Soils 39 



plants. Water is taken up by the roots and either becomes part 

 of the plant without change or it is decomposed and the ele- 

 ments of which it is composed are available to become part of 

 new compounds of the plant body. In addition water keeps 

 the cells distended, transfers food from one part of the plant to 

 another, and by evaporation from the leaves tends to equalize 

 the temperature of the plant. 



Soil moisture is one of the limiting factors in crop produc- 

 tion, for without sufficient water there can be no profitable 

 crop growth. The quantity of water required by a growing 

 crop is much larger than might be thought. The soil solutions 

 taken up by roots are very dilute ; consequently large quanti- 

 ties of water must be carried up into the plant for every pound 

 of growth produced. In fact it has been found that in humid 

 climates from two hundred to five hundred pounds of water 

 are transpired from the leaves for every pound of dry plant 

 material produced in a crop. In addition to the water taken 

 up by the plants, there is much lost from the soil by run-off 

 from the surface, by percolation to a depth below the reach 

 of roots, and by evaporation from the surface. These condi- 

 tions make it necessary in most sections to conserve the soil 

 moisture. 



20. Forms of water in soil. Water is held in the soil in 

 three forms, known as hygroscopic, capillary, and gravita- 

 tional water. 



Hygroscopic water. This water is held as a very thin film 

 around each soil particle ; it is absorbed from the air and con- 

 densed on the surfaces of the soil particles. Even in very dry soil, 

 this film- water surrounds each particle. The quantity increases 

 according to the moisture-content of the air and there is more 

 in fine-grained than in coarse-grained soil, because of the larger 

 area of film-surface exposed. Also, the more humus in a soil, 

 the greater is the quantity of hygroscopic water present. The 

 moisture held hygroscopically in a soil cannot be absorbed by 

 plants. In fact plants wilt for lack of water if only hygro- 



