138 Humus: Nature and Formation 



especially evident when the soil is kept fallow or free from all plant 

 growth and no plant residues in any form are added to it. 



In the cultivation of soil, the top layer, including both living and 

 dead materials to a depth of 4-8 inches, is turned over. The rapidly 

 decomposing organic residues, which previously were on the surface, 

 are now placed under the surface. Root systems filling the upper 

 layer of soil are thus killed and mixed with the residues already dead 

 and decomposing. The lower layer of soil, which was hitherto pro- 

 tected by a surface layer, is in its turn brought to the surface. 

 Harrowing, hoeing, or other cultivating brings this fresh soil into 

 more or less intimate contact with air, sunlight, and the daily varia- 

 tion of heat, cold, and drought. 



The whole mass of soil from the surface to the lower layer of the 

 furrow-slice becomes an aerobic environment in which microorgan- 

 isms find favorable conditions for development. This results in great 

 changes in the soil flora and fauna and in the soil organic matter. 

 Smith and Humfeld, in their studies of decomposition of green ma- 

 nure, showed that great activities were localized wherever plant 

 remains were left to decompose in such a mixture of materials. 



Humfeld demonstrated that the whole mass above the green 

 manure is quickly flooded with carbon dioxide, due to the respiration 

 of the dying vegetation and to the microbiological reactions. When 

 optimum moisture was present, the process was accompanied by the 

 disappearance of the various fungi, and even by the partial suppres- 

 sion of the whole fungus flora. This was true especially of the brown- 

 and black-walled fungi. With a low moisture content, certain fungi 

 developed; the mycelium was colorless, however, and soft-walled; 

 this was associated with a low content of lignin or related substances, 

 which are characteristic of the flora of the surface decomposition 

 process. 



Under conditions of regular cultivation, as demonstrated by King 

 and Doryland in Kansas, the organic matter content of the soil was 

 gradually decreased by cultivation. Conditions were made favorable 

 for decomposition of the soil organic matter. This was accompanied 

 by a release of nitrogen, phosphorus, and potash. As a result, crop 

 production increased. Continuance of this system of cropping, how- 

 ever, without compensating return of organic manures, might be 

 expected to deplete the humus supply of the soil. 



Difficulties encountered in continuous cropping of soils in prairie 

 areas led Alway and others to make extensive comparisons of the 

 organic matter in various soils. 



