PROBLEMS OF DRY-FARMING 7 
as to its power to receive and retain the water from 
rain and snow. In fact, a knowledge of the soil is 
indispensable in successful dry-farming. Only by 
such knowledge of the rainfall and the soil is he able 
to adapt the principles outlined in this volume to 
his special needs. 
Since, under dry-farm conditions, water is the 
limiting factor of production, the primary problem 
of dry-farming is the most effective storage in the 
soil of the natural precipitation. Only the water, 
safely stored in the soil within reach of the roots, can 
be used in crop production. Of nearly equal impor- 
tance is the problem of keeping the water in the soil 
until it is needed by plants. During the growing 
season, water may be lost from the soil by downward 
drainage or by evaporation from the surface. It 
becomes necessary, therefore, to determine under 
what conditions the natural precipitation stored in 
the soil moves downward and by what means surface 
evaporation may be prevented or regulated. The 
soil-water, of real use to plants, is that taken up by 
the roots and finally evaporated from the leaves. 
A large part of the water stored in the soil is thus 
used. The methods whereby this direct draft of 
plants on the soil-moisture may be regulated are, 
naturally, of the utmost importance to the dry- 
farmer, and they constitute another vital problem 
of the science of dry-farming. 
The relation of crops to the prevailing conditions 
