CHAPTBE XVIII 

 SOIL ACIDITY 



A CHEMICAL or physieo-cliemieal viewpoint regarding the 

 soil and its solution is essential in explaining many of the phe- 

 nomena, especially those relating to higher plants and their 

 nutrition. Since plants respond so markedly to their chemical 

 environment, the importance of soil reaction has long at- 

 tracted much attention. Two conditions are popularly recog 

 nized in this respect — soil alkalinity or alkali and soil acidity. 

 The former condition can only occur where soluble salts may 

 concentrate in the soil and is confined largely to arid and semi- 

 arid regions. Soil acidity, on the other hand, is common only 

 in humid sections. So widespread is it occurrence and so 

 marked is its influence on crop yields that its importance in 

 a practical way surpasses that of soil alkali. 



183. General nature of soil acidity.^ — The nature of soil 

 acidity is so little understood that it is impossible to define 

 or explain it except in the most general terms. So-called soil 

 acidity may be considered for practical purposes as a more 

 or less unfavorable condition for plant growth, arising in the 

 soil through a lack of certain active bases such as calcium and 

 magnesium and which in practice is alleviated by the addition 

 of some form of lime.^ 



Technically three reasons may be suggested as accounting 

 for the harmful effects of soil acidity: (1) unfavorable hydro- 



^Maclntire, W. H., Tlie Nature of Soil Acidity witli Begard to its 

 Quantitative Determination; Jour. Amer. Soe. Agron., Vol. 13, No. 4, 

 pp. 137-161, 1921. 



^Lime in an agricultural sense refers to all of tlie compounds of cal- 

 cium and magnesium commonly utilized in correcting soil acidity. 



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