118 IRRIGATION PRACTICE 



water will be spread over a much larger surface of soil 

 particles and the film, therefore, will be thinner; hence, the 

 water will be absorbed at a slower rate than from a coarse- 

 grained soil, which exposes a smaller surface and over 

 which the same quantity of water forms a much thicker 

 film. It may be demonstrated that, with a given quantity 

 of water, the thickness of the film that forms over soil 

 particles varies as the radius of the soil grains. That is, 

 if in a given soil the particles are twice as large in diame- 

 ter as in another, a given quantity of water added to 

 these soils will form a film twice as thick in the coarse 

 soil as in the fine one. Consequently, plants growing on 

 fine-grained soils will use water at a lower rate than 

 those growing on coarse-grained soils. In other words, 

 under conditions otherwise uniform, the more clay a 

 soil contains the less rapidly does the plant draw water 

 from it. 



84. Chemical composition of soils. The chemical 

 composition of the soil also determines the rate at which 

 plants take moisture from the soil. This factor is of 

 especial importance because it is within the power of the 

 farmer to change, at least in a small way, the chemical 

 composition of the soil, by proper methods of tillage, or by 

 the direct addition to the soil of manure or commercial 

 fertilizers. As explained in Chapter V, the chemical sub- 

 stances of which the soil is composed are gradually dis- 

 solved by the soil moisture. The soil solution of different 

 soils varies, therefore, with the composition of the soil 

 and the quantity of water added. The root-hairs, through 

 which soil moisture is absorbed, lie immersed in the soil 

 solution. The rate at which water is taken from the soil 

 by these plant roots depends largely upon the relative 

 strength of the solution inside and outside of the root- 



