376 SOILS: PROPERTIES AND MANAGEMENT 



had time partially to decay. The acidity of soil that 

 arises from the presence of free acids has been termed 

 positive acidity. 



It is to be presumed that soils in which free acids exist 

 are rather deficient in basic material, and that the bases 

 are held so firmly combined that some of the relatively 

 weak organic acid present is not capable of forming salts 

 with them. Plummer 1 has shown that dihydroxystearic 

 acid when added to an acid soil had a distinctly toxic 

 effect on wheat plants, but when added to the same soil 

 previously treated with lime there was no toxic effect, 

 indicating that this substance retained its acid properties 

 in the unlimed soil. 



282. Negative acidity. A soil deficient in basic 

 material but containing no soluble free acids may be 

 sour as regards its relation to plant growth. At least 

 such a soil may be greatly benefited by liming, although 

 it shows no acidity to most of the ordinary indicators of 

 acidity when these are used in the customary way. This 

 condition has been termed negative acidity and is really 

 not acidity according to a correct use of the word. Such 

 acidity does not have a direct effect on the plant, but 

 an indirect one arising from a lack of bases. Soils that 

 are acid in this sense always have a large capacity for 

 absorbing lime or other bases, before exhibiting an al- 

 kaline reaction. Calcium being, as has already been 

 seen (par. 264), the base most liberally released to solution, 

 there is a tendency toward the formation of calcium 

 carbonate in any soil dependent on the equilibrium be- 



1 Plummer, J. K. The Isolation of Dihydroxystearic Acid 

 from Volusia Silt Loam. Thesis presented in partial fulfillment 

 of the requirements for the degree of Master of Science. Cornell 

 University Library (not published). 1911. 



