No. I.] IRRIGATION. 403 



or the losses of moisture from the soil. Only a part of the 

 water which, falls becomes available to growing crops. A 

 considerable part flows off on the surface of the ground and 

 finds its way into streams, while in the case of heavy rains 

 quite a portion is leached away and carried beyond the roots 

 of plants. However, in soils that are not too porous much 

 of the water which is leached into the lower strata may find 

 its way back to the surface soil by capillary attraction. 

 One of the most important matters to be considered in the 

 culture of crops is that a large and variable quantity of 

 water is evaporated directly from the soil. The amount 

 of this depends upon several conditions, the chief of which 

 are the state of the weather, the kind of crop on the soil, 

 the amount of cultivation, and whether or not the soil is 

 mulched. In times when rainfall is insufficient for the best 

 growth of crops the atmospheric conditions are usually such 

 as to favor the evaporation of 'moisture from the soil. The 

 amount of evaporation that takes place depends upon the 

 amount of wind that may be blowing over the soil and 

 the degree of saturation of the air. Meteorological data 

 showing the relative humidity of the air indicate that on 

 hot, dry days the air may contain as low as from 20 to 50 

 per cent of its water-holding capacity. Under such condi- 

 tions, especially in connection with winds, the moisture 

 evaporates from the soil very rapidly. The shade afforded 

 by crops like grass and small grains tends to lessen the 

 amount of evaporation from the soil, w r hile crops which do 

 not shade the ground as much furnish conditions more 

 favorable for the escape of moisture. 



It is a well-known fact that mulch on the soil, in the form 

 of coarse hay, straw, etc., tends to prevent the escape of 

 moisture. This, together with the cleaner fruit that is thus 

 obtained, is one of the reasons for using such materials on 

 strawberry fields. Frequent stirring of the surface soil by 

 cultivation has much the same effect in preventing the escape 

 of moisture as the direct use of mulch. In the experiments 

 by the writer, on the evaporation of moisture from heavy 

 loam and from light loam soils, part of the soils were fre- 

 quently stirred at the surface, while the others were not 



