146 



PRINCIPLES OF AGRICULTURAL, CHEMISTRY 



The annual evaporation from the bare soil in the Rothamsted 

 rain-gauges is apparently unaffected by the amount of rainfall. 

 During nine years the rainfall varied from 22.9 to 42.7 inches ; 

 the evaporation from the shallow gauge varied from 16.6 to 18.4 

 inches, while the percolation varied from 5.6 to 25.5. inches. 



The losses of water from soil carrying vegetation is greater 

 than from a bare soil. In the rotation of crops, it is necessary 

 to consider this fact. For example, King 1 determined the per- 

 centage of water in two portions of a field about to be planted to 

 corn ; one portion had previously been a bare fallow, the other 

 had carried clover. The clover land contained much less water 

 than the bare fallow land, and the corn on the bare fallow would 

 thus be far better able to stand a summer drought. One injurious 

 effect of weeds is to remove water from the soil. 



Relation of Water Content to Evaporation. In the following 

 table (by Schubler) the amount of water absorbed by soils is com- 

 pared with the amount of evaporation during four hours : 



It is evident that the finer soils have not only a greater water 

 capacity, but also allow less evaporation to take place. 



Control of Water. The control of water, so that plants may 

 at all times receive the optimum amount, or as near it as possible, 

 is one of the most important parts of agricultural practice and 

 the chief object of many operations of tillage. 



Control of the water supply is exercised by storing water in the 

 1 The Soil, I9r. 



