PLANT GROWTH IN DRY AREAS 109 



the depth of 8 feet was at the outset 17.57 and 16.55 per 

 cent., which shows very conclusively that the larger the 

 proportion of the water in the surface soil, the greater rel- 

 atively will be the loss, and vice versa, the treatment of 

 the soils being the same in both instances. When the sur- 

 face soil is kept in proper condition, the loss of moisture 

 from the subsoil is very little, save through transpiration 

 in growing plants. 



The most potent agency in preventing the escape of 

 moisture from the soil which it has entered is surface cul- 

 tivation. The influence of cultivation, mainly through 

 the maintenance of the dust mulch is discussed elsewhere 

 (see p. 165). The same is also true of the maintenance of 

 humus in the soil, which also is discussed more fully in 

 another chapter (see p. 413). Of course no system of cul- 

 tivation can be adopted that will entirely prevent the 

 loss of moisture or even the loss of more than half of it 

 on the average, during the year, but beyond all question 

 a system of cultivation may be followed that will safely 

 hold a large share of the moisture that would be lost 

 to the soil in the absence of such cultivation. 



Reference is made elsewhere to the loss of moisture 

 to the soil through the growth of weeds (see p. 386). This 

 loss cannot be entirely prevented but it may certainly be 

 very greatly reduced. Ordinarily, it is possible to prevent 

 such loss in the summer-fallow and also in crops that are 

 cultivated. But it is not possible to prevent it entirely in 

 cultivated crops, although by a judicious system of till- 

 age it may be kept so low that it may not be a serious 

 loss. In humid areas, loss of moisture from this source is 

 much less serious than in dry areas, but in either case 

 the aim should be to prevent it. 



In semi-arid as in humid areas the shading of the soil, 

 as already intimated, may under some conditions tend 

 materially to reduce the amount of evaporation. The 

 shade may be natural or artificial. Natural shade is 



