98 SOIL WATER AND ITS RELATION TO SOIL FERTILITY 



strawy rye. Rye is coromonly 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 fiom the subsoil 

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

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

 turned ^mder 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 lollmg it. 



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

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

 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 fi.fteen 

 poimds 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 



Soils 



A coarse sand 



A fine sand 



A hght colored silt loam 



A black silt loam 



A well-decomposed peat 



Water-holding 



capacity 



(capillary and 



hygroscopic 



water) 



per cent 



15 

 22 

 30 

 45 

 134 



The equivalent 



m terms of 



inches of 



water m one 



acre one foot 



deep* 



3 2 inches 



4.2 inches 



4 4 inches 



6 inches 



10 2 inches 



The approx- 

 mxate amount 

 that crops can 

 use from one 

 acre one foot 



deept 



(Available or 



capillary 



water) 



3 inches 

 3 6 inches 

 2 6 inches 

 4.0 inches 

 6 8 mches 



* 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 m soils, because when the 

 water films become very thm 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 m silt loams 

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

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



