THE WATER OF SOILS. 221 



ABSORPTION AND MOVEMENTS OF WATER IN SOILS. 



The phenomena and laws of capillary ascent of water in 

 soils, as discussed in the preceding chapter, serve best to de- 

 monstrate the general behavior of liquid water within differ- 

 ent soils and their several grain-sizes ; because measurably in- 

 dependent of the physical changes that almost unavoidably ac- 

 company the percolation of water from above downward; 

 whether such water comes in the form of rain, or irrigation, 

 or even when applied with the utmost precautions in the labor- 

 atory. The " beating " of rains quickly compacts the surface 

 to a certain extent, varying with the nature of the soil, its con- 

 dition of more or less perfect tilth, and the degree of violence 

 with which the rain strikes the surface. When the latter has 

 been compacted by a previous rain and then dried, " baking " 

 or incrusting the surface, the latter may almost wholly shed a 

 rain of brief duration, which, had the surface been loose, would 

 have been wholly absorbed, materially benefiting the crop. 

 Such surface-crusting is, therefore, injurious in preventing the 

 absorption of water from above; and in addition, it serves to 

 waste, by evaporation, the moisture contained in the under- 

 lying soil and subsoil. For the crust being of a finer (single- 

 grain) texture than the tilled portion beneath, it will forcibly 

 abstract from the latter, by absorption, its capillary moisture, 

 and evaporating it at the upper surface, continue to deplete the 

 land, to the great injury of crop growth, until destroyed by 

 cultivation. 1 



The flow of irrigation water produces the same compacting 

 effect, but to a less extent; the more as, unlike rain water, irri- 

 gation water usually contains a certain amount of alkaline and 

 earth salts, which tend to prevent the diffusion of clay and of 

 fine sediments, and therefore the disintegration of the soil-floc- 

 cules into single grains. Nevertheless, it is in some soils as 

 necessary to cultivate after surface-irrigation as after rains, 

 in order to prevent great waste of moisture by evaporation. 



Determination of rate of percolation. When water is al- 



1 This effect is well illustrated by the behavior of a dry brick laid upon a wet 

 sponge. It will quickly absorb all the liquid moisture contained in the latter, 

 while the sponge will be wholly unable to take any moisture from a fully-soaked 

 brick. 



