344 SOIL FERTILITY 



Both soil moisture and temperature have a more or less direct relation to soil 

 aeration. As has already been pointed out, the proper circulation of air and water in 

 the soil is essential for the normal activities of both plants and micro-organisms. 

 Temperature affects this circulation by influencing the rate at which inorganic and 

 organic substances become soluble. In its turn, concentration of the soil solution will 

 affect surface tension and the movement of capillary water. Temperature affects 

 also the diffusion of gases and influences the oxygen pressure in the soil solution. 

 Hence we recognize that moisture and temperature both help to determine the 

 prevalence and activities of obligate and facultative aerobes and anaerobes, and the 

 extent and intensity of oxidation and reduction processes. In localities where the 

 winters are long and the spring and summer temperatures relatively low, evaporation 

 is retarded, and the activities of aerobic bacteria are slowed down. Conditions become 

 favorable then for the accumulation of peat as it is found in North America, Siberia, 

 Northern Europe, and elsewhere. 



Topography, like moisture and temperature, must receive consideration as a factor 

 capable of influencing soil fertility. Weathered material is constantly being moved to 

 lower levels. The steeper the slope, the more rapid is the downward movement of 

 such material. Hence, topography comes into play as a factor in determining the 

 depth of the soil, not only on the hillsides, but also in the valleys. Soil depth, water 

 storage, qualitative and quantitative differences in the soil solution, and microbio- 

 logical activities are interdependent and find expression in the size and quality of the 

 crops. The depth of the mantle of soil on the hillsides is a matter of interest not alone 

 for the student of soils, but for the forester, the hydrauHc engineer, and the economist. 

 Erosion and its control, floods and droughts, alluvial deposits and their agricultural 

 use, are matters of general interest, yet not too remote from the special interests of 

 the microbiologist. 



SOIL TREATMENT 



Aside from the natural forces that directly affect soil productivity, land utiUza- 

 tion must reckon with tillage, crop rotations, lime, green manures, animal manures, 

 commercial fertilizers, irrigation, and drainage as important soil-fertility factors. 

 Tillage as an art has evolved from the use of the burned stick by the primitive farmer 

 to the manipulation of the tractor-pulled gang plows. But whether it be human, 

 animal, or mechanical power that furnishes the energy for stirring the surface soil the 

 principle involved is the same. It has been said aptly that the soil is not a simple 

 culture medium, but a collection of culture media. Indeed, it may be claimed with 

 propriety that each soil particle with its surrounding moisture film is a culture 

 medium. Since the solutions surrounding different inorganic and organic fragments 

 in the soil must differ in their composition, they differ also as to the microflora and 

 fauna colonized upon them. 



The stirring of the soil tends to establish a greater degree of uniformity in the 

 distribution of its micro-organisms. It modifies the circulation of air and water and, 

 consequently, also the intensity of the oxidation and reduction processes. The varying 

 depth of tillage involves differences in the distribution of plant roots, of organic 

 residues, and of micro-organisms. It should be expected, therefore, that for both 

 micro-organisms and crops a surface layer of 3 inches, on the one hand, and of 10 



