UNIVERSITY OF CALIFORNIA PUBLICATIONS IN AGRICULTURAL SCIENCES Vol. 3, No. 2, pp. 25-36, plates 9-11 September 29, 1917 OPTIMUM MOISTURE CONDITIONS FOR YOUNG LEMON TREES ON A LOAM SOIL BY L. W. FOWLEE and C. B. LIPMAN Among the numerous problems emanating from the use of irriga- tion water on land is the important one of maintaining as nearly as possible an optimum moisture content in the soil. While much research work has been done in an attempt to determine what consti- tutes such an optimum moisture content, it seems that our knowledge is still too indefinite for accurate application to specific cases. For that reason it has appeared to the junior author that some specific information should be gathered concerning the moisture needs of soils which are used for growing crops under field conditions and also the variations in such moisture needs occurring through changes in soil type and changes in the kind of crop grown. Obviously the task just mentioned is too great to be disposed of quickly, and in one series of experiments, and it has, therefore, seemed wise to start the work with one crop and one soil type first. Owing to the fact that the Limoneira Company of Santa Paula, California, expressed its willing- ness to co-operate in such an experiment and to give to it the time and attention of the senior author as well as the necessary equipment, the experiment was started with young lemon trees on a loam soil, characteristic of much of the large ranch in the possession of the company. It was further hoped that the results obtained from the experiment, along with contemporaneous results of careful moisture determinations at short intervals in the lemon orchards, would give a basis for planning a scientific system of irrigation in the orchards in question. A plan for the specific experiment, and one for the field work were arranged by the junior author and they were executed under the direction of the senior author. The detailed results of the 26 University of California Publications in Agricultural Sciences [Vol.3 field work cannot be given in this paper but will need discussion separately elsewhere at some future time. The experiment proper, however, has now been in operation for more than two years and the results obtained have been so interesting as to more than justify their presentation and discussion here. Plan of the Experiment It was decided to grow the young lemon trees in galvanized iron cylinders, 24 inches in depth and 15 inches in diameter. The cylin- ders were painted with a heavy coating of asphalt. The soil used in them is a loam having the following mechanical analysis (Bureau of Soils method), which was furnished us through the courtesy of Professor C. F. Shaw: First foot Second foot Third foot Fine gravel 1.45 1.14 1.71 Coarse sand 3.24 3.13 4.27 Medium sand 3.32 3.25 4.13 Fine sand 12.77 12.33 12.58 Very fine sand 42.99 44.93 43.22 Silt 18.74 19.31 16.61 Clay 17.49 15.91 17.48 On the basis of this mechanical analysis the Bureau of Soils would classify the soil as a fine sandy loam, but owing to its relatively high clay and silt content it should, in the junior author's opinion, be classified as a light clay loam, but certainly as no less than a loam. Mr. Chas. A. Jensen of the Bureau of Plant Industry, United States Department of Agriculture, was good enough to furnish the ' ' moisture equivalents" and "wilting coefficients" of the first three feet in depth of the soil used, as it occurs under field conditions. Mr. Jensen's determinations follow : Wilting per cent Moisture equivalent First foot 9.3 17.0 Second foot 8.7 15.9 Third foot 8.1 14.0 The soil used in the cylinders was obtained from a lemon orchard now twenty-three years of age in which the trees have always shown good vigor and high productivity. The soil from the first and second feet in depth was thoroughly mixed in preparation for use in the cylinders. The same amount of soil was weighed into every cylinder. The variety of lemons selected for the test was the Lisbon. The trees were one vear old and as nearly uniform as could be obtained. 1917] Fowler-Lipman : Optimum Moisture Conditions for Lemon Trees 27 Before planting, the roots of the trees were entirely freed from soil and the tops were pruned to a whip. The planting was done on March 17, 1915, after which the cylinders were placed in a row (see plate 9) in a trench 24 inches deep, in order to prevent the undue heating of the soil from the exposure of the cylinders to the direct sun. From the time of planting until June 1, 1915, a soil moisture content of 20 per cent based on the dry weight of the soil was maintained in all the cylinders in order to give all the trees the same start. At the last date mentioned the growth of all the trees was sufficiently good and uniform enough to allow of the arrangement for the variation in moisture content in the different cylinders. In order to allow for individual variations among the trees, every moisture content was employed on triplicate trees and the moisture percentages tested were as follows : 10 per cent based on the