CHEMICAL CONSTITUENTS OF THE SOIL l6l 



There is thus not sufficient experimental evidence to support 

 the theory that low yields are ordinarily due to toxic substances in 

 the soil, rather than to deficiency of plant food. The injurious 

 effects observed in the water culture experiments may be due to 

 acidity. It is, of course, possible that some soils may contain 

 organic toxic substances other than acids, but this fact has not 

 been established. 



Assimilation of Organic Compounds by Plants. Experiments 

 have been made to ascertain whether organic compounds may be 

 assimilated by plants. The organic compounds most important 

 are those which may enter in the soil in animal excrements 

 or plant or animal residues, or may be formed in the decay of 

 these. Uric acid and urea are found in urine, hippuric acid 

 occurs in the urine of cows, sheep, etc., leucin, tyrosin and 

 asparagin, are found in plants or formed in their decay. Other 

 organic bodies formed in the decay of plants or animals, are prob- 

 ably of little importance. 



The question as to the assimilation of these substances as such 

 is complicated by the fact that they are for the most part 'easily 

 transformed into ammonium salts, which may be assimilated. 



Baeyer found oats to grow well in a solution of urea, and the 

 plants contained considerable amounts of urea. But ammonia 

 had formed in the solution. Hampe 1 grew corn with urea and 

 found the leaves to contain 0.25 to 0.8 1 per cent. urea. The 

 solution was changed every day to avoid error by decomposition, 

 and, though the corn possibly assimilated some ammonia, the 

 evidence is that it utilized urea also. A. Thomson 2 compared 

 sodium urate, sodium hippurate, urea, and sodium nitrate, on oats 

 and barley in water cultures, and found that uric acid and urea 

 have the same value as nitric acid, but hippuric acid has not. This 

 investigator did not correct for decomposition of the compounds. 

 Other experiments 3 could be quoted tending to prove that leucin, 



1 Landw. Versuchs-stat. , 1867, p. 79. 



2 Exp. Station Record 13, p. 919. 



3 Hutchinson and Miller, Jour. Agr. Sci., 1912, p. 283, references being 

 given. 



