TRANSFORMATION OF ORGANIC MATTER 



consider "humus" to be a nitrogen containing substance of an acid type, 

 the acidity due partly to adsorption by the colloidal "humic acid" 

 and partly to the presence of carboxyl groups. The salts of "humic 

 acid" are not considered as true salts formed in stoichiometric propor- 

 tions, but as complicated chemical and adsorption compounds. The 

 nitrogen containing part of the "humic acid" is protein in nature and 

 gives hydrolytic products as proteins do; these proteins are at least 

 partly of microbial origin. The "humic acid" contains benzene ring 

 compounds with hydroxyl groups in the form of side chains. "Humic 

 acid" contains about 62 per cent carbon, 3.2 per cent nitrogen, and 4.2 

 per cent hydrogen, calculated on an ash-free basis. These and more 

 recent 81 investigations point definitely to the lignins as the mother sub- 

 stances of these so-called "humic acids." In addition to the lignins, 

 the cell substance synthesized by microorganisms is another source of 

 the soil "humus." A complex mixture of these accompanied by vari- 

 ous decomposed and partly decomposed constituents of plant and micro- 

 bial origin form the soil organic matter or soil "humus." Some of these 

 constituents are amphoteric in nature and increase the buffer content of 

 the soil. The mycelium of fungi, for example, has an isoelectric point at 

 pH 4.9 to 5.5. 82 



Soil organic matter and the activities of microorganisms. Organic 

 matter influences the growth and activities of microorganisms in the 

 soil by forming a more favorable physical and chemical environment and 

 by offering a source of energy and other nutrients. The different con- 

 stituents of the organic matter and the degree of their decomposition 

 influence to a large extent the nature of the organisms which are capa- 

 ble of developing. 



The favorable influence of small amounts of soil extract upon the 

 activities of various bacteria, like Azotobacter, led to various assump- 

 tions that we are dealing with certain vitamines (auximones). 83 It 

 was also suggested that the favorable influence of organic matter upon 

 the growth of various microorganisms is due to the chemical improve- 

 ment of the medium. Others 84 consider soil "humus" as a reservoir of 

 nutrients required by the soil organisms. 



81 Waksman, 1926 (p. 694). 



82 Robbins, W. J. Isoelectric points for the mycelium of fungi. Jour. Gen. 

 Physiol., 6: 259-271. 1924; Jour. Agr. Res., 31: 385-399. 1925. 



"Bottomley, 1915 (p. 580); Mockeridge, 1915 (p. 580). 



84 Kaserer, H. Einige neue Gesichtspunkte Liber die Rolle des Humus in der 

 Ackererde. Inter. Mitt. Bodenk., 1: 367-375. 1911. 



