A comparative study of the effect of cumarin and vanillin on wheat grown in soil, sand, and water cultures
1915
Davidson, J.
It is apparent from the foregoing analysis that the evidence which is offered in favor of the theory of soil toxicity is neither direct nor conclusive. The facts on which it is based can be interpreted in a variety of ways other than the existence of toxic substances. The question would be considered definitely settled if toxic substances isolated from a poor soil, when applied in the same quantity in which they are there present, caused a soil which does not contain them to produce a poor crop similar to that produced by the poor soil. Indirect or circumstantial evidence is of less value in problems of soil fertility than in many other problems. So many factors known and unknown affect the soil that several interpretations of the same phenomena are frequently possible. The real significance of these phenomena may often escape us because of our lack of knowledge of the processes taking place in the soil.
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