The evidence offered in favor of the theory of soil toxicity is not sufficient to establish the fact that roots of higher plants excrete substances harmful to themselves or to other plants. Neither is the evidence sufficient to establish the presence in the soil of organic substances harmful to plants under normal field conditions. The concentrations of 600 parts per million of cumarin and of 3,000 parts per million of vanillin, figured on the basis of the total moisture content of the soil, depressed to some extent the yield of wheat grown to maturity in pots. There are indications, however, that the effect was rather on the soil than on the plant. The addition of small quantities of soil to water cultures entirely destroyed the toxic effects of cumarin, while it did not affect the action of vanillin. It is possible that vanillin is less readily decomposed by the microorganisms of the soil than cumarin, or that the decomposition products of vanillin are as toxic in water cultures as vanillin itself. In quartz cultures, cumarin has proved to be as toxic as in water cultures, while vanillin behaved approximately the same way as in the soil. Vanillin is evidently toxic only in a liquid medium when it is applied in mass, but not when it is distributed as films over quartz grains or soil particles. The ameliorating effect of phosphoric acid on the action of cumarin reported in Bulletin 77 of the Bureau of Soils would not seem to be due to its antagonistic behavior with reference to that toxin, since it did not behave in the same way in a balanced solution. The ameliorating effects reported might be due either to the residual effects of the base after the phosphate radical was used up, or to the fact that cumarin does not interfere with the absorption by the plant of phosphoric acid while it does interfere with the absorption of the other food elements. The behavior of toxic substances is so different in the soil than in water cultures, that one is hardly justified in drawing conclusions from results obtained with water cultures as to what might take place under actual field conditions.