134 AERATION AND AIR-CONTENT. 



leaching of basic elements in drainage-water, or the removal of bases 

 by crops, by the decay of carbonaceous and nitrogenous substances, 

 and by the hydrolysis of mineral compounds and organic matter. 



Hoagland and Sharp (1918 : 139) define soil acidity as that con- 

 dition of the soil in which its aqueous solution contains H-ions in 

 excess of OH-ions. The H-ion concentration of suspensions of acid 

 soils is not markedly affected by increasing the content of carbon 

 dioxid up to 10 per cent, but it is slightly increased in alkaline soils, 

 and a notable increase occurs in soils containing alkali carbonates. 

 No treatment with carbon dioxid produced an alkaline reaction in 

 the suspension of an acid soil. When the original conditions were 

 restored, no permanent change in the soil reaction could be ascribed 

 to the carbon dioxid. 



Sharp and Hoagland (1919 : 197) have found large inversion of 

 sugar only in soils of distinctly acid reaction, the greatest inversion 

 coinciding with the highest H-ion concentration of the soil suspen- 

 sion as well as of the water and the sugar extracts. Direct evidence 

 was also obtained that acid soils do give acid filtrates, the acid re- 

 actions of which were generally of a magnitude very similar to those 

 obtained with the suspensions. 



Noyes and Yoder (1918 : 151) state that organic matter is acid in 

 reaction at certain stages of decay and that apparently no work 

 absolutely proves that this acidity is other than carbonic acid weakly 

 held by the organic matter. They find that acid soil increases in 

 acidity by standing in the greenhouse with one-half its water-holding 

 capacity satisfied. Cropping brought about a slight increase in 

 acidity, while applications of carbon dioxid increased it further, 

 constant treatment giving the greatest amount. Thus, carbon 

 dioxid added to cropped soil increased its acidity, whether it was 

 treated with lime alone or lime and fertilizer. The results support 

 the chemical theories of soil acidity, since different applications of 

 carbon dioxid gas, which is not only soluble in water but also com- 

 bines with it, yielding hydrogen-ions, caused differences in acidity. 



Plummer (1918 : 30) has found that ammonium sulphate materially 

 increases the aci dity of the soil as measured by the hydrogen-ion con- 

 centration, while the effect of potassium sulphate is somewhat less. 

 Sodium nitrate reduced acidity slightly, acid phosphate seemed 

 without effect, and lime materially increased the OH-ion concentra- 

 tion of field plots. 



Rice and Osugi (1918 : 354) state that "soil acidity" is the term 

 customarily applied when infertility of the soil can be corrected by 

 the use of a free base such as lime. There are many factors involved 

 in causing this condition in soils, the presence of real acid being but 

 one of them. The methods used for determining "soil acidity" 

 generally do not measure the acid, but depend upon properties of 

 the soil-mass unrelated to acidity. The power of a soil to catalyze 



