Factors influencing soil microbes

Several environmental conditions affect the density and composition of the microflora and frequently alter their activities in soil. Primary factors include moisture, aeration, temperature, pH, organic matter, inorganic fertilizers. Lesser variables are the secondary factors which include crop rotation, season, soil depth, cultivation practices etc.

I. Influence of Primary factors

Soil moisture: Soil moisture is one of the important factors influencing the microbial population. Moisture is present in the form of film in soil pores. The amount of water increases with increase in porosity of soil. Water is the major component of protoplasm, an adequate supply must be available for vegetative development. But, when it becomes excessive, proliferation is suppressed due to limitation in gaseous exchange and lowers the availability of O₂ supply creating anaerobic environment. Most of the organisms prefer a moisture percentage between 20 and 60 per cent. Many bacteria and fungi are able to adjust themselves to different moisture conditions. Under dry conditions, the bacteria may form spores which can resist the drought conditions. The fungi may sporulate or form chlamydospores to tide over adverse conditions. The protozoans also may form cysts and can survive under dry conditions. Actinomycetes are the chief group of organisms that prefer dry conditions. At high moisture, it is believed that the concentrations of nutrients are diluted and also the aeration is very much limited and hence only the anaerobic and microaerophilic organisms can develop better.

Soil air: It is directly linked up with the moisture level of the soil. Most of the forms are active in aerated soil and under waterlogged soils, anaerobic and microaerophilic forms develop.

Soil temperature: exhibit considerable influence on the microbial population. Though microorganisms have been found to exist under extreme temperature conditions, such as –60°C and +60°C the soil temperature usually does not reach such extremes. Microbial population varies both quantitatively and qualitatively under extreme conditions. In tropical and subtropical regions, temperatures vary widely in summer and winter and the population may also be varied. In temperate regions, not much variation in temperatures of summer and winter and not much variation in the soil population. Soil temperature influences the temperature of air, water and solid phase of the soil. Soil water and temperature exert a combined influence on the microbial population.

Soil organic matter: Community size is related to the organic matter content, so that humus rich localities have the largest biological numbers. Organic matter content varied with soil types from less than 0.5 per cent in desert soils to 40 per cent in peaty soils. Organic matter being the chief source of energy and food for most soil organisms, it has great influence on the population. Nature of organic matter is responsible for the differential stimulation of the population. There are several indirect effects of the organic matter on soil microflora. It influences the structure and texture of soil, besides enriching with nutrients for plants and microorganisms. Such influences on the soil also greatly affect the activity of the soil microorganisms. However, upon incorporation of green manures, crop residues, etc. in soil, the community size of microorganisms gets increased.

Soil pH: It is a key factor influencing the microflora of soil. It influences enzyme system and thus plays an important role in microbial activity. In general fungi thrive better than bacteria and actinomycetes in acid soils. Bacteria flourish well in neutral and alkaline soils. The saline and alkaline soils have different microflora. Salinity is due to excess salts, alkalinity is due to high H-ion concentration. Several direct and indirect effects of H-ion and salt concentration in soil are exerted on microbial population. The optimum pH for the microbial life falls between 6.5 to 7.5.

Fertilizer application: Application of fertilizers, to the soil improves the microbial activity because of the availability of more readily obtainable nutrients. Some fertilizers, may however have inhibitory effect on specific bacterial types. Continuous application of ammonical fertilizers favour the growth of fungi due to the formation of nitric acid and which inhibits the growth of bacteria and actinomycetes. Addition of nitrate inhibits the activity of free living N₂-fixing bacteria like Azotobacter. Some of the autotrophs are encouraged by the addition of fertilizers.

Cropping and vegetation: Two kinds of effects are exhibited by the crop plants. One is through root exudates, which may have different compounds with reference to the crops grown. Vegetation has selective stimulation over population. The dominance of one or the other groups is related to the type of vegetation and growth stages of a plant. This selective action of plants is attributed to microbial response either to specific root-exudates or chemical constituents of sloughed-off tissues that undergo decomposition. Continuous cultivation leads to more microbial activity than the uncultivated land.

II. Influence of secondary factors

Crop rotation: Crop rotation with different species like legumes, graminaceous plants, etc. brings about different stimulatory effect on the microflora. Some crops have deeper roots than others and some are more fibrous. Such variations bring about physical changes in soil which in turn may have direct and indirect stimulatory effects on the soil microflora.

Cultural practices: Various cultural practices, such as tillage operations and irrigation have several physical and chemical changes in soil which are reflected on the soil microflora. Through subsoil ploughing deeper layers may get better aeration and there may be quicker multiplication of aerobic organisms. Weeding, irrigation etc. may influence the microbial populations in the soil.

Soil depth: Most of the organisms are almost in top layers, largely in upper few centimeters and decline with greater depth, more active at few cm down and less active at deeper layers. Low O₂ and less sunlight in deeper layers reduce population.

Season: Cell number are greatest during the spring and autumn and a decline occur during hot, dry and winter, the cells remain in a state of dormancy for biochemical inactivity. The influence is due to mainly the alterations in temperature, moisture as well as availability of organic matter.

III. Specific influences

Some of the factors had specific influence on the microbial communities

1. Sunlight favours algae and other autotrophs.

2. Herbicide application had devasting effect on algae.

3. Attach by neighbours – protozoa, nematode, earthworms consume algae.

4. Presence of parasites and predators – eg. viruses eat on specific bacteria

5. Increase in bacterial population increases protozoans also.

IV. Other factors

1. Burning of top soil : Leads to partial sterilization of the top soil and may kill protozoan population, which may lead to the increase in bacterial population. This condition may affect biological equilibrium in soil.

2. Application of nematicide, fungicide and bactericide may exhibit partial sterilization. Thus every change in crop production direct or indirect, alters the soil microflora, still many more are not clearly understood.