398 NATURE AND PROPERTIES OF SOILS 



through the action of organisms of various kinds, the inter- 

 mediate and final products of decomposition assisting plant 

 production by contributing nitrogen, and certain mineral 

 compounds that are a directly available source of plant nutri- 

 ents, 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 plants. 



Through these operations the supply of carbon and nitro- 

 gen 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 condition higher plants 

 cannot use them, are, under the action of micro-organisms, 

 converted by a number of stages into the 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 fortunately this may be recovered and even 

 more nitrogen taken from the air by certain other organisms 

 of the soil. 



Higher fungi and actinomyces are particularly active in 

 the early stages of decomposition of both nitrogenous and 

 non-nitrogenous organic matter. Molds are capable of am- 

 monifying proteins, and even re-forming complex protein 

 bodies from the nitrogen of ammonium salts. Certain of the 

 molds and of the algae are apparently able to fix atmospheric 

 nitrogen, and contribute in addition a supply of carbohy- 

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

 While the higher fungi are important in such transforma- 

 tions, their activities in almost every stage are excelled by 

 those of the bacteria. Because of this, the vital biological 

 transformations within the soil are generally ascribed to bac- 

 terial action, the bacteria receiving the greatest attention of 

 the numberless organisms making up both the soil flora and 

 fauna. 



218. Biological cycles.— Because of a lack of knowledge 

 regarding the flora and fauna of the soil, it is obviously im- 



