CHAPTER XVII 



ACID, OR SOUR, SOILS 



Some soils are known as acid, or sour, soils. The 

 property of acidity is of practical significance because 

 some plants do not grow so well on sour soils as they do 

 on soils that are neutral or alkaline ; on the other hand, 

 some crops prefer an acid soil. Sour soils are rarely met 

 with in arid regions, but in humid sections of the United 

 States they are commonly found. 



280. Nature of soil acidity. — Soils may be acid, or 

 sour, so far as their relation to plant growth is concerned, 

 (1) when free acids are present, (2) when no soluble free 

 acid exists, but when there is a deficiency of basic material 

 in the soil. Decomposition of organic matter in certain 

 soils under an inadequate supply of oxygen often results 

 in the formation of considerable quantities of organic 

 acids, as has already been explained (par. 93). 



281. Positive acidity. — The formation of organic 

 acids under conditions of insufficient oxygen supply is 

 frequently seen in muck and other soils high in organic 

 matter that are saturated with water and that are also 

 deficient in lime. In such cases an acid condition is very 

 likely to be found, but when the land is drained the 

 acidity usually disappears because of the better aeration 

 resulting. When a large quantity of green vegetation 

 is plowed under, as is done in green-manuring land, a 

 soiH condition sometimes appears after the material has 



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