60 DRY-FARMING 
decayed vegetable matter and modified by plowing, 
harrowing, and other cultural operations. The sub- 
soil has been profoundly modified by the action of 
the heavy rainfall, which, in soaking through the 
soil, has carried with it the finest soil grains, espe- 
cially the clay, into the lower soil layers. 
In time, the subsoil has become more distinctly 
clayey than the topsoil. Lime and other soil ingre- 
dients have likewise been carried down by the rains 
and deposited at different depths in the soil or wholly 
washed away. Ultimately, this results in the re- 
moval from the topsoil of the necessary plant-foods 
and the accumulation in the subsoil of the fine clay 
particles which so compact the subsoil as to make 
it difficult for roots and even air to penetrate it. 
The normal process of weathering or soil disinte- 
gration will then go on most actively in the topsoil, 
and the subsoil will remain unweathered and raw. 
This accounts for the well-known fact that in humid 
countries any subsoil that may have been plowed up 
is reduced to a normal state of fertility and crop 
production only after several years of exposure to 
the elements. The humid farmer, knowing this, is 
usually very careful not to let his plow enter the sub- 
soil to any great depth. 
In the arid regions or wherever a deficient rain- 
fall prevails, these conditions are entirely reversed. 
The light rainfall seldom completely fills the soil 
pores to any considerable depth, but it rather moves 
