SOIL ACIDITY WHERRY. 251 



Table 5. — Methods of measuring acidity present and producible in soils. 



1. A salt solution is added to the soil; for this purpose there have been used 



sodium chloride, potassium chloride and nitrate, calcium chloride, nitrate 

 and acetate, zinc sulphide plus calcium chloride, etc. The quantity of 

 acid in the resulting solution, -which represents that originally present in 

 the soil plus a much greater amount produced indirectly by the processes 

 outlined on a previous page, is then determined by titration or other 

 means. 



2. No salt solution but some pure water is added to the soil. 



A. The mixture is titrated with lime water, using either an indicator or 



observation of the freezing point to determine the end point. This 

 gives the amount of lime needed to neutralize the acid originally 

 present in the soil plus that produced indirectly by the action of 

 lime (which is likely to differ from that produced by a neutral salt 

 solution), as well as the amount of lime required to satisfy the 

 adsorptive power of the soil colloids for calcium-ion under the given 

 conditions. 



B. The mixture is filtered and the filtrate titrated with standard alkali. 



This gives the quantity of acid present in the soil. 



C. The hydrogen-ion concentration or specific acidity is determined : 



(a) By catalysis of an ester. 



(6) By measurement of the potential clue to hydrogen-ion with the 



potentiometer. 

 (c) By observation of color changes of indicators whose relations 



to hydrogen-ion concentration are known. 



The methods listed under 1 in Table 5 are not methods of de- 

 termining the acidity originally present in a soil. The results they 

 yield are composite, representing both acid originally present and a 

 usually greater quantity produced by the treatment. That the re- 

 sults obtained would differ widely with the salt used and with the 

 conditions of the experiment would naturally be expected, and has 

 been demonstrated by actual trial. Even when soils are neutral or 

 alkaline at the start they may develop a considerable acidity when 

 treated with a salt solution; and any method which indicates an 

 alkaline soil to be acid is certainly valueless for the determination of 

 the effect of acidity on plant growth. 



By way of analog} 7 , suppose a man has one pocket full of coins, 

 and another full of slugs. In so far as his ability to purchase com- 

 modities is concerned the contents of the former pocket is alone of 

 significance. Even though the slugs can, by appropriate procedure, 

 be converted into coins, they have only potential and no direct pur- 

 chasing power. Determination of the total number of metal objects, 

 or of the total weight of metal, which the man carries, can, of course, 

 be carried out with as high a degree of precision as desired; but 

 what bearing will this data have on his actual wealth ? 



Since the methods in this group yield composite, variable, and con- 

 tradictory results, and furnish no information as to the soil acidity 

 nor as to the lime requirement, it can only be concluded that they 



