288 DRY-FARMING 
mit a deeper penetration of the natural precipitation 
and a freer movement of the soil-water result in a 
larger quantity of plant-food reaching the upper 
two or three feet from the lower soil depths. Such 
concentration near the surface, when it is not exces- 
sive, favors the production of increased yields of 
crops. 
The characteristic high fertility and great depth 
of arid soils are probably the two main factors 
explaining the apparent increase of the fertility of 
dry-farms under a system of agriculture which does 
not include ‘the practice of manuring. Yet, there 
are other conditions that contribute largely to the 
result. For instance, every cultural method accepted 
in dry-farming, such as deep plowing, fallowing, and 
frequent cultivation, enables the weathering forces 
to act upon the soil particles. Especially is it made 
easy for the air to enter the soil. Under such condi- 
tions, the plant-food unavailable to plants because 
of its insoluble condition is liberated and made avail- 
able. The practice of dry-farming is of itself more 
conducive to such accumulation of available plant- 
food than are the methods of humid agriculture. 
Further, the annual yield of any crop under con- 
ditions of dry-farming is smaller than under condi- 
tions of high rainfall. Less fertility is, therefore, 
removed by each crop and a given ainount of avail- 
able fertility is sufficient to produce # large number 
of crops without showing signs of cleficiency. The 
