PHYSIOGRAPHIC FACTORS 171 



ability of soil water to the plant may be obtained with porous soil 

 point cones, 158 whose rate of absorption is taken as the basis for 

 evaluating water supplying povcer of the soil. 



Availability of Soil Moisture to Plants.— Gravitational water is 

 readily available to plants only when present in a saturated soil, a 

 condition that rarely continues long enough to be of importance. 

 Normally, then, readily available water is that capillary water in 

 the range between field capacity and the permanent wilting per- 

 centage. It is usually lowest in sand, and highest in clay. The fol- 

 lowing values for readily available water are found in some North 

 Carolina soils : 149 sand, 2 percent, sandy loam, 14 percent, clay, 19 

 percent. However, this generalization does not always hold, for 

 some clays may have high field capacities but also have high wilt- 

 ing percentages. A California clay with a moisture equivalent of 

 31 percent was found to have a wilting percentage of 25 percent, 

 and, therefore, it could contain only 6 percent of available water. 

 Such a soil would store less water for plant use than many sandy 

 soils, and plants growing in it would suffer from drought much 

 sooner than its soil texture would indicate. This also explains why, 

 in contrast to the usual situation, sand dunes in deserts have more 

 favorable moisture conditions than the surrounding clay soils. 

 When both are at or near the wilting percentage, as they fre- 

 quently are, a typically light rain provides little or no available 

 water in the clay but does provide some in sand, in addition to 

 penetrating more deeply, because of the lower wilting percentage 

 of sand. 



Whether or not all available water is equally available to plants 

 is not entirely agreed upon. The evidence from a variety of 

 sources seems to favor a decreasing availability as the supply is 

 reduced toward the permanent wilting percentage and particu- 

 larly in the lower half of the range of available water. Another 

 factor affecting the availability of soil water is the concentration 

 of the soil solution, which, if high, may have a toxic effect on 

 plants and also modify their osmotic activity. Soil temperature, 

 too, may be effective. Water supplying power may be reduced by 

 half when soil temperature is lowered from 77° E to 32° F. Prob- 

 ably the increase in viscosity of water at low temperatures reduces 

 the rate of movement from soil to absorbing surface. 



