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most efficiently carried out in the presence of only a moderate 

 amount of moisture, under the influence of a more or less rapid 

 circulation of air and in the presence of calcium carbonate to 

 neutralize any acids which may be formed. Under these conditions 

 the vegetable substance is converted into black, neutral, insoluble 

 humus compounds. 



He believed that the nitrogen of plant debris which has become 

 an integral part of the soil must first be converted by humifying 

 bacteria and fungi into humus before the nitrogen can become avail- 

 able to the nitrifying bacteria and thus rendered available for the 

 use of the higher plants. His views of the persistence of plant ma- 

 terials in soils are contained in the following statement: 



As a matter of course, the several organic compounds contained in plants 

 may continue to exist in soils for some time, varying according to conditions 

 of temperature and moisture. Thus dextrin, glucose, and even lecithin and 

 nuclein have been reported to be found. The activity of the numerous fungus 

 and bacterial ferments under favoring conditions, will of course, limit the 

 continued existence of such compounds somewhat narrowly so that they can 

 hardly be considered as active soil ingredients save in so far as they favor 

 the development of bacterial flora. 



Suzuki (1906-08 a) made a study of the formation of humus by 

 treating oak leaves with a humus soil and various inorganic com- 

 pounds and concluded that not only calcium carbonate but also 

 magnesium carbonate promoted the decomposition of moist oak 

 leaves by fungi, judging from the amounts of carbon dioxide 

 evolved. He further states that the opinion of Hilgard corresponds 

 closely to the natural conditions of humification. Further studies 

 of Suzuki (1906-08 b) indicate that protein, starch, and pentosans 

 contribute to the formation of the black matter of humus, but 

 neither fat nor cellulose, and that protection from air is essential. 



One of the latest additions to the idea of specific humificatio.il 

 of plant materials in the soil is that of. Trusov (1915). He inoculat- 

 ed various types of organic compounds with soil bacteria for vari- 

 ous lengths of time and concludes that humus has its origin in 

 lignin, albumen, starch, chlorophyll, and tannic substances; while 

 cellulose, hemicelluloses, mono- and disaccharides, glucosides, or- 

 ganic acids including amino acids, and wax forming substances 

 do not appear to have any part in its formation. He also finds that 

 the organic nitrogenous compounds used as nutrients for the micro- 

 organisms may serve as an indirect source of humus. 



Weir (1915) has recently questioned the idea that the soluble 

 humus of the soil is an indication of the fertility of that soil and 

 that the humus nitrogen plays an important role in the nutrition 

 of plants. He removed 40 per cent of the nitrogen of the soil by 

 extracting the humus with sodium hydroxide and then used the 

 extracted soil for pot experiments. However, Gortner (1916 b) 

 has shown that in all probability a very considerable portion of the 

 humus nitrogen still remained in Weir's extracted soil, for he was 

 able to extract 90.3 per cent of the original nitrogen content of 

 the soil. This would indicate that nearly all of the soil nitrogen 

 could be extracted with sodium hydroxide. 



Snyder (1897) prepared artificial humus by mixing a subsoil 

 with certain organic substances and allowing these to remain in a 

 moist condition for one year. At the end of the year humus was 



