Relation of Soils to Moisture and Air 43 



and left to the action of these films of water gradually 

 releases its phosphorus to the roots, though the chemist 

 finds that it is soluble only in strong acids. But Nature's 

 laboratory, though working slowly, works all the time 

 and often puts at naught the results of the laboratory of 

 the chemist. Few who have not studied the matter sci- 

 entifically realize the large amounts of water used in the 

 production of our cultivated crops. For every ton of dry 

 matter in a crop of Indian corn the plants used 309.8 tons 

 of water, and similar amounts are used by other crops. 

 A clover crop uses a great deal larger amount than com. 

 This water is not only taken as a means for the carrying 

 of plant food, but is itself necessary for the life activities 

 of the plant ; for no matter how much food may be present 

 and within the plant its activities must cease if there is 

 not a sufiiciency of water present for the living matter to 

 continue its work. 



Soils vary greatly in their capacity for the 

 Retentive retention of moisture. Since the water in 



Capacity of ^-^^ ^^.j j^ ^^^^ ^^ ^^^^ surrounding each 

 Various . 



Soils grain, it is evident that a coarse-gramed soil 



will hold less moisture than one made up of 

 very fine grains. Then, too, the capacity of the soil for 

 drawing water from below will vary with the fineness of 

 its particles, for we have seen that the finer the capillary 

 tubes in the soil the higher the water will rise from below. 

 This capillarity will also depend on the depth at which 

 the permanent water-table in soil is from the surface. 

 Also, in a finely divided soil the water that falls in rain 

 percolates more slowly downward and moisture is re- 

 tained longer after a rain than in a coarsely granulated 



