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 soii 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 early 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 lies 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 evaporation, shallow cultivations often cause 
a greater evaporation by disturbing the almost per- 
fect naturai mulch. Clay or sand soils, which do not 
so well formanatural mulch, will respond much better 
to shailow cultivations. In general, however, the 
deeper the cultivation, the more effective it is in re- 
