100 DECOMPOSITION OF ORGANIC SUBSTANCES IN SOIL 



the soil organic matter, giving to the soil its characteristic proper- 

 ties, and frequently spoken of as soil humus. 



When a ton of fresh organic matter (on a dry basis), in the form 

 of plant stubble, green manures, stable manures, or organic fer- 

 tiUzers, is plowed under or worked into the soil, the microorganisms 

 immediately become active, decomposing first the water-soluble 

 substances, then the starches, the proteins, the hemicelluloses, and 

 cellulose. Within 10 to 20 days, under favorable conditions 

 of moisture, aeration, and temperature, only about 1,000 to 1,200 

 pounds may be left out of the 2,000 pounds originally added. 

 These 1,000-1,200 pounds comprise only a part of the cellulose, pen- 

 tosans and fats, most of the lignin, a large part of the waxes and 

 cutins, and a large quantity of substance synthesized by the micro- 

 organisms of the soil. Most of the original protein has disap- 

 peared, and in its place there has been formed fresh microbial 

 protein, some ammonia and nitrate, depending upon the relative 

 nitrogen content of the original plant material. If the nitrogen 

 was present in amounts less than 2 per cent, only a small amount of 

 the nitrogen becomes mineralized; if the nitrogen content of the 

 original plant material was greater than 2 per cent, a large part 

 may become liberated as ammonia or nitrate within 20 days 

 under favorable conditions. 



Another month has passed, and the residual organic material 

 represents a new picture. If the original material was in a green 

 state, or if there was an abundance of nitrogen available, only 400 

 to 600 pounds may be left out of the original ton of dry material. 

 This will consist largely of lignin from the original plant or its 

 transformation products, some fats and waxes, considerable syn- 

 thesized microbial cell substance in the form of proteins, hemi- 

 celluloses, chitin, etc. This mass of residual and synthesized 

 materials is referred to as soil organic matter or humus. If the 

 original plant material was in the form of cereal straw or corn- 

 stover, which contain very little nitrogen (about 0.5 per cent), the 

 decomposition of the plant cellulose and hemicelluloses would have 

 been slower, since the transformation would have been controlled 

 by the amount of available nitrogen and by the rapidity with which 

 the soil microorganisms were able to use the nitrogen in the soil and 

 in the plant. 



The organic matter in the soil is not uniform in composition. 

 It constantly undergoes numerous changes. The condition of this 



