338 Recent Developments in Soil Microbiology 



made available again for the growth of new roots and new plants. 

 In this process, the microorganisms build up extensive cell material, 

 comprising bacterial cells and slimy substances produced by bac- 

 teria, mycelium of fungi and of actinomycetes and their products, as 

 well as numerous other living and dead bodies of microscopic forms 

 of life. All these contribute to the formation of soil humus. They 

 not only serve as reservoirs for further activities of microorganisms, 

 but also exert various physical and chemical effects upon the soil, as 

 by binding the soil particles and interacting with the various cations 

 and anions of the soil organic and inorganic constituents. 



As a result of these microbiological activities in the soil, a continu- 

 ous stream of carbon dioxide, ammonia, nitrate, phosphate, and 

 other nutrient elements is made available for plant growth. The 

 humus supply of the soil may either increase or be gradually de- 

 stroyed, depending on the rate of formation of new plant material 

 and its decomposition. This dark-colored, amorphous, highly char- 

 acteristic soil constituent possesses certain important physical and 

 chemical properties which give to the soil its specific characteristics. 

 The formation and disintegration of humus are closely bound with 

 the activities of the microbiological population of the soil, on the 

 one hand, and with soil conditions and plant growth, on the other. 



Many attempts have been made to develop inocula for various 

 nonleguminous plants; these comprise the so-called all-crop soil 

 inocula, and the inocula of nonsymbiotic nitrogen-fixing bacteria. 

 All these have failed to accomplish useful results. The suggestion 

 that the favorable effect of small amounts of stable manures upon 

 plant growth is due to the introduction of large numbers of bacteria 

 into the soil has likewise remained unsubstantiated. When soil con- 

 ditions are not favorable to the development of particular organisms, 

 mere introduction of these organisms will not result in their estab- 

 lishment in the soil. When conditions are made favorable for the 

 development of new organisms, as by drainage of salt lands and peat 

 bogs, by liming of acid soils, and by planting specific host crops, 

 certain organisms may be introduced to advantage. This is particu- 

 larly true of the legume bacteria, and occasionally of nitrifying bac- 

 teria, sulfur-oxidizing bacteria, and mycorrhiza fungi. 



The preparation of composts represents another important process 

 in which considerable improvement has resulted from knowledge of 

 the microbiological population. When stable manures or plant resi- 

 dues supplemented with inorganic fertilizer are placed in a compost 

 and conditions made favorable to the activities of aerobic micro- 



