152 
ELEMENTARY AGRICULTURE 
Sour Soils.—Acid or sour soils are found in the humid 
regions and are formed by the decay of organic matter 
without a sufficient supply of free oxygen. Such crops 
as cowpeas, clover, soy beans, and alfalfa do not thrive 
well on acid soils. But the acidity of the soil may be 
corrected by applying two tons of finely ground lime¬ 
stone per acre. 
Alkali Soils.—In semi-arid regions we find alkali 
spots which were formed by the evaporation of water 
Fig. 115.—Soil and subsoil. The shovel points to the line where the light 
clay begins. The upper portion is top soil. 
from the soil which was charged with alkali salts leav¬ 
ing the deposit in the surface soil. Since the quantity 
in these places does not generally prevent crop growth 
during years of sufficient rainfall it is advisable to 
improve the land by plowing under a heavy application 
of barnyard manure. 
Water in Soils.—The water which enters the soil 
after a heavy rain or the melting of snow and ice which 
passes down through the loose earth and drains away 
in the form of springs, is known as a free, or gravity 
