14 OKGANIC CONSTITUENTS OF SOILS. 



these units for building up the complex proteins as far as it is possible 

 to do so. Nitrate is usually considered as the best form of nitrogen 

 for plant food. In order to use nitrate, a highly oxidized form of 

 nitrogen, to form the amido and imido groups of the protein molecules, 

 a reduction must take place. . 



It is obvious that the plant must expend considerable energy in 

 making this transformation. What is more reasonable than to 

 suppose that the unit parts of the complex protein molecules, when 

 presented to the plant, will be used by it in preference to preparing 

 these units from the nitrate ? If a soil be liberally supplied with all 

 of these units it is conceivable that good plant growth will result, 

 even without nitrate. If only a limited amount or kind of the units 

 be present the plant must have nitrates from which to supply the 

 missing units. The knowledge, therefore, that such compounds 

 exist in soils and play such a prominent part in the metabolism of 

 the growing plant is of fundamental significance in the study of soil 

 fertility and gives a breadth of view to the subject which in its horizon 

 can not be compared with the restricted vision imposed by the 

 purely mineral-requirement theory of Liebig as this is used by his 

 followers. 



Nitrates are not produced in these cultures. Ammonia, if formed, 

 is insignificant in amount, nor does ammonia produce the striking 

 results shown by these organic compounds. There is a limit to the 

 amount of any one of these soil constituents which the plant can 

 profitably use and show increased growth. When a mixture of sev- 

 eral of these units is presented at once the growth is better than if 

 an equivalent or even larger amount of any single one is presented. 



These compounds are all nitrogenous, but it must not be inferred 

 that all nitrogenous compounds are beneficial to plant growth. 

 Tyrosine, also a degradation product of protein, is distinctly harm- 

 ful, and picoline carboxylic acid, isolated from soils, is moderately 

 toxic. Guanidine, a compound not yet isolated from soils, but 

 whose presence is indicated, has also been rather thoroughly studied 

 and is decidedly harmful to plants, producing an effect on the cul- 

 tures similar to that shown by certain plant diseases. The leaves 

 become spotted with bleached dots, which spread and ultimately 

 coalesce, producing a wilting of the plant and finally death. Not 

 only is the guanidine harmful in contradistinction to the beneficial 

 nitrogenous substances just discussed, but it differs from these com- 

 pounds in its behavior when nitrates are present. The latter 

 fertilizer ingredient very greatly increases the toxic action of this 

 compound. This is especially striking when a large series of cultures 

 with and without guanidine in the presence of many ratios of phos- 

 phate, potash, and nitrate is set up and the cultures arranged accord- 

 ing to the nitrate content. Both groups of cultures with and without 



