140 AERATION AND AIR-CONTENT. 



sorrel, owing to the fact that it brought about conditions favorable 

 to clover and other plants, which were then successful in the com- 

 petition. 



White (1915), in dealing with soil rendered very acid by am- 

 monium sulphate, showed that sorrel was largely replaced by clover 

 where limestone was present in slight excess, but that it also gave 

 the highest yield with the maximum amount of hmestone. The 

 calcium-content of sorrel in acid soil was but 10 per cent of that in 

 alkaline soil. The conclusion was reached that sorrel is not an 

 acid-loving plant, but that it is able to adapt itself to conditions 

 unfavorable to most field crops. It invaded the acid plat not 

 because of preference for an acid soil, but because of the opportunity 

 for establishment afforded by the failure of the clover. 



Pipal (1916) has found that hme exerts no harmful effect upon the 

 growth of sorrel. Wherever acidity or other conditions unfavorable 

 to crop plants occur, sorrel takes possession at the expense of the 

 crop, but if the unfavorable factor is corrected by means of limestone, 

 manure, drainage, etc., crop plants are enabled to compete success- 

 fully with sorrel. Frear (1915 : 109) has discussed the results of 

 White and others with respect to sorrel and various crop plants. 



Wherry (1920) has studied the distribution of plants around salt 

 marshes with respect to soil acidity, and concludes that the latter 

 is a factor of considerable if not fundamental importance. It is not 

 regarded as the only factor of importance, nor is it implied that the 

 acid or alkali acts directly upon the plant. It is thought that some 

 plants may require a soil of definite acidity or alkahnity for them- 

 selves or for symbiotic organisms, while others may be favorably 

 affected by some property of the soil that accompanies acidity, and 

 still others may find competition less severe in soils of a certain 

 degree of acidity. The presence of species in soils with an acidity 

 of 300 is assumed to be a matter of preference and not merely of 

 tolerance, since the majority of them are not known to grow in 

 much lower degrees of acidity. However, it is interesting and per- 

 haps significant that at least one-third of the species listed often 

 grow in soil but weakly acid or even somewhat alkaline. 



Summary. — The causes of soil acidity are still a subject of earnest 

 debate, and much more investigation is needed before a final decision 

 is possible. However, there is a strong tendency to recognize multi- 

 ple causes, as has been done by Blair and Macy, Frear, and Wherry 

 especially, though there is great difference of opinion as to which of 

 these is paramount. Of all the authors considered, Frear is the only 

 one that regards humus as a factor in acidity. The view that acidity 

 is due to adsorption is maintained by Cameron, Harris, and Bogue, 

 and this process is regarded as one of the factors by Frear and by 

 Wherry. Veitch, Daikuhara, Conner, and Rice consider the re- 



