SOIL REACTION 415 



possibly a precipitation of harmful toxic substances may occur, 

 especially salts of Cu, Zn, and the heavy metals when these are 

 present. Truog, in fact, generalises thus : " Nearly all the 

 chemical reactions which take place in soils are affected unfavour- 

 ably in regard to fertility by an acid condition." Thus, it has 

 been stated, an acid condition will bring about a decreased 

 availability of phosphates due to {a) the formation of the less 

 soluble ferric and aluminium phosphates from the more soluble 

 calcium phosphates, and (b) the formation of complex phosphorus 

 compounds with acidic organic matter which have a very slow 

 rate of decomposition under acid conditions. In general, and 

 from a chemical point of view, the elements present in soils may 

 be divided into two groups according to whether their solubility 

 or availability is increased or diminished by an acid condition 

 of the soil : ( i ) Ca., Mg., Na., and K tend to become less soluble, 

 while (2) Fe, Al, Mn, Cu, and the heavy metals tend to become 

 more soluble. As the metals of the first group become less 

 soluble and available those of the second group become more so 

 because the carbonates or bi-carbonates of the first group act 

 as precipitating agents for the second group, which might 

 otherwise be in solution as other salts. These relationships 

 are important not merely because of the increased availability 

 of the phosphates, but on account also of the toxic action of 

 most trivalent metal ions, e.g. Al and Mn. Moreover, the metals 

 of the former group, together with P2O5, form the great chemical ^ 

 " reaction regulators " of the soil solution, to which reference 

 will be made later. 



Effect of Acidity on the General Soil-Plant Economy 



These considerations bring out more or less clearly how the 

 general fertility of the soil is affected by an acid condition on the 

 one hand and by an excess of lime on the other. There are, 

 however, many other far-reaching effects of soil acidity on the 

 general interrelationships comprised within the soil-plant 

 economy. These have been discussed at some length by 

 Truog,* who classifies the various effects under the following 

 heads ' : 



A. The general and indirect effects of soil acidity on plant 

 growth : 

 I . Effect on the general fertility of the soil ; 



* Chemical, because soil colloids are also very ef&cient reaction regulators. 



3 Soil Sci., 5 (1918), p. 169. 



^ This scheme does not represent the actual known effects of soil reaction 

 on the plant-soil economy, but rather the possible effects that would have 

 to be considered in any really comprehensive study of the subject. 



