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 imjDortant as any of the 

 differences above outlined are those which depend 

 definitely upon the leaching pijwer 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 nKjvement of soil-water thrcjughout the 

 year. As is clearly shown by the ajjpearance, taste, 

 and chemical composition of drainage waters, this 

 process leaches out considerable cjuantities 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 



