SOIL MICROBIOLOGICAL EQUILIBRIUM 765 



dioxide and consume a great deal of nitrogen which they transform 

 into microbial protein. They multiply rapidly when large quantities 

 of energy material, like undecomposed organic matter, are added to 

 the soil. They are also less resistant to the action of heat and anti- 

 septics, but multiply very rapidly when reintroduced into partially 

 sterilized soils. In such a case, the fungi, together with the bacteria, 

 contribute the large amount of carbon dioxide formed, but they use 

 up, for structural purposes, a part of the available nitrogen, which 

 otherwise would remain in the soil as ammonia or nitrates. 



The actinomyces develop very slowly in the soil, but are more re- 

 sistant to the action of antiseptics like toluene and air drying of soil. 

 When the soil is kept under uniform conditions and the flora gradually 

 comes to an equilibrium, there is generally an increase in the proportion 

 of actinomyces. When the equilibrium is shifted either by air drying 

 of soil, volatile antiseptics, heat, etc., the actual number of actinomyces 

 may actually diminish only slightly, but after the disinfectant is re- 

 moved, or when moisture is added to the air dry soil, the actinomyces 

 regain their previous numbers only very slowly. In comparison with 

 the rapid increase of the bacteria, they diminish rapidly so that, two 

 years after treatment with carbon bisulfide, they have not fully regained 

 their former numbers. They do not use up much available nitrogen 

 unless an available source of energy is introduced, but a number of 

 them produce substances distinctly toxic to certain bacteria and fungi. 



The bacteria are such an heterogeneous group of organisms that 

 their activities cannot be classified together. The spore formers are 

 very resistant to the treatments that result in partial sterilization of 

 soil and may develop abundantly afterwards. The non-spore-forming 

 bacteria are very sensitive to the treatment and are much diminished 

 in numbers as a result of partial sterilization of soil; but soon afterwards 

 they begin to multiply very rapidly and may bring up the numbers to 

 hundreds of millions per gram. They use very little of the energy for 

 structural purposes, and, therefore, consume only very little of the 

 ammonia liberated from the decomposition of the proteins. The 

 ammonia can thus accumulate in the soil, unless the fungi are reintro- 

 duced. The temporary suppression of the nitrifying bacteria tends 

 to intensify the accumulation of the ammonia. 



The soil protozoa probably also play some part in this group of 

 processes. At least some of the protozoa consume bacteria as food, 

 perhaps even large numbers of them; their selective feeding may affect 

 only certain groups of bacteria. They may thus store away consider- 