water-free soil. io It it it it -lAti i i it t I i a 1 1 it it it 1Q II t i it 11 20 " ' ' " ' ' 22 ' ' { t 1 1 ii ■ > i tt 1 1 tt tt 26 " " " " 28 " " " " 30 " " " " The cylinders are weighed three times per week and the losses of moisture due to evaporation are replaced by additions of the necessary amounts of the ordinary irrigation water employed on the ranch. The weighing is done on steelyards and a derrick is available for raising and lowering the cylinders as desired. The water is added in a depression in the surface soil corresponding in nature to an irri- gation furrow and is applied by means of a very small stream flowing from a hole in a can used for the purpose. This method is employed to obviate puddling. During rainy weather the cylinders kept at less than 20 per cent moisture are protected by canvas roofs. Between irrigations the surface of the soil in all the cylinders is kept cultivated. Results of the Experiment Seventeen months after the experiment was started or when the trees were two years and five months old, measurements were made and a diagram showing their relative heights at the time is given in 28 University of California Publications in Agricultural Sciences [Vol. 3 figure 1. While, however, the measurements show clearly enough the effects of the different soil moisture percentages on the growth of the young lemon trees, they do not really tell the whole story, since the general vigor and abundance of foliage are naturally as much and perhaps more affected than the height by the moisture conditions in the soil. For that reason photographs taken at about the time the measurements were made are submitted herewith to show the actual condition of the trees. By whatever criterion the results are gauged, it is at once clear that the effects of the soil moisture content on the development of the young lemon trees are most striking. For the soil and plant in question, 20 per cent of moisture based on the dry weight of the soil seems to be optimum in so far as the total growth and the height of Fig. 1. Showing the relative heights of lemon trees grown with different quantities of moisture. Trees 29 months old; in experiment 17 months. The relative heights are shown on the ordinates and the percentages of moisture under which they were produced are given in the abscissae. The broken line shows the height of all the trees at the beginning of the experiment. the trees are concerned. A fact which is not brought out by either the measurements or the photographs is that the general tone and color of the trees growing in the 20 per cent cylinders is somewhat inferior to that of the trees growing in the 16 per cent and 18 per cent cylinders. The optimum moisture content of the loam studied for young Lisbon lemons seems to be therefore between 18 and 20 per cent, if we may judge from the experiment described and from the time given it. The trees at or near the optimum moisture content doubled in height and general size during the period mentioned, while the trees at 10 per cent or at 30 per cent moisture contents have scarcely gained more than half of their original height in the period named. Other important points deserve mention in connection with the results obtained. It appears from the data given that the range of 1917] Fowler-Lipman: Optimum Moisture Conditions for Lemon Trees 29 soil moisture percentages within which the young Lishon trees will grow satisfactorily in the soil studied is, relatively speaking, a wide one, since for practical purposes there is probably little difference between the growth obtained at moisture percentages varying from 16 to 22, both inclusive. This is a fortunate circumstance from the point of view of orchard practice since it allows of considerable leeway in the control of irrigation operations. It does not follow, however, that as regards fruit production the same wide range of moisture per- centages in the soil would be similarly effective as in the case of general vegetative growth. On either side of the range of moisture percentages just discussed, there can be no question that conditions are far from proper for good tree growth. This is especially true, however, for moisture percentages in excess of 22 per cent, at which the light-colored foliage and general lack of vigor, increasing with increase of moisture, accompany the slow growth. In the case of the cylinders receiving less than 16 per cent of moisture while the growth is also slow owing to lack of moisture, the leaves and branches appear to be normal in color and the trees appear to be suffering less from untoward conditions. It seems to be very clear at this stage of the experiment, therefore, that, in practice, there is very much more danger to young lemon trees from too much than from too little moisture in the soil. The harmful effects of the former seem to be always more sharply denned and more intense ; small additions of water beyond the optimum produce large and sudden changes, whereas small decreases of moisture below the optimum show their effects only gradually with the continued