n8 RELATIONS OF SOIL TO WATER 



soil. From this point of view, salts of little solubility, as 

 gypsum, should be those which most effectually hinder 

 evaporation. Investigations on this subject are much needed ; 

 but it appears quite likely that the results obtained from 

 manuring experiments in dry seasons are a good deal com- 

 plicated by the effect of the manures on the water contents of 

 the soil, an effect which may be as strongly manifested by a 

 saline manure supplying little plant food as by one rich in 

 such constituents. 



The effect of farmyard manure is complicated by a variety 

 of circumstances. Applied as a top dressing it acts excellently 

 as a mulch, diminishing the evaporation from the soil. 

 Ploughed in in spring or early summer, during dry weather, its 

 effect may be to dry the surface soil considerably, its bulky 

 nature and loose texture greatly aiding the drying effect of 

 wind. The permanent effect of the manure is decidedly to 

 increase the capacity of the soil for retaining water, owing to 

 the humus produced by its decay (p. 72); but this effect is 

 confined almost entirely to the surface soil with which the 

 manure is mixed. 



Influence of a Crop on Evaporation. We have considered 

 hitherto the case of a bare soil, and have discussed the various 

 circumstances influencing the amount of evaporation from it. 

 The factor, however, which more than any other determines 

 the rate of evaporation is the presence or absence of 

 vegetation. 



When the soil is covered by vegetation, a portion of the 

 rain does not reach the ground, but remains on the leaves 

 and is evaporated from them ; this loss probably reaches its 

 maximum in the case of a forest. According to Weber's 

 observations in Switzerland, Prussia, and Bavaria, the per- 



