SOIL ACIDITY 353 



veloped may have a nutritive relationship as well as a toxic 

 effect. 



When fertilizer salts are added to the soil, the basic ions 

 are usually absorbed to a greater degree than the acid radi- 

 cals. This tends to develop actual acidity in the soil solution, 

 which may in itself be toxic or may facilitate the development 

 of detrimental ions. If the crop utilizes the basic ion of the 

 fertilizer added to a greater extent than the acid radical, it 

 will aid in the development of acidity. If the plant, on the 

 other hand, absorbs the acid radical, it will tend to counter- 

 act the selective absorption by the soil. The combined influ- 

 ences of soil and crop on ammonium sulfate tend to develop 

 acidity, while the effect on sodium nitrate is toward alkalinity. 

 A salt such as potassium nitrate should leave no residue. 



The decomposition of organic matter, especially when green- 

 manures are plowed under, is often considered as increasing 

 the acidity of the soil. Such may be the case at the beginning 

 of the decomposition process, but the data 1 available on the 

 subject seem to indicate that organic matter, if it exerts any 

 influence on acidity, tends to reduce rather than accentuate 

 it. This result may occur through the liberation of bases 

 from the organic matter as decomposition proceeds. 



189. Relative tolerance of acidity by plants. — Since so 

 many intermediate influences are possible in acid soils, and 

 since plants respond so differently to these influences, it is im- 

 possible to forecast the relative resistance of different crops 

 on the same soil. The response of the same crop on differeu x 

 acid soils is likewise difficult to foretell. 



It is known that certain crops are often more tolerant to 

 soil acidity than others. Of the common weeds sheep sorrel, 



1 White, J. W., Soil Acidity as Influenced by Green Manures; Jour. 

 Agr. Res., Vol. XIII, No. 3, pp. 171-197, 1918. 



Stephenson, E. E., The Effect of Organic Matter on Soil Beaction; 

 Soil Sci., Vol. VI, No. 6, pp. 413-439, 1918. 



Howard, L. P., The Beaction of the Soil as Influenced by the De- 

 composition of Green Manures; Soil Sci., Vol. IX, No. 1, pp. 27-38, 

 1920. 



