BOG XEROPHYTES AND ACID SOILS. 131 



num cells. This view has been vigorously assailed by Tacke and 

 Siichting (1911), as well as many other chemists, who maintain that 

 acid soils exhibit phenomena to be explained only by the presence of 

 true acids. Schreiner and Shorey (1909) have discussed the results 

 obtained by workers in this field, and lend their support to the con- 

 clusion of Cameron and Bell (1907) that the existence of humic 

 acids has never been demonstrated and no satisfactory descriptions 

 have ever been given of their physical or chemical properties or of 

 their salts or characteristic derivatives. Their effect upon plants, 

 whether harmful or otherwise, is considered to be as doubtful as 

 their constitution or composition. 



Blair and Macy (1908) concluded that agencies are at work pro- 

 ducing acids in practically all cultivated soils, and these tend to be- 

 come permanently acid unless bases are present in sufficient quantity. 

 The acids may be the result of decomposing organic matter, of bac- 

 terial action, or of the breaking-up of commercial fertilizers. They 

 may exert a directly injurious effect upon the plants themselves, or 

 upon beneficial bacteria, or they may bring into solution mineral 

 compounds that are injurious to the plants or the bacteria. 



Abbott, Conner, and Smalley (1913) have found that peat soils 

 typically dominated by huckleberries are excessively acid and poorly 

 supplied with lime. The water extract contained very large amounts 

 of aluminum salts, highly poisonous to corn seedlings. These facts 

 seem to warrant the conclusion that the unproductivity of such acid 

 marsh soils is due to the toxic action of soluble salts of aluminum, or 

 rather to the acid conditions that permit these salts to exist. 



Ruprecht (1915 : 128) has shown that aluminum sulphate is very 

 toxic to clover seedlings in culture solutions containing more than 40 

 parts per million and that ferrous sulphate is toxic in concentrations 

 greater than 4 parts per million. The toxic effect of both can largely 

 be remedied by calcium carbonate up to a certain point, but not by 

 calcium sulphate. It is assumed that the carbonate precipitates the 

 aluminum and the iron as hydroxids, thus removing them from solu- 

 tion and counteracting their harmful effect, the difference between 

 the two being due to the different solubility of the hydroxids. The 

 toxic action is restricted to the outer layer or two of the apical region 

 of the root, thus retarding or arresting the growth of the latter. 



Truog (1916) has found considerable amounts of manganese in 

 the soil solution of certain acid soils, and thinks that this element 

 may sometimes act injuriously as a result of high absorption. 



Wilcox and Kelley (1912), White (1916), and Johnson (1917) have 

 similarly shown that manganese is toxic under certain conditions. 

 When lime is added to such soils, the amount of manganese in the 

 soil solution is inconsiderable, as the alkaline condition favors its 

 precipitation. 



