TOXIC EXUDATES AND SOIL TOXINS. 161 



of the crop will be determined by the concentration of the soil solution in 

 phosphoric acid and potash, which in its turn is determined by the amount 

 of these substances in the soil, their state of combination, and the fertilizer 

 appUed. The net result of these investigations is to restore the carUer theory 

 of the direct nutrition of the plant by fertiUzers." 



In conformity with the view that roots do not excrete toxic sub- 

 stances, it must be recognized that the value of crop rotation does not 

 depend upon getting rid of the toxic exudates of a particular crop. 

 Where such toxins as dihydroxystearic acid are present as the result 

 of the partial decomposition of organic matter, the effect w^ould be 

 produced by all crops that leave residues in the soil. As a conse- 

 quence, fallowing, tillage, or fertilizing, alone or in combination, 

 would suffice to get rid of the toxin, regardless of the crop sequence. 

 The fact that the absence of rotation for 60 years does not result 

 in the appearance of specific toxins in normal cultivated soil has been 

 proved by Hall and his associates (1913, 1919), in connection wdth 

 the growth of continuous crops of wheat and barley on the Rothamsted 

 plots. 



Russell has also shown that the growth of 6 successive crops was 

 insufficient to cause appreciable reduction in the yield of the next 

 crop, and concludes that this rules out toxins as an explanation of the 

 advantages of rotation, when there is a lengthy interval between 

 crops. As King and Hall maintain, rotation still appears to rest 

 upon the different nutrient and tillage relations of the succes- 

 sive crops, though in many soils the relation of the various crops to 

 acidity may be an important factor, as indicated by Hartwell and 

 his associates. 



The early 'assumption that root secretions were a factor in plant 

 communities and in succession is no longer vahd, but apparently this 

 is still thought of as a possibiHty in connection with soil toxins. In 

 all successions, except for the early stages of the hydrosere, the 

 amount of organic matter in the soil steadily increases, but the ab- 

 sence of any toxic effect is demonstrated by the fact that the number 

 of individuals and often the number of species also increases to 

 the subclimax or climax stage. Since a plant community regularly 

 returns its material to the soil, the question of nutrients enters only 

 in cases of intense competition, though their increasing availability 

 is a factor in succession. Field studies of germination and growth 

 and of community development, as well as competition cultures 

 under control, have show^n that w^ater-content, air-content, nutrient- 

 content, and temperature are normally the primary factors, more or 

 less modified by competition. Innumerable seedlings have been 

 found to grow as well in parent communities as in those of other 

 species, and consocies of annuals have been known to maintain them- 

 selves for 10 to 20 years, and to yield only when invasion became 

 overwhelming. 



