HOW SOILS HOLD CAPILLARY WATER 93 



"How and to what extent can farmers conserve the rainfall and 

 control the moisture in the soil to prevent any reduction in yield 

 because of short dry periods or to lessen the damaging effects 

 of drought? ^^ 



Before taking up the study of moisture conservation and con- 

 trol, it is necessary to consider first the forms of soil water, how 

 water is retained by soils for crop use, the movements of water in 

 the soil, and the capacity of soils for holding water for crops. 



SOIL WATER 



Forms of Soil Water. — Water in relation to soils may occur in 

 three forms or conditions; as — 



(a) Gravitational or free water. Water which, if opportunity 

 be given it, will flow off or run down through the soil and away 

 because of gravity. 



(6) Capillary water. Water which is held by the soil against 

 gravity after all free water is allowed to drain out, but which is 

 free to move from soil particle to soil particle. 



(c) Hygroscopic water. Moisture which exists in air-dried soil, 

 or in soil in which plants permanently wilt. Usually a small amount 

 of capillary water is also present in soils when plants wilt. 



Capillary Water Most Important. — ^Free or gravitational water 

 in wet lands should receive first consideration, since that isthe water 

 which should be gotten rid of before such lands can be brought 

 under cultivation. In all soils having good drainage, however, 

 capillary water is the most important, because this is the form which 

 constitutes the soil moisture reserve from which crops draw their 

 water requirements. Free water is detrimental to most culti- 

 vated crops. Rice and cranberries are crops which are peculiarly 

 adapted to wet or saturated soils. 



How Soils Hold CapiUary Water. — Capillary water may be 

 held in soils in three ways, viz.: 



(a) In form of films around the soil grains. 



(6) In organic matter as in a sponge. 



(c) In some of the pore spaces within soil crumbs or granules 

 (Fig. 6A). 



A mineral soil can hold its greatest amount of capillary water 

 when moisture can be retained in all the three ways mentioned. 



A clean sand can hold capillary water only as films around the 

 sand grains. A coarse sand is capable of retaining a compara- 

 tively small percentage of capillary water because of the less amount 



