There are many physical factors which affect the activity of the soil biota (Killham, 1994). The main ones are;

• Temperature
• pH
• Moisture
• Soil mineralogy
• Light

Temperature

Temperature directly affects the activity of the soil biota by determining the rate of physiological activity such as enzyme activity and indirectly by affecting physico-chemical properties such as diffusion & solubility of nutrients, mineral weathering and evaporation rates and so on.

Within defined limits biological activity increases with increasing temperature. Organisms have a specific range of temperature at which their biological activity operates. For common soil organisms the temperature range at which they can be active ranges from about 0°C to about 60 °C although no single species is likely to be active throughout the entire range.This temperature range is largely determined by the temperature at which soil can reach although organisms may be able to survive outside the soils' temperature range or produce survival structures to allow them to survive under adverse conditions.

There are extreme environments such as volcanic hot springs & vents which can reach considerable higher temperature. It was once thought that the highest temperature that could be survived by microorganisms was around 400°C but this has been reduced to around 121°C. 

Within an organisms' temperature range, there is a temperature optimum at which biological function performs best. Beyond this value, cellular processes do not work so efficiently and as the temperature increases away from their upper limit there are irreversible changes to the cell properties leading to cell death. Those organisms which have a high temperature limit would tend not to be so active at the lower extreme compared to organisms more suited to the lower limit.

Soil pH

In a similar manner to temperature, organisms have a range of pH at which they are active. Within certain limits, organisms can tolerate extremes but this normally requires the cell to use energy in maintaining the correct internal cellular pH (pH 7.0). A few organisms (bacteria & archaea) can tolerate very extreme pH values such as pH 1 or pH 11 but these are extreme conditions are not found in agricultural soils. Larger soil animals may be more sensitive to pH than microorganisms. Earthworms being sensitive to low pH are not active in forest or peatland systems (pH <5) their function is replaced by other organisms (enchytraeids in forests) or the soil forms in a different manner (peatlands).

pH directly affects the solubility of elements. At acidic pH, aluminium becomes more soluble and hence more available to the organisms with increased toxicity. Essential minerals can become unavailable at extremes of pH. For example, phosphorous and managanese become increasingly unavailable at high pH values.

Soil moisture

Soil moisture affects the soil biota in two ways. Biologically water is essential for life and for enzyme activity and metabolism and, is a solvent for biological nutrients and other chemicals. Physically, soil moisture affects soil temperature (water is good conductor of heat) and soil aeration. The degree at which soil pores are filled affects the movement and predation of microorganisms in soil. In very dry soil, plants may not be able to extract sufficient water through the roots because of the energy it takes to remove water from the small pores. This is known as the permanent wilting point, beyond which the plant cannot recover. Conversely, under wet conditions, oxygen does not diffuse through the soil as readily so the levels available to organism may become depleted leading to anaerobic conditions. Fungi tend to be more resistant to water stress than bacteria although under extreme conditions, some bacteria can form resistant endospores which allow the organism to survive until more suitable conditions arise. Chemicals may be produced by the cell to allow survival under drought. Actinomyces which can tolerate drier conditions than fungi can protect themselves from drought by synthesising the amino acid proline.

Soil mineralogy

Soil minerals are weathered by bacteria and fungi  either as a direct source of energy for the soil biota (chemoautotrophs) or a consequence of the production of acids such as sulphuric acid (Uroz et al., 2009). As well as weathering minerals, bacteria have been shown to form minerals (Parraga et al., 1998). Depending on the pH, some nutrients particularly ammonium may become bound up with the clay an unavailable. Clays have a number of sites on their surface which can bind enzymes and whole organisms thereby affecting the movement of cells through soil and the breakdown of organic matter.

Light

In soil, light directly affects those organisms on or just below the surface and indirectly by heating the soil surface. Phototrophs, such as plants, algae and cyanobacteria, use the energy from sunlight to synthesis carbohydrates. In plants, some of this material (the photosynthate) finds its way into the soil biota via the roots and from leaf fall. With algae and cyanobacteria, the organic matter is directly inputted into soil either from release of material from the cell or when the cell dies.

Parts of the light spectrum are more damaging to organisms than others. Ultraviolet light can damage DNA which induce mutations in the organisms resulting in death of the organism or biochemical changes through for example changes of enzymes and metabolic pathways. Photochemical degradation will lead to structural changes which organic molecules making them more or less easy to be degraded.

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