98 SOIL WATER AND ITS RELATION TO SOIL FERTILITY 



strawy rye. Rye is commonly plowed under to add organic 

 matter to soils. For best results it should be plowed under before 

 it develops stiff straw. In either case cavities are formed which 

 impede or prevent the capillary rise of water from the subsoil 

 to the seed bed. This is especially true when sod is improperly 

 plowed just prior to a dry period. When much coarse litter is to be 

 turned under it is best to plow in the fall, and whenever it is done a 

 short time before planting good contact should be secured, if possible, 

 between the soil and subsoil by working the land and rolling it. 



The Soil a Reservoir. In the preceding chapter it was learned 

 that soils act as reservoirs in storing a portion of the water supplied 

 to them and giving it up again to growing crops. Soils vary in 

 this capacity because of differences in texture, content of organic 

 matter and structure. Thus it is that we speak of "water-holding 

 capacity" of soils. 



Water-holding Capacity of Soils. Water-holding capacity of a 

 soil is generally understood to mean the greatest amount of water 

 it can retain when all free water is given a chance to drain out. 

 The water thus held includes capillary and hygroscopic moisture, 

 and is expressed in per cent of the dry weight of the soil; for 

 example, if fifty pounds of a perfectly dry soil can hold fifteen 

 pounds of water after allowing all free or gravitational water to 

 drain out, that soil has a water-holding capacity of thirty per cent, 

 the greater portion of it being capillary water. 



The following table shows how different soils vary in the amount 

 of water they can hold against gravity: 



Water-holding Capacity of Soils 



* Obtained by multiplying the weight of soil by the per cent water-holding capacity, 

 and reducing result to inches. 



t Roots cannot absorb the last trace of capillary water held in soils, because when the 

 water films become very thin the attraction between the soil grains and the thin films becomes 

 as great or greater than the absorptive power of the roots. The finer the soil the more 

 hygroscopic or unavailable moisture held. Crops may wilt and cease to grow in silt loams 

 and clays when still carrying 12 and 14 per cent moisture, respectively, while they may 

 grow well in coarse sand possessing but 1 to 3 per cent water. 



