EXPERIMENT STATION BULLETINS. 4lt 



It may be observed from Table 3 that, according to these methods, 

 there are no direct relationships shown between soil class, active bases, 

 excess acids and pH. The term lime requirement refers in this case to 

 the quantity of calcium as Ca(OH), required to neutralize 2,000,000 

 lbs. of soils and expressed as lbs CaCOs. 



Reference is here given to the work of Swanson, Latshaw and Tague 

 (10) who state that the quantity of Ca(0H)2 necessary to raise the 

 pH of soils to a definite point does not depend on the pH of the un- 

 treated soils, and that the quantity of soil calcium soluble in dilute 

 acids, or the total soil calcium, is not related to the pH of Natural soils. 



A direct relationship, however, was found to exist between the 

 base-acid ratio and pH, below the neutral point. A logical conclusion 

 to be drawn from these results is that the pH of soils, or the concentra- 

 tion of hydrogen ions, is controlled by the proportion of active soil 

 bases to the active soil acids. This is not a direct proportion between 

 the pH and base-acid ratio as is shown by the curve in Fig. 11, where 

 pH is plotted against base-acid ratios; but the concentration of hydro- 

 gen ions , decreases more rapidly above pH 4.00 as the quantity of 

 active bases becomes greater in proportion to the quantity of active 

 acids. This curve is typical of hydrogen ion titration curves obtained 

 by titrating weak acids with alkalis and strongly supports the conclu- 

 sion previously given that Ca(0H)2 and other alkalis may form salts 

 with soil acids. 



The first point on the curve in Fig. 11 was determined in the fol- 

 lowing manner: 20 gm. portions of the four soils were treated with an 

 excess of a 0.2 N acid solution of AloCl^, shaken for three hours, washed 

 until washings were free from chlorine and the pH determined in the 

 regular manner. The results found were: Medium sand pH 4.00, 

 sandy loam pH 4.06, silt loam pH 3.92, and clay loam pH 3.96. Since 

 the pH was nearly the same on the four soils after their active bases 

 had been leached from them, it was assumed that the average of these 

 figures, pH 4.00, was close to the pH of the soil acids, and this point 

 was plotted on the curve in order to compare with the other points 

 given. 



It may also be seen from Table 3 that when either the quantity of 

 active bases or the quantity of excess acids is known the other may be 

 computed by using the base-acid ratio and pH factors. Points on the 

 curve in Fig. 11 are determined on a series of different soil classes 

 taken from various soil formations and when these differences are con- 

 sidered the correlation between base-acid ratio and pH seems remark- 

 able, unless these soil acids, or the major portion of them, have the 

 same chemical composition. Provided soil acids of soils formed under 

 different climatic and geologic conditions are not the same chemical 

 compounds, variations from base-acid ratios and pH from those given 

 here, are to be expected. 



14 



