JACOB G. LIPMAN 343 



add more or less substantial amounts of combined nitrogen to the reserves of this 

 constituent. Under some conditions nitrates may be reduced and thus withheld from 

 the crop. In the presence of sufficiently large amounts of cellulose, starch, sugar, etc., 

 bacteria and fungi may use up available nitrogen compounds fast enough to offer 

 serious competition to higher plants. The kinds, numbers, and distribution of the 

 micro-organisms in the soil are affected by the food supply and the nature of the soil 

 environment. In their turn the micro-organisms modify the soil environment of crops 

 by increasing or decreasing the food supply of the latter, and it is the task of the skil- 

 ful farmer to treat his soil in such manner as to reduce to a minimum the negative 

 effects and raise to a maximum the positive effects of soil microbiological activities, 



EXTERNAL FACTORS 



The amount and distribution of rainfall, the conservation of soil moisture, and 

 the use of irrigation water are related in an important way to the growth of plants. 

 Optimum moisture conditions for plants are established when the pore space in the 

 soil, that is, the space not occupied by the soil particles, is filled with water to the 

 extent of about 60 per cent. When there is too much or too little water in the soil, 

 reduction and oxidation processes and the diffusion of gases may be shifted in the 

 direction unfavorable to normal growth. Micro-organisms, like higher plants, are 

 directly affected by the thickness of the moisture films surrounding the soil particles, 

 the movement of capillary water, the concentration of the soil solution, and the diffu- 

 sion of soil gases. Fine-textured soils may at times contain an excess of water, while 

 coarse-textured soils may frequently contain an excess of air. Hence in the former 

 the mineralization of organic matter is retarded, and in the latter it is hastened often 

 to the detriment of the farmer. This will account for such agricultural practices as 

 drainage, subsoiling, tillage, liming, green-manuring, etc., intended to speed up the 

 circulation of water and air in fine-textured soils. It will account, also, for marling, 

 claying, liming, green-manuring, and other practices as a means of retarding the 

 movement of water and air in the more open soils. By controlling the water and air 

 supply the farmer can influence the microbiological activities in the direction most 

 favorable for crop growth. 



Temperature, like water and air, is a major soil-environment factor for both higher 

 plants and micro-organisms. There is a direct relation between the range of soil 

 temperature and the amount of chemical change produced by the soil microflora. 

 Similarly, there is a direct relation between the intensity of microbiological activity 

 and the growth and yield of crops. In tropical and subtropical regions microbio- 

 logical activities are never dormant wherever the moisture supply is sufficiently 

 abundant. This accounts for the rapid decomposition of plant residues as an offset 

 to more luxuriant vegetation and the more rapid accumulation of organic matter 

 both in the soil and on its surface. In such environments the circulation of carbon 

 and nitrogen between soil and atmosphere is manifestly more active than it is in 

 regions farther north. On the other hand, in temperate and subarctic regions the rate 

 of both accumulation and dissipation of organic matter is less intense. There must be, 

 therefore, a certain relation between temperature, the microbiological activities in the 

 soil, the supply of carbon dioxide to vegetation, and the rate of growth of the latter. 



