the solution before passing through the soil, and 
 the break in the curves gives the point where the solution was replaced 
 by distilled water and the removal of the absorbed potassium was 
 begun. Only the results for the clay and clay loam are given in this 
 figure, as these are the two soils in whicli both the absorption and 
 removal of potassium was studied. The absorption curve for the 
 loam would lie between those of the clay and clay loam, the curves for 
 the sandy loam and fine sand would lie above that of the clay loam. 
 It is apparent from the figure that the general run of the absorption 
 and removal of the potassium is very similar to the phosphate curves 
 given in figure 2. The curves show that the concentration of the first 
 few hundred cubic centimeters of the solution is materially reduced, 
 in the one case approximately to one-half and in the other to one-fourth 
 the original strength. With increase in volume of the solution passed 
 the curves rise slowly and approach the upper boundary line of the 
 figure (that is, the ordinate of the original strength of the solution) 
 apparently as an asymptote. 
 
36 
 
 ABSORPTION OF PHOSPHATES AND POTASSIUM. 
 
 The removal of the absorbed potassium is very rapid at first, but the 
 solutions soon reach a constant concentration, as is very strikingly 
 brought out by the horizontal position of the removal curve in both 
 cases. A comparison with figure 6 also shows that the concentration 
 of the solution becomes constant at a point where only about one-third 
 of the absorbed potassium has been removed. It is also noteworthy 
 that the clay, although it contains throughout a much larger quantity 
 of absorbed potassium, gives a lower concentration of potassium in 
 solution than does the clay loam with its smaller potassium content. 
 It follows, therefore, that the relative concentration of the potassium 
 
 I 200 
 ^ 160 
 
 ^ 100 
 
 S3 30 
 
 Fig. 5.— Solution curves. Ab.sorption of potassium by soils from a solution of potassium chloride and 
 the removal of the absorbed potassium by water. 
 
 
 0.5 1.0 
 
 1.5 
 
 EO 
 
 
 2.5 
 
 
 
 3>0 
 
 T^ 
 
 ^_ctflr^^^---Tr^ 
 
 \^ 
 
 
 
 
 
 
 
 - - 
 
 
 " 
 
 
 
 
 
 CLBY SOIL 
 
 LITERS 
 
 Fig. 6. — Soil curves. Absorption of potassium by soils from a solution of potassium chloride and the 
 removal of the absorbed potassium by water. 
 
 in the percolates gives no indication of the quantity of absorbed potas- 
 sium present in the soil, although it must be admitted that this is in a 
 form readih^^ soluble in water, as is shown by its continued removal 
 by water in the above experiments. The magnitude of these absorp- 
 tion results and the removal of the absorbed material show very con- 
 clusively that in experiments dealing with the solubility of finely 
 powdered substances of slight solubility, such as soils or rock-forming 
 minerals, it is not so much the solubility itself with which one is deal- 
 ing as with the decomposition and absorption phenomena and the 
 removal of the absorbed products of decomposition. 
 
 In figure 6 are shown the results expressed in terms of the soil itself, 
 the abscissas being the volume of solution or water passed through 
 the soil, and the ordinates the amount of potassium absorbed by the 
 
RESULTS OBTAINED WITH POTASSIUM CHLORIDE. 
 
 37 
 
 soil. It is at once apparent that the different soils show marked dif- 
 ferences in their absorptive power for potassium. The clay loam, for 
 instance, is approaching a saturated condition with a concentration of 
 potassium in the soil at which the clay soil is still absorbing- at a rapid 
 rate. Similar differences in the extent of the absorption are shown by 
 the other soils, but the general run of the curves is the same as those 
 of the more thoroughly studied clay soil and clay loam. The removal 
 curves drop rapidly at first and then run downward in a straight line 
 in the case of both of the soils studied. 
 
 The similarity of the absorption curves to those of the phosphate 
 given in figure 3, in that thej tend to approach a horizontal asymptote, 
 is striking. It has been found that these results are quite accurately 
 described by the same differential equation which describes the phos- 
 phate-absorption curves. 
 
 dv 
 
 K{A-y) 
 
 or integrating 
 
 \og {A—y)=\og A— Kv 
 
 where ^is a constant and A is the maximum amount of potassium 
 the soil can absorb under the conditions of the experiment — i. e. , ^ is 
 the specific absorptive capacity of the soil for potassium, y is the 
 amount of potassium the soil has absorbed when the volume v of solu- 
 tion has passed through the soil. In these experiments, however, as 
 in the case of the sodium phosphate, on account of the fact that the 
 curves do not pass through the origin, it is necessar}^ to substitute log 
 {A—y^ for log JL, where y^ is the ordinate of the point taken as the 
 first reading, and also to substitute the value of {v—v^ for v where v^ 
 is the volume corresponding to the y^ taken as the first reading. The 
 formula has been applied to the results obtained with the clay and the 
 clay loam. When the following values are used a very good agree- 
 ment between the experimental results and the calculated results is 
 found to exist. 
 
 Clay soil . . 
 Clay loam , 
 
 1,000 
 650 
 
 log 
 
 (^-2/o). 
 
