SOIL MICROBIOLOGICAL EQUILIBRIUM 739 



teria; others use up the ammonia for the building up of microbial 

 protoplasm. Some produce soluble substances as intermediary or by- 

 products of metabolism, which are either distinctly beneficial and 

 stimulating to other organisms, or distinctly injurious. The soil also 

 harbors a number of fungi, actinomyces and bacteria which are 

 causative agents of plant diseases, although they may be able to live 

 in the soil saprophytically. 



When a soil is left undisturbed for a long period of time the numbers 

 and activities of the various groups of organisms come to a condition 

 which may be termed unstable equilibrium. This equilibrium is not 

 static but dynamic, in a chemical sense, especially under field condi- 

 tions. Sunshine and rain, freezing and thawing, plowing and 

 cultivating, fertilizing and manuring and a host of other factors which 

 affect the soil will bring about a change in this equilibrium. If the 

 numbers of bacteria and protozoa are determined daily for a period of 

 time, constant fluctuations are found. 1 The same is true of the numbers 

 of fungi, nitrate and carbon dioxide content of the soil. 



However, when a soil is kept under constant optimum conditions 

 and undisturbed, the daily variability is very small and there is a 

 constant gradual diminution in the numbers and activities of the 

 microorganisms, as shown 2 in figure 63. This figure indicates that the 

 rapid rise of the numbers of bacteria and evolution of carbon dioxide, as a 

 result of moistening of an air-dry soil, was followed by a gradual drop 

 for about 200 days, when the drop became hardly perceptible but was 

 still present. The soil was kept in pots and the soluble products, 

 resulting from the decomposition of the organic matter in the soil, 

 were not removed by drainage, nor by growing plants, nor by any 

 microorganisms, since no fresh sources of energy were added. For 

 this reason nitrates continued to accumulate. This equilibrium in 

 microbiological activities is not due to a lack of nitrogen, but to a 

 lack of available energy. Rahn 3 found that the addition of straw to 

 such a soil will result in a rapid increase in the growth and develop- 

 ment of microorganisms, lasting as long as the available energy does 

 and followed again by a decline. The same increase in the activities 



1 Cutler, et al., 1923 (p. 32). 



2 Waksman, S. A., and Starkey, R. L. Partial sterilization of soil, microbio- 

 logical activities and soil fertility. Soil Sci., 16: 137-156, 247-268, 343-357. 

 1923. 



' Rahn, O. Die schadliche Wirkung der Strohdungung und deren Ver- 

 hiitung. Ztschr. techn. Biol., 7: 172-186. 1919. 



