326 Journal of Agricultural Research voi. xv. no.6 



The soluble aluminium decreased when the fully saturated soils con- 

 taining much organic matter were dried. With soils B and D, one- 

 fourth and one-half saturated with water, the soluble aluminium in- 

 creased on air-drying. This is in accord with the acidity, which likewise 

 increased when these soils were dried. Different investigators have 

 endeavored to correlate the amounts of soluble aluminium and iron 

 with the degree of acidity as obtained with the potassium-nitrate method. 

 The results given in Tables II and III show a certain correlation along 

 this line, but it is very apparent that the titrated acidity can not be 

 entirely explained on the basis of the amount of potassium-nitrate solu- 

 ble aluminium and iron. This acidity is apparently partly due to solu- 

 ble acid organic compounds in addition to the iron and aluminium 

 compounds. 



The amount of calcium in solution varied to a large degree in inverse 

 relation to the aluminium and iron. In all the soils, except the peat 

 (E), the solutions from the air-dried soils contained more calcium than 

 did those from the undried soils. Magnesium and soluble silica showed 

 no striking variations due to the varied moisture conditions. 



The changes shown in the degrees of acidity and also the differences in 

 the amounts of soluble bases occurring when the soil samples are air-dried 

 indicate the importance of further study of soils and soil reactions on 

 samples which are kept under field moisture conditions. Some of the 

 reactions which occur when soils are dried are apparently very rapid 

 and so slowly reversible that the composition of dried soils may be quite 

 different from that of field soils. 



MOISTURE REACTIONS OF ACID SOILS 



It has been noted by different investigators that carbonated water 

 will extract from a mineral soil a solution that on boiling to drive off 

 the carbon dioxid will be alkaline to phenolphalein. This fact can 

 hardly be taken as proof that the soil moisture is not acid or that the 

 soil acidity has been regulated by the formation of carbonates. Such 

 an extraction of bases by an acid is, of course, to be expected from the 

 laws of chemistry, but it does not tell in what state of equilibrium the 

 soil bases may have been before they were extracted. Recent re- 

 searches would indicate that the soil moisture of acid soils is distinctly 

 acid and not basic in reaction. Gillespie (j), working with the hydrogen 

 electrode, has found that solutions of acid soils are distinctly acid in 

 reaction. Sharp and Hoagland (6) likewise found that there is an excess 

 of hydrogen ions in solutions of acid soils. In addition they say : 



Soils containing calcium in equilibrium with HCO:^ and CO2 have a very slightly- 

 alkaline reaction 



and 



The figure for Ca (HC03)2 is almost identical with those obtained for the alkaline 

 soils. 



