210 BIOLOGICAL CHEMISTRY 



its concentration exercises a predominant influence on the reac - 

 tion, similar deficiency of one constituent in the soil causes the 

 amount of this substance to exert a predominant influence on 

 the rate of growth of the plant. This is expressed as the 

 Law of the Minimum, namely, that no matter how great an 

 excess of food material there is, if one substance is relatively 

 deficient the rate of growth is small and is regulated by the 

 amount of deficient substance in the soil.* 



The supply of nitrogenous substances is not so simple as that 

 of the inorganic salts such as phosphates, potassium, calcium, 

 etc. Nitrogenous substances can be supplied in the form of 

 animal or vegetable waste matters. Proteins and amino 

 acids are decomposed, giving rise to ammonia. Urea is rapidly 

 hydrolysed to ammonium carbonate by urease, which is formed 

 by micrococcus ureae (cf. p. 182). The main supply of nitro- 

 genous material is therefore in the form of ammonia. Am- 

 monia, however, is not used directly by plants but it must be 

 oxidised to nitric acid. This oxidation is accomplished in two 

 stages, namely, the formation of nitrous acid from ammonia 

 and the oxidation of nitrous acid to nitric acid. 



The first stage is accomplished by nitrosomas and the second 

 stage by nitrobacter. The energy liberated by the reaction 

 is made use of by these organisms. A supply of oxygen is 

 necessary, and if this is not forthcoming nitric acid is not 

 formed. Under anaerobic conditions or by the overgrowth of 

 organisms that prevent the growth of the normal nitrifying 

 organisms some of the ammonia is decomposed with the 

 liberation of atmospheric nitrogen, which causes a distinct 

 loss to the fertility of the soil. 



There are, on the other hand, organisms which "fix" 

 atmospheric nitrogen and furnish a supply for plants. These 

 organisms were first v shown to be associated with certain 

 nodules on the roots of leguminosse ; hence the fertilising 

 value of growing such crops as clover, f Later still organisms 

 independent of a symbiotic relationship have been found to 

 oxidise atmospheric nitrogen. 



The organisms which cause loss of nitrogen in the form of 

 gas are provisionally believed to be protozoa. These organ- 

 isms are more easily destroyed than bacteria, hence the value 

 of partial sterilisation of soil consists in killing the protozoa 

 without killing the bacteria. The bacteria afterwards com- 



* J. Liebig, Chemistry in its Application to Agriculture and Physi- 

 ology, edited by L. Playfair, 3rd ed., London, 1843. 



t H. Hellriegel and H. Wilfarth, Zeit. des Vereins f. d. Rilben- 

 ziicher Indusdrie, 1888, cit. nach Russell. 



