434 Mineral Nutrition of Plants 



condition. Roots of trees on alluvial soils in California may penetrate 

 20 feet in depth, so that depletion of the soil moisture in the surface 

 two feet of soil, under such conditions, would not indicate exhaustion 

 of the total moisture reservoir to the tree. Veihmeyer and Hendrickson 

 (724) point out that on compact or dense soils, plants may show symp- 

 toms of moisture stress even though "available" water is still present 

 in the soil. They attribute this to the poor permeation of roots in such 

 soils. Aldrich et al. (j) also noted that irrigated Anjou pears on a 

 Meyer clay adobe showed a decrease in growth before the moisture 

 was depleted to the wilting percentage. They regarded the poor root 

 permeation observed in this soil as a partial contributor to this result. 

 It may be shown by a study of moisture sorption curves for different 

 soils that when 75 per cent of the available water is removed, the 

 remaining water may be under a tension of only 1 atmosphere in some 

 soils, but 5 atmospheres in others. Consider the five soils represented in 

 Figure 1. The data are replotted in Figure 4 in such manner that the 

 moisture content at the 15-atmosphere value is designated as "O" 

 available water; the moisture content of the soils at 0. 15-atmosphere 

 tension (approximately field capacity) is designated as 100 per cent 

 available water. In other words, the relative scale is simply the moisture 

 present in excess of the 15-atmosphere percentage divided by the total 

 available range as just defined. When moisture retention curves are 

 plotted on this relative basis, it is quite evident that their respective loci 

 vary considerably, depending on the specific nature of the soil. For 

 example, it is shown in Figure 4 that when the moisture tension reached 

 1 atmosphere in the Indio sandy loam, about 84 per cent of the "availa- 

 ble moisture" had been removed; whereas, at this same tension, only 

 about 50 per cent of the "available water" was removed from the 

 Olympic clay. It is reasonable to assume that variations in plant growth 

 associated with variations in soil moisture content above the wilting 

 percentage are more likely to prevail on the latter type of soils than on 

 the former. And there is considerable evidence (128, 129, 36, 44, 64) 

 that growth of plants may decrease with a concomitant decrease in soil 

 moisture content of the root zone at moisture levels above the wilting 

 percentage. 

 The status of prevailing weather conditions may be a determinant as 



