TRANSPIRATION AND SOIL SOLUTION 181 
is increased greatly; but when alkalies of any kind 
are added, transpiration decreases. This is of special 
interest in the development of dry-farming, since 
dry-farm soils, as a rule, contain more substances 
Fig. 43. Ideal tilth of soil. 
that may be classed as alkalies than do soils main- 
tained under humid conditions. Sour soils are 
very characteristic of districts where the rainfall is 
abundant; the vegetation growing on such soils tran- 