reduction in the moisture percentage. About six months have passed since the measurements and photo- graphs discussed above were obtained. The effects of the different moisture percentages continue to stand out as clearly or more so than ever before, indicating the probability that they may continue so for a long period of years. It should be mentioned here that small but uniform applications of sulphate of ammonia have been made to all the cylinders during the past year to maintain a more nearly normal growth than is possible without additional nitrogen in such a limited volume of soil as that at the disposal of the trees in the cylinders. In the soil under study in this experiment it was found that the theoretical wilting point was very close to, if not identical with, the actual wilting point, as both the field moisture determinations and the 10 per cent moisture cylinders have on very dry days attested. It will be observed, moreover, that the moisture equivalent and the 30 University of California Publications in Agricultural Sciences [Vol. 3 optimum moisture percentage in the same soil are not far apart. While the height of the trees is greatest at moisture percentages in excess of that of the moisture equivalent, the most vigorous appearance of the trees is obtained with percentages of soil moisture very close to the moisture equivalent. As above stated, it is not possible now to discuss the detailed results of the moisture determinations in the orchard, but in general it was true that the soil moisture percentages rarely fell to the wilting point in the orchard soil and very infre- quently rose to the optimum under the system of irrigation practiced. In the orchard under consideration, therefore, a lack rather than an oversupply (a common condition elsewhere in California) of water seems to be the rule. 1917] Fowler— Lipman: Optimum Moisture Conditions for Lemon Trees 31 Summary In attempting to determine the optimum moisture content of a rather heavy loam soil for young Lisbon lemon trees grown in cylin- ders, at the Limoneira Ranch, Santa Paula, California, the following information was obtained in the course of the first two years of the experiment : 1. A moisture percentage of 20 per cent based on the dry weight of the soil has produced the tallest trees. 2. Trees grown with 16 and 18 per cent of moisture, while not as tall as those grown with 20 per cent of soil moisture, show better color and more vigor. The differences are not very marked, however. 3. The foregoing facts seem to show that the range of optimum or nearly optimum moisture percentages for the soil and plant in question is a relatively wide one. 4. Much more visible damage results to the young lemon trees from moisture percentages in excess of the optimum than from those below the optimum. 5. Every successive increment of moisture beyond the optimum is accompanied by a sharp depression in growth, color, and general vigor of the trees. 6. Every successive decrement of moisture from the optimum shows only a relatively slight depression in growth. 7. The theoretical wilting point and the moisture equivalent for the soil studied are in close accord respectively with the actual wilting point as determined in the soil of the orchard and the optimum moist- ure content as determined in the experiment discussed above. The authors wish to acknowledge their sincerest sense of obligation to Messrs. C. C. Teague and J. D. Culberson of the Limoneira Com- pany, who have so kindly cooperated with them in the experiment above described and who have at all times been willing to place at their disposal all possible facilities for the prosecution of the work. PLATE 9 Fig. 1. Showing arrangement of cylinder experiment to study water needs of young lemon trees. Fig. 2. From right to left, cylinders 1, 2, and 3 maintained at 10 per cent of soil moisture; cylinders 4, 5, and 6 maintained at 12 per cent of soil moisture. [32] UNIV. CALIF. PUBL. AGR. SCI., VOL. 3 [FOWLER-LIPMANI PLATE 9 (?'■- :-.#-"-- Fig. 1 Fiar. Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/optimummoisturec32fowl PLATE 10 Fig. 1. From right to left, cylinders 1, 2, and 3 maintained at 14 per cent of soil moisture, and cylinders 4, 5, and 6 maintained at 16 per cent of soil moisture. Fig. 2. From right to left again, cylinders 1, 2, and 3 maintained at 18 per cent of soil moisture, and cylinders 4, 5, and 6 maintained at 20 per cent of soil moisture. Part of cylinder 7 showing 22 per cent of soil moisture. [34] UNIV. CALIF. PUBL. AGR. SCI., VOL. 3 'FOWLER-LIPMAN | PLATE 10 *m Fig. 1 MRU £&*W. «&$&¥&■ ■** *3iSSfc Fig. PLATE 11 Fig. 1. From right to left, cylinders 1 and 2 maintained at 22 per cent of soil moisture, cylinders 3, 4, and 5 at 24 per cent of soil moisture, cylinders 6 and 7 at 26 per cent of soil moisture. Fig. 2. From right to left, cylinder 1 at 26 per cent of soil moisture, cylinders 2, 3, and 4 at 28 per cent of soil moisture, cylinders 5, 6, and 7 at 30 per cent of soil moisture. [36] UNIV. CAL F. PUBL. AGB. SCI., VOL. 3 [ FOWLER-LI PM AN | PLATE 11 3 Fig. 1 Fig.