DIFFUSION PRESSURE DEFICIT OF WATER IN SOILS 259 



as determined with sunflower plants shows ver}' little difference over a wide 

 range of climatic conditions. In general this quantity seems to be controlled 

 almost entirely by soil conditions and is only slightly influenced by the species 

 of plant used, or by the climatic conditions to which that plant is exposed. 



The significance of the wilting percentage lies in the fact that it is essen- 

 tially a measure of that fraction of the soil water which is unavailable for 

 plants. However, it should not be assumed that absolutely all movement of 

 water into the roots has ceased when the soil water content has been reduced 

 to its wilting percentage. Wilted plants continue to reduce the water content 

 of the soil but at such a slow rate that restoration of turgidity is impossible 

 since the rate of transpiration even from a wilted plant exceeds the rate of 

 absorption of water from a soil which is at its wilting percentage or lower. 



Actually, plants in the field often exhibit permanent wilting at soil water 

 contents somewhat in excess of the wilting percentage as determined by 

 the usual technique. In laboratory determinations of the wilting percentage 

 the plants are ordinarily rooted in relatively small containers in which the soil 

 mass becomes effectively penetrated by roots resulting in a fairly uniform 

 reduction in the water content of the soil. In the field, however, such con- 

 ditions do not often prevail. The soil is usually less effectively penetrated 

 by the root systems of the plant growing therein, and some portions of the 

 soil may be less completely depleted of water than others before permanent 

 wilting ensues. Although the soil in the immediate vicinity of the absorbing 

 zone of each root is at the wilting percentage more remote portions of the 

 soil may still be at the field capacity. Hence the average water content of 

 such a soil at permanent wilting may be considerably in excess of the wilting 

 percentage as determined by the usual laboratory procedure. 



The Diffusion Pressure Deficit of the Water in Soils. — In preceding 

 discussions of the water relations of plant cells and tissues it has been shown 

 that the most significant unit for the expression of the dynamics of water rela- 

 tions of plant cells is the quantity which has been termed in this book the 

 diffusion pressure deficit. Since all problems of the absorption of water by 

 plants involve a consideration of the relation of the water in the soil and the 

 water in the plant, it is desirable that this concept of the diffusion pressure 

 deficit of water be extended to the soil water if the absorption of water is to 

 be interpreted in terms of dynamic units. 



One of the first successful attempts to measure the retentive capacity 

 of the soil for water was made by Shull (1916). In an ingenious approach 

 to this problem air-dry seeds of the cocklebur {Xanthium penTisylvanicuni) 

 were used as the "instrument" for measuring the diffusion pressure deficit of 

 the soil. Seeds of this species were first placed in salt solutions of various 



