128 



The Plant World 



poor soils. Other investigators in various parts of the world 

 have already taken up this question, and corroborative evidence 

 is rapidly accumulating, so that, although the exact nature of 

 the toxic bodies remains still to be defined, yet the fact of their 

 causation by growing roots, whether directly or indirectly, can 

 not at present be doubted. 



Along with the development of these ideas, and supported 

 by other entirely different lines of evidence, has grown up the 

 at first somewhat startling hypothesis, that the good or poor 

 qualities of agricultural soils are not usually dependent upon 

 the salt contents of these soils, and are usually to be related to 

 physical conditions or to the absence or presence of toxic ma- 

 terial. A corollary to this hypothesis is, logically, that the 

 almost universally accepted Liebig theory of fertilizer practice 

 is in most cases untenable. According to this theory, if calcium 

 nitrate proves beneficial upon a certain soil, that soil must have 

 been deficient in either calcium or nitrate, or both. It is sup- 

 posed by the usually accepted theory of crop rotation, that one 

 species of plants removes from the soil more of certain elements 

 than another, and conversely, and that thus a more vigorous 

 growth is produced when the two species are alternated than 

 when the same form is grown continuously for a number of 

 seasons upon the same land. 



Now it is incontestable that certain salts are beneficial upon 

 certain soils, but it is equally evident to one conversant with 

 the disagreeing literature of the subject, that the effect of fer- 

 tilizer salts can not be predicted from the chemical analysis 

 of a soil solution obtained with one or another solvent. 



The need see apparent, then, for a new hypothesis, • 

 which will explain, or aid in the investigation of, the properties 

 of unproductive soils, on the one hand, and the effects of fer- 

 tilizer salts, on the other. Vaguely suggested in previous bul- 

 letins, such an hypothesis is partially formulated in Bulletin No. 

 56 of the Bureau of Soils, which has recently appeared. 



Various lines of previous research, by the Bureau of Soils 

 and others, have pointed to the conclusion that toxic organic 

 substances are often rendered less toxic to plants by oxidation. 

 Of course any poisonous body will be rendered harmless by 

 complete oxidation into carbon dioxid and water. The natural 



