PHYSIOGRAPHIC FACTORS 179 



Under ordinary conditions, the hydrogen ions themselves prob- 

 ably have little direct effect upon plants, but degree of acidity of 

 the soil may have a regulatory effect upon chemical processes that 

 do influence growth. Increased acidity may reduce availability of 

 nutrients, as when phosphorous combines with aluminum and iron 

 to form insoluble phosphates. High acidity may, apparently, pro- 

 duce toxic effects, but these are not caused by H ions. It is more 

 likely that they result from soluble compounds of aluminum and 

 iron, which form in increasing amounts as the H ion concentration 

 rises. Since lime is a required nutrient, its characteristically low 

 content in acid soils may be of more importance than the degree 

 of acidity. Numerous soil organisms are sensitive to changes of 

 acidity, and, if their activities are inhibited, decomposition of or- 

 ganic matter may be retarded, nutrients may not be released, and 

 nitrification and nitrogen-fixation may be checked. 



With such a variety of things that may be affected by soil 

 acidity, it should be suspected that a simple relationship between 

 pH and plant responses does not exist. Studies of soil pH and plant 

 distribution bear this out, for, in general, if the environment is 

 favorable and necessary nutrients are available, most species can 

 tolerate a rather wide range of pH. At the same time, many of 

 these species reach their best development or are most abundant 

 within a restricted portion of that range of pH. It should be clear 

 that, even under such conditions, pH alone cannot be the limiting 

 factor. 



Determinations of pH may be made colorimetrically by the use 

 of indicator solutions or electrometrically with a potentiometer 

 and a glass electrode. 206 A very useful approximation may be made 

 with a universal indicator, which, when placed in the soil solution, 

 takes on a color corresponding to a particular pH value. This is 

 handy in the field since it requires no more than a small bottle of 

 indicator and a pocket-size porcelain plate on which permanent 

 color standards are painted. More accurate colorimetric deter- 

 minations require a series of indicators whose colors correspond 

 to overlapping pH ranges. When electrometric equipment is avail- 

 able, it is preferable because of its accuracy. 



Exchangeable Bases.— Ecologists have given relatively little at- 

 tention to the ways in which the mineral nutrients of the soil 



