ORGANISMS IN THE SOIL 427 



otherwise accumulate to the exclusion of other plants. 

 These are decomposed through the action of organisms 

 of various kinds, the intermediate and final products of 

 decomposition assisting plant production by contributing 

 nitrogen and certain mineral compounds that are a 

 directly available source of plant nutriment, and also by 

 the effect of certain of the decomposition products on the 

 mineral substances of the soil, by which they are rendered 

 soluble and hence available to the plant. 



Through these operations the supply of carbon and 

 nitrogen required for the production of organic matter is 

 kept in circulation. The complex organic compounds 

 in the bodies of dead plants or animals, in which condi- 

 tion plants cannot use them, are, under the action of 

 microorganisms, converted by a number of stages into 

 the very simple compounds used by plants. In the course 

 of this process a part of the nitrogen is sometimes lost 

 into the air by conversion into free nitrogen, but fortu- 

 nately this may be recovered and even more nitrogen 

 taken from the air by certain other organisms of the soil. 



The slime molds, bacteria, fungi, and algse all play a 

 part in these processes, but none of them so actively 

 during every stage of the processes as do the bacteria. 

 Molds and fungi are particularly active in the early stages 

 of decomposition of both nitrogenous and non-nitrogenous 

 organic matter. Molds are also capable of ammonifying 

 proteins, and even re-forming the complex protein bodies 

 from the nitrogen of ammonium salts. Certain of the 

 molds and of the algae are apparently able to fix atmos- 

 pheric nitrogen, and contribute a supply of carbohydrates 

 required for the use of the nitrogen-fixing bacteria. Among 

 these are Aspergillus niger and Penicillium glaucum. 



It also seems probable that the fungi associated with 



