TILLAGE AND EVAPORATION 157 



Depth of cultivation 



The all-important practice for the dry-farmer who 

 is entering upon the growing season is cultivation. 

 The soil must be covered continually with a deep 

 layer of dry loose soil, which because of its looseness 

 and dryness makes evaporation difficult. A leading 

 question in connection with cultivation is the depth 

 to which the soil should be stirred for the best results. 

 Many of the earl}" students of the subject found that 

 a soil mulch only one half inch in depth was effective 

 in retaining a large part of the soil-moisture which 

 noncultivated soils would lose by evaporation. 

 Soils differ greatly in the rate of evaporation from 

 their surfaces. Some form a natural mulch when 

 dried, which prevents further water loss. Others form 

 only a thin hard crust, below which Hes an active 

 evaporating surface of wet soil. Soils which dry out 

 readily and crumble on top into a natural mulch 

 should be cultivated deeply, for a shallow cultivation 

 does not extend beyond the naturally formed mulch. 

 In fact, on certain calcareous soils, the surfaces of 

 which dry out quickly and form a good protection 

 against evaj:)oration, shallow cultivations often cause 

 a greater evaporati(jn by disturbing the almost per- 

 fect natural mulch. Clay or sand soils, which do not 

 so well form a natural mulch, will respond much better 

 to shallow cultivations. In general, however, the 

 deeper the cultivation, the more effective it is in re- 



