LEACHING IN DRY-FARM SOILS 65 
every other year, so that a large quantity of water 
may be stored in the soil to open and destroy the 
hardpan. 
Of course, in arid as in: humid countries, it often 
happens that a soil is underlaid, more or less near the 
surface, by layers of rock, marl deposits, and similar 
impervious or hurtful substances. Such deposits 
are not to be classed with the hardpans that occur 
normally wherever the rainfall is small. 
Leaching. — Fully as important as any of the 
differences above outlined are those which depend 
definitely upon the leaching power of a heavy rain- 
fall. In countries where the rainfall is 30 inches or 
over, and in many places where the rainfall is con- 
siderably less, the water drains through the soil into 
the standing ground water. There is, therefore, in 
humid countries, a continuous drainage through the 
soil after every rain, and in general there is a steady 
downward movement of soil-water throughout the 
year. As is clearly shown by the appearance, taste, 
and chemical composition of drainage waters, this 
process leaches out considerable quantities of the 
soluble constituents of the soil. 
When the soil contains decomposing organic 
matter, such as roots, leaves, stalks, the gas carbon 
dioxid is formed, which, when dissolved in water, 
forms a solution of great solvent power. Water 
passing through well-cultivated soils containing much 
humus leaches out very much more material than 
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