THE QUESTION OF ROOT EXCRETIONS 263 



the middle of the boxes kept the roots of the weeds separate from those 

 of the corn. When the partition was present, each sort of plant was 

 confined to half the soil in the box, but, where lacking, one sort of 

 roots appeared to exercise a noxious influence upon the other. 



In studying the general question of soil fertility, the laboratories 

 of the Bureau of Soils in the United States Department of Agriculture 

 have obtained some instructive evidence bearing upon this problem of 

 plant excretions. The data obtained from different lines of experi- 

 mentation all go to prove the truth of the general assumption that 

 plants do excrete substances which may have a toxic action on the 

 species of plants producing them. 



The Bureau of Soils has recently published the outcome of some 

 experiments in which several sets of wheat plants were grown in rapid 

 succession on the same soil. The soil was kept in pots and each set 

 of wheat plants was smaller than the one immediately preceding, as 

 might have been predicted, since it is well known that the continuous 

 culture of a given crop on a soil " exhausts " it more rapidly than the 

 culture of diversified crops. In the experiments cited, the growth of 

 the fourth set of plants was in some cases only 30 per cent, of the 

 growth of the first set. 



Such cases of apparent exhaustion have generally been assumed to 

 be due to the removal of plant nutrients from the soil by previous 

 crops. Accordingly, experiments were made in which mineral plant 

 nutrients in the form of pure chemicals were added to the soil at the 

 time of planting the successive crops in amounts equal or greater than 

 those removed by the preceding crop. The addition of these supplies 

 of plant food constituents failed, however, to produce a growth of 

 plants equal to the first crop, although they helped the growth of the 

 plants. This possibility of soil exhaustion is still further rendered 

 improbable by the fact that the wheat plants grew only three weeks 

 and during that time they derived much of their food supply from the 

 reserve nutrients stored in the seed ; hence the amounts taken from the 

 soil were very small. 



To emphasize the action of root excretions upon the soil, experi- 

 ments were made in which the first set of wheat plants was allowed to 

 grow for only five days, or until the plumules were just beginning to 

 appear above the surface of the soil in the pots, a length of time obvi- 

 ously insufficient to " exhaust " the soil. All the plants were carefully 

 removed and the soil again planted with wheat, simultaneously with 

 the same number of pots of fresh soil. The plants following the five- 

 day crop made only 80 per cent, of the growth of the plants in fresh 

 soil. 



The outcome of these experiments is quite strong proof that the 

 cause of these decreased yields is not the depletion of mineral nutri- 



