Biological fertility is the bridge between physical and chemical fertility - it is vital to sustaining a stable structure in many soils.
Biological fertility depends on the amount of organic matter available, the soil micro-organisms present and the interactions between them. Soil organisms, comprising invertebrate animals and microbes, which account for most of the living tissue in a pasture system, are involved in the decomposition of organic matter and the nutrient recycling system.
Soil organic matter
Organic matter is the primary food source for soil organisms. Most organic matter in a pasture system is of plant origin, made up of decaying plant material, dead roots, and animal waste. It contains nutrients, which slowly become available to plants as decomposition proceeds.
The final breakdown product of organic matter is humus, and while it has no nutritive value, it can contribute to the ability of soils to retain and supply nutrients to plants (cation exchange capacity).
Removing organic matter through cultivation, stubble burning, overgrazing or clearing, reduces soil biological activity by reducing the food source. Any activity that improves soil fertility for plant growth, such as fertiliser or organic manure application, will increase biological activity and the rate of nutrient ’recycling’.
Any actions that improve plant growth clearly benefit animal production and the organic recycling systems.
Soil organisms are largely responsible for decomposition of organic matter and the subsequent release of plant nutrients. Larger soil organisms, such as earthworms, play a major role in breaking down plant material into smaller pieces more prone to microbial attack, incorporating decaying pasture and dung, and stabilising soil structure by building networks of soil macropores.
Earthworms are a well-recognised indicator of soil condition and move up to the soil surface when organic matter (their food source) and moisture are plentiful.
There are more than 200 native earthworm species, however a few introduced species dominate southern Australian agricultural soils.
Earthworm faecal pellets are stable, and the mucus secretions from their skin line that their burrows may last for decades, further improving soil structure.
The more diverse and abundant the soil biota, the more fertile the soil.
Herbicides and drenches
Some older herbicides (such as the active constituent simazine) are toxic to soil organisms such as mites, springtails, fly larvae and earthworms.
Drenches and backliners that pass through livestock can also harm important soil fauna (such as dung beetles and fly larvae) that feed on the dung pats.
However, the effect on overall dung decay and nutrient recycling may be small during a full year. Any adverse effects on some specialised dung feeders must be balanced against health and production benefits to livestock.