A healthy and functioning soil ecosystem contains a wide diversity of soil organisms. Diversity is indicated by being able to observe or count more than six different species of soil organisms in a spadeful of soil. An ‘earthy’ smell suggests active and healthy actinomycetes, which are beneficial soil bacteria.
Not all soil organisms are beneficial — some are pests or cause disease in valuable pasture plants. Taking action to promote beneficial soil organisms will help strengthen the overall grazing system.
Litter or mulch not only helps protect the soil from wind, rain and temperature extremes, it is also the main food source for soil organisms. Litter quality is just as important for soil bugs as pasture quality is for grazing animals. Soil organisms readily consume highly digestible, high protein feed and recycle nutrients, but it takes much longer for dead, fibrous stems and leaves to break down and be recycled. When the bugs die, nutrients such as nitrogen, phosphorus, potassium, and sulphur are released and become available again for plant growth.
Increasing soil fertility generally increases both pasture growth and quality, which leads to more and better-quality litter and, therefore, more of the beneficial soil bugs with increased activity.
The presence of earthworms indicate soil conditions are suitable for other soil microorganisms. Sample moist soil at the end of winter or early spring to check for earthworms. In the high-rainfall zone, more than 10 earthworms per spadeful (20 cm by 20 cm by 10 cm deep) is equivalent to a stocking rate of more than 2.5 million earthworms per hectare and indicates an active biological system.
Many factors impact soil biology:
- Soil temperature: Soil temperatures above 25°C or below 15°C have a negative impact on soil organisms. It is impossible to always keep the soil temperature in this range, but a dense litter layer can help.
- Soil moisture: Most soil microorganisms live permanently in the water film around soil particles and like their food moist. Dry conditions can reduce the levels of living organisms in the soil.
- Cultivation: Regular cultivation physically disturbs the soil and reduces soil biota levels by decreasing aggregate stability and depleting organic matter.
- Grazing and compaction: Overgrazing and soil compaction can reduce soil health and starve microorganisms of much needed food, reducing their activity and the rate at which they recycle nutrients.
- Burning: Burning destroys litter and reduces inputs of soil organic matter, which are vital to the survival of soil organisms.
- Waterlogging: Waterlogging can reduce the oxygen content of soil, impacting the health and activity of beneficial soil organisms and slowing the rate of organic matter decomposition. This can be caused by compaction by grazing, cultivating wet soils or sodicity.
- Acidity: Like plants, most soil microorganisms prefer a pH (CaCl2) range of 5.5 – 6.5.
- Agricultural chemicals: Herbicides generally have less impact on soil organisms than insecticides, which, in turn, are less harmful than fungicides and nematicides. Most soil microbes tolerate herbicides with few persistent effects. However, simazine, 2,4-D, paraquat, diquat and MCPA can be more harmful than glyphosate and sulphonyl ureas. Anti-parasitic drenches can adversely affect soil organisms that colonise affected dung.