Wadleigh and Richards 445 



roots from surface soil at or below the wilting percentage when mois- 

 ture was available to the roots below the 2-foot depth. 



The foregoing observations agree with those obtained by Volk (125) 

 using a comparable technic. He also found that corn roots would pene- 

 trate a soil when at lower moisture content than the wilting percentage; 

 and, what is especially noteworthy, Volk (125) demonstrated that these 

 corn roots were able to absorb appreciable quantities of nitrogen from 

 the dry soil. Furthermore, he found good evidence that significant quan- 

 tities of potassium and phosphorus may be absorbed from a soil at the 

 wilting percentage, even though the amounts were relatively small as 

 compared with nutrient entry from moist soil. 



Although there is some evidence that plants may absorb small quan- 

 tities of nutrients from dry soils, this source appears to be inadequate 

 for a thriving plant. If moisture is available in unfertile lower horizons 

 of a soil and if most of the fertility is in the surface horizon that has 

 become very dry, the plant may suffer from inadequate mineral nu- 

 trients. 



SUMMARY 



Consideration is given to the physical condition of moisture through- 

 out the plant-growth moisture range, including typical curves showing 

 soil moisture content at moisture tensions up to 20 atmospheres. Princi- 

 ples governing the flow and distribution of water in soils are discussed, 

 together with the relation of field-capacity to the unsaturated perme- 

 ability of soils. 



The mineral nutrition of plants is reviewed under three soil moisture 

 categories: (a) excess moisture above field capacity, (b) moisture vari- 

 ations within the field moisture range, and (c) at moisture content 

 below the wilting percentage. 



Excess moisture affects mineral nutrition by loss of nutrients through 

 surface runoff, excessive leaching, and by decreased availability of nu- 

 trients under the poor aeration prevailing in wet soils. 



Within the field moisture range, soil moisture supply may itself be- 

 come the limiting factor to plant growth so that nutrients available are 

 not efficiently utilized. Drying of soils .intensifies potassium and phos- 

 phorus fixation, and lowers their availability. Since soil moisture vari- 



