194 THE PRINCIPLES OF SOIL MANAGEMENT 



capacity of the soil for water is somewhat greater in 

 this case. (2) It has been estimated that the annual 

 run-off of the streams in the eastern half of the United 

 States amounts to about 50 per cent of the rainfall; 

 but in the basin of the Missouri river the run-off is not 

 over 20 per cent of the rainfall, and in the Great Basin 

 it is practically nil. 



These figures give some idea of the total amount 

 of water lost by percolation through the soil, and repre- 

 sent a supply which it is the aim of good soil manage- 

 ment to lessen or eliminate, according to the needs of 

 the crop. Loss from percolation may be reduced in two 

 ways, which depend upon the fact that the rapidity 

 of such loss is directly proportional to the size and volume 

 of the pore spaces in the soil. These are (a) by modifi- 

 cations of texture, (b) by modifications of the structure 

 of the soil. The primary method is, or course, that 

 modification of structure which breaks down the granular 

 arrangement and permits a greater compactness. When 

 rain falls on the soil, its fall is not stopped. It continues 

 to fall through the soil at a reduced rate as gravitational 

 water. And, as the movement of this gravitational water 

 is directly determined by the fineness of the soil spaces, 

 it is possible to very greatly reduce this type of move- 

 ment by compacting the soil structures. The greater 

 compactness of the soil lengthens out the period during 

 which the soil contains hydrostatic water, and, if the 

 roots of growing plants are distributed througli the soil, 

 they are able to make a larger use of this free water 

 than would be possible if the wave of saturation, as a 

 result of rainfall or irrigation, quickly passed beyond 