 2.886 
 2.740 
 
 0. 000864 
 . 000622 
 
 The method of calculation is exactly the same as already described 
 with the phosphate absorptions. The calculated results for the respec- 
 tive soils are given in the fourth columns of the absorption tables. 
 In figure 6 the absorption curves for the clay and the clay loam are 
 drawn through the calculated points, the experimental points being 
 indicated by the respective signs and the plots show, therefore, how 
 well the calculated and found figures agree. 
 
38. ABSOEPTION OF PHOSPHATES AND POTASSIUM. 
 
 Acknowledgments are due Messrs. H. C. Keith and R. M. Goss for 
 assistance in carrying out analytical and other experimental details of 
 the work. 
 
 SUMMAET. 
 
 The data presented in the foregoing pages throw much new light on 
 the behavior of phosjDhates and potassium in the soil. The successive 
 solutions obtained by slow percolation of water through the four soils 
 have been shown to have a concentration in phosphate which is prac- 
 tically a constant for any given soil, thus substantiating the experi- 
 ments of Schloessing and other authorities, as well as the conclusions 
 advanced in former publications from this Bureau deduced from 
 entirely different lines of evidence. The soils studied, moreover, 
 yielded solutions which differed little in concentration, except in the 
 case of the Podunk fine sandy loam, which has a low absorptive 
 capacity and is acted upon by water to an unusual extent. 
 
 The absorption experiments with the phosphates have shown that at 
 the start this is very rapid and complete, the strong phosphate solu- 
 tion being reduced to the concentration characteristic of the water 
 solution for that soil. This seems to show that the application of con- 
 siderable quantities of a soluble phosphatic fertilizer would not mate- 
 rially increase the concentration of the phosphate dissolved in the free 
 soil moisture. On tracing the absorption of phosphate further, by 
 the percolation of more phosphate solution, it has been found that 
 while absorption continues it is becoming less marked and finall}^ a 
 saturated condition will be reached. The law which appears to govern 
 this change has been found to be that the quantity absorbed from a 
 unit volume of the phosphate solution as it passes through the soil is 
 proportional to the quantity which may yet be absorbed. This 
 absorption process is mathematically represented by the differential 
 equation 
 
 where A" is a constant, A the maximum quantity the soil can absorb, 
 and y the quantity it has absorbed when the volume v of phosphate 
 solution has passed through the soil. The quantity A is defined as 
 the specific absorptive capacity of the soil. The value of A differs 
 considerably for the different soils, being highest in the clay and lowest 
 in the sand}^ soil. In the case of the clay and the clay loam the same 
 values were found with the sodium phosphate as with the monocalcium 
 phosphate, but in the fine sandy loam and fine sand these two phos- 
 phates gave entirely different results, the specific absorptive capacity 
 with the monocalcium phosphate being over twice as great as with the 
 sodium phosphate. 
 
SUMMARY. 39 
 
 The removal of the absorbed phosphate appears to be in general the 
 same as the removal of the phosphate from the original soil. The 
 concentration of the percolates becomes constant when onl}'^ a 
 fractional part of the absorbed phosphate has been removed, and this 
 concentration is practically that yielded by the original soil with a 
 far less phosphate content. It would therefore seem that the con- 
 centration of the phosphate in the soil solution is practically the same, 
 whether the soil contains a large or a small quantity of absorbed phos- 
 phate, and that it is this absorptive power of the soil which controls 
 the concentration of the phosphate in the free soil moisture. It fol- 
 lows that with change in the absorptive power of the soil the concen- 
 tration of the phosphate in the free soil moisture would also change. 
 Attention has been called to the unusual extent to which the Podunk 
 fine sandy loam is acted upon b}^ water. It is therefore interesting 
 to note that the concentrations of the phosphate in the aqueous perco- 
 lates from this and the other three soils came closer together after the 
 soils had been treated for some time with the calcium or sodium phos- 
 phate solution than they did before this treatment. This seems to 
 indicate that by the similar treatment of the soils with considerable 
 quantities of the same chemical substances the soils have been made 
 more alike in their chemical behavior toward water, so far as phos- 
 phate — the only constituent studied in this case — is concerned. It is 
 also noteworthy that the absorbed phosphate is not insoluble, but is 
 slowly and continuously diffusing into the free soil moisture and 
 becoming, therefore, directly available to plants. 
 
 The results obtained in the potassium experiments, while not so 
 comprehensive, nevertheless show the same general tendencies. The 
 five soils studied for their absorption all show a more or less marked 
 absorptive power for potassium, which is, however, considerabl}^ less 
 than the absorptive power of these soils for phosphate. The law gov- 
 erning the absorption process appears to be the same with potassium 
 as with phosphate, and the curve of the absorption is quite accu- 
 rately described by the same differential equation: 
 
 , The constancy of the removal of the absorbed potassium by water 
 is even more striking than in the case of the phosphate and the con- 
 clusion that the concentration in the free soil moisture is dependent 
 on the absorptive power of the soil is well sustained by these results. 
 The absorbed potassium, like the absorbed phosphate, is continually 
 diffusing into the free soil moisture and becoming, therefore, directly 
 accessible to plants. hr 
 
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