104 THE SOIL AND ITS IMPROVEMENT 



depends on the extent of surface of the soil particles. A 

 cubic foot of coarse sand will hold one third its bulk of 

 water. Only a part of this water, however, is capil- 

 lary water, that is, water which clings to the surface of the 

 grains. We have learned in arithmetic that surfaces in- 

 crease as the squares of their like dimensions while solids 

 increase as the cubes of their like dimensions. A grain of 

 sand has a greater surface in proportion to its size than 

 a pebble has. Therefore a cubic foot of sand will hold 

 more capillary water than a cubic foot of pebbles. 



63. Preventing Loss of Capillary Water. Farmers give a 

 great deal of attention to preventing the needless escape 

 of capillary water from the soil, since nearly all the water 

 absorbed by plants is of this sort. When the sun shines 

 bright again after a heavy rain, the surface of the soil soon 

 becomes hard and compact, forming a crust. Pick up a 

 small block of such crust and look for air spaces in it. 

 They are exceedingly small. In other words, the soil par- 

 ticles are fine and close together. Such a condition (ac- 

 cording to the statements in the last paragraph above) 

 hastens the movement of soil water to the surface, from 

 which it is evaporated and lost to the crop. 



The way to prevent this rapid loss is to break up the 

 crust into loose particles with the hoe, harrow, or cultivator. 

 By the use of these tools a dry mulch is formed at the 

 surface of the soil, and the widely separated particles of 

 this mulch draw up the capillary water from below very 

 slowly. Of course, some moisture continues to evaporate; 

 but, by renewing the mulch every few days, the loss can 

 be kept very small. Top dressings of straw or manure, 

 and cover crops like rye and clover, are also effective 

 mulches. Moreover, when plowed under and turned into 

 humus, such substances enable the soil to hold more 

 capillary water. 



