254 AGRICULTURE 



fully estimated that one ton of humus will absorb two tons 

 of water and give it up as needed by growing plants. 



3. Tillage and Soil Water 



One of the chief problems of agriculture is to conserve 

 the capillary water of the soil and make it available for 

 plant growth. Capillary water is removed from the soil 

 in two ways, (1) by evaporation, and (2) by being absorbed 

 by the roots of growing plants. What is lost by evapora- 

 tion is wasted so far as crops are concerned. 



Tillage for conserving capillary moisture. All loos- 

 ening of the soil increases its absorbing power, and thus 

 causes rain to soak into the ground instead of running off 

 the top. The deeper the plowing the greater this effect will 

 be. Fall plowing, by opening the soil for the absorption of 

 the winter snows, adds to the amount of soil water. 



The most effective tillage for conserving capillary water, 

 however, is the frequent cultivation during the growing sea- 

 son which results in a fine soil mulch over the surface. To 

 see the truth of this, make the following experiment : 



Effect of a soil mulch. Fill two glasses nearly full 

 of the same soil ; if the soil is dry, add an equal amount of 

 water to each, making the soil fairly damp, but not soaked ; 

 pack them equally by striking the glass gently down on the 

 table. Now put a half inch of fine, dry road dust over the 

 top of one, leaving the other without covering. Set the two 

 glasses side by side, and note the time it requires for each 

 to dry out by losing its capillary water through evapora- 

 tion. 



The fine mulch made by frequent harrowings and culti- 

 vatings has precisely the same effect on our fields. In dry 

 regions summer fallowing is used for the purpose of col- 

 lecting a supply of capillary water. Whatever rain falls is 



