Some Plant and Soil‐Moisture Relations

Conditions prevalent in California during the growing season are propitious for the study of the effect of varying soil-moisture conditions upon plants, the use of water by plants, and the losses of moisture from soils. For more than a decade the writers have been concerned with various phases of plant and soil water-relations. This article is intended to summarize in a brief way some of the essential results obtained from these studies. Since the cost of irrigation is one of the principal items of expense in the production of crops under arid or semi-arid conditions, the economical use of water is very important. One of the first of the studies was to ascertain the relation between the amount of water lost by surface evaporation and that transpired by plants. The amount of water transpired by growing plants was found to be tremendously greater than that evaporated directly from the surface of the soil. These results were based not only on comparisons of losses of water through plants growing in large tanks, but also on the results of sampling soil for moisture* determinations in fallow and cropped areas in the lield (34) . They are in accord with the previously published statements of Alway (1), Burr (0;, Miller (20) , Young (41) , Baker ( 2 ) , and Rotmistrov (24) to the ef ic*ct that li t t le of the water which passes below the first, foot of soil is lost by evaporation. Our results show that the loss of moisture by direct evaporation was conf ined very largely to shallow depths of soil. Moisture below the upper 8 inches of soil was lost at an extremely slow rate. Shaw (28) from a study of soils in columns concludes that upward movement of moisture to supply evaporation does not cccur. At the time our experiments were started, most California farmers placed reliance upon cultivation to conserve moisture through the maintenance of a dust mulch on the theory that the upward movement of moisture by capillarity would be interrupted and a saving of water thereby effected. Young ;* (41) , Call and Sewell (8 ) , Sewell (25) , Chilcott and Cole (10), Cates and Cox ( 9 ) , and Rotmislrov (24) had seriously questioned the efficacy of a soil mulch in controlling moisture loss. Since then many of the Agricultural Experiment Stations in this counti'y have issued reports on the effect of cultivation on conserving soil moisture and on crop yields. The! results in almost every case indicate that cultivation for the purpose of simply stirring the soil is wasted effort. Our results covering a wide range of soils and climatic conditions clearly show that the soil mulch was not effective in control l ing the evaporation losses from soil. Furthermore, cultivation did not materially inf luence the distribution of moisture in the soil. The soil dried out to the same extent and to the same depth whether it was cultivated or not. By the time% the surface soil had dried enough to be tilled properly, so large a portion of the total amount of water evaporated had been lost that cult ivation was not effective. After the water applied to the soil had become distributed, further movement of moisture was found to be extremely slow (36) . Neither upward, downward, nor lateral movement was appreciable. The very slow movement of moisture from moist soils to dry soils was demonstrated both in the field and in columns of packed soils in which moist soils remained in contact with dry soils for periods of more than four months. The results indicated that the capillary movement of moisture from moist soil to dry soil is too limited to be effective for use by plants. Sometimes the movement of water through soils when they are in contact with free water may be fairly rapid. It naturally follows that roots must extend into a body of soil to utilize its moisture since capillarity cannot be counted on to move moisture from moist soil into dry soil. A continuous supply of moisture can be obtained uniy by the elongation of the absorbing portion of the roots into moist; soil, unless the supply be replenished by ram or irrigation. Shull (32) and Livingston (18) have expressed the same general idea. It has been shown (11) th.,t soil-moisture data may Le used with a fair degree 01 accuracy to show the presence or absence of roots, binee our experiments (13) lead us to believe that root3 will not grow into dry soil, it seems logical to conclude that if a soil is wet at the beginning of the growing season to the full depth to which roots of the plants normally penetrate, subsequent applications of water, unless adverse conditions for growth are brought about thereby, have little influence on the extent of the root system developed. On the other hand, Snantz (27), Magistad and Breazeale (19), and Breazeale (3) believe that some plants do have the ability to push their roots through dry soil. The idea that a small amount of water applied at any point to a certain quantity of soil will wet that soil uni formly to a lower moisture content than would a larger amount of water has been, in the opinion of the writers, the cause of misinterpretation of the results of many investigations on the effect of water on plants. This is especially true of the work done with plants in containers which were supposedly grown under controlled conditions of soil moisture. Shantz (26) from theoretical considerations, put forth the same idea at about the time the experiments of the writers were completed. Our experiments showed that when a definite quantity of water was applied, the soil was wet to a certain depth depending upon its water-holding capacity and its initial water content. Shortly after the application, usually within two or three days, the rapid downward movement of water ceased and the soil throughout the wetted portion was at a moisture content which in the case of finer textured soils closely approximated the moisture equivalent. The moisture content shortly after the water is applied, which we call the field capacity, is a rather definite soil-moisture content. The field capacities of some sandy soils have been found to be appreciably higher than their moisture equivalents (38) (22). The moisture equivalent technique as originally defined by Briggs and McLane (4) has been modified to some extent (39). The moisture equivalent results obtained in one laboratory may not be the same as those from an-