ABSORPTION OF ASH-CONTITUENTS 99 



If a particular kind of plant is grown for several years in succession upon the 

 same soil, the crop gradually decreases, in spite of the addition of plenty of 

 fertilizers. This is the well-known phenomenon of "soil sickness." In this 

 case we do not have to deal with an inadequate supply of mineral nutrients, 

 but with something entirely different. The work of Whitney and Cameron, 

 and that of Livingston, Schreiner, and other American investigators, 1 has indi- 

 cated that plants produce poisonous substances (toxins) in the soil. 7 ' These 

 toxins appear, in many cases, to be poisonous only to the particular kind of plant 

 in connection with which they were produced, and this may explain the fact that 



» Whitney, Milton, and Cameron, F. K., Investigations in soil fertility. U. S., Dept. Agric. Bur. Soils- 

 Bull. 23. 48 p. Washington, 1904. Livingston, B. E., Britten J. C, and Reid, F. R., Studies on the prop- 

 erties of an unproductive soil. Ibid. Bull. 28. 39 p. Washington, 1905. Livingston, 1907. [See 

 note 6, p. 83.] Schreiner, Oswald, Reed, Howard S., and Skinner, J. J., Certain organic constituents of 

 soils in relation to soil fertility. Ibid. Bull. 47. 52 p. Washington, 1907. Schreiner, Oswald, and 

 Shorey, Edmond C, The isolation of picoline carboxylic acid from soils and its relation to soil fertility. 

 Jour. Amor. Chem. Soc. 30: 1295-1307. 1908. Idem, The isolation of dihydroxy-stearic acid from soils. 

 Ibid. 30: 1590-1607. 1908. Idem, The isolation of harmful organic substances from soils. U. S. Dept. 

 Agric, Bur. Soils. Bull. 53. 53 p. Washington, 1909. 



' A discussion of some of the earlier literature regarding this general idea of soil toxins is 

 given by Livingston, 1907. [See note b, p. 83.] This earlier literature (not considered by 

 Whitney and Cameron, 1904, nor by Livingston ei al., 1905 [note 1, just above]) is rather 

 extensive. The idea that plants may excrete into the soil substances that may be poisonous 

 to other plants, appears to have originated with A. P. DeCandolle (Physiologie vegetale. 

 Paris, 1832), but the experimentation invoked by this writer's suggestion seemed to dis- 

 prove the hypothesis, and the whole matter was laid aside until it was taken up again, in 

 a modern way, by the Duke of Bedford and S. U. Pickering (at the Woburn Experimental 

 Fruit Farm, near Bedford, England) and by the American students mentioned above. On the 

 Woburn work see : Pickering, Spencer U., The effect of grass on apple trees. Jour. Roy. Agric. 

 Soc. England 64 (of entire series): 365-376. London, 1903. Also see the Reports of the 

 Woburn Experimental Fruit Farm after 1897. 



In later years the general hypothesis that unproductiveness in agricultural soils is frequently 

 due to soil toxins has been well established by workers in various parts of the world, and it is 

 now generally accepted. Evidence that agricultural plants do actually excrete toxic substances 

 into the soil is not very strong in any of this work, however. Better than to assert that they 

 are so excreted is to state that there is evidence that the soil frequently contains toxins and 

 that these sometimes result, directly or indirectly, from the growth of higher plants. As to the 

 manner in which these poison substances arise in the soil, no definite statements can yet be 

 made, but they are surely not generally excreted as such from plant roots.. There is physio- 

 logical evidence, however, that such substances are given off by living roots when the latter are 

 practically deprived of oxygen. (See p. 1 26.) It seems highly probable that soil microorgan- 

 isms play an important part in the production of the toxic substances here considered. Ex- 

 creted substances, the materials of dead root-cap cells, root-hairs, roots, etc., or even substances 

 carried down into the soil by rain (as from the bark of trees and fallen leaves) may become 

 altered by the action of microorganisms so as to produce poisons. That such poisons are 

 present in many soils has now been established without question by Schreiner and his co- 

 workers, and also that their deleterious effect upon plants may often be removed by oxidation, 

 or by the addition of proper substances. 



The general acceptance of the hypothesis of toxic soil constituents as a frequent cause of 

 unproductiveness was much retarded by the form of its original statement, by Whitney and 

 Cameron (1904), which emphasized actual root excretion at the expense of all the other logical 

 possibilities. It was of course to be expected that such poisons might arise in the soil in a 

 great variety of ways, and the theory of soil toxins is not to be considered without continual 

 reference to the microbiology of the soil. Russell (1921. [see note i, p. 73.]) gives a clear dis- 

 cussion of this whole matter, from the standpoint of field experiments. — Ed. 



