168 



PLANT SOCTOLOGV 



0.02 pH unit. Accurate directions for use always accompany these 

 instruments. 



Table 13. — Specific Acidity Equivalents for pH Values 

 (After Wherry) 



It has been shown recently by Pallmann (1930) that the in verso- 

 metric method agrees with the electrometric. 



Treatment of Soil Samples for pH Determinations. — The samples are 

 examined as soon as possible after collection, although Gustafson 

 (1928) has shown that they may be kept for weeks in air-dried condi- 

 tion without effecting any appreciable change in their H ion concentra- 

 tion. To 20 g. of fine soil 50 cc. of distilled water are added, and the 

 mixture left in a stoppered flask for 24 hr., shaking frequently.^ From 

 this solution a required amount is taken after thorough shaking. This 

 precaution is necessary in order to obtain a uniform concentration in 

 the suspension. Pallmann (1930) and others have shown that such 

 suspensions have, in general, a different pH value from the clear solu- 

 tion. Suspensions of acid systems react more acid, while suspensions 

 of alkaline systems are more alkaline than the dispersion medium itself. 



' Peat soils which take up much water may be diluted with more water. 

 Indeed, according to Gustafson, the amount of dilution usually makes little 

 difference to the result. 



