Soil Biological Attributes
Soil Organic Matter
Organic matter is the partially decomposed remains of soil organisms, animals and plant life. It makes up only a small fraction of the soil (normally 2 to 10%), but is essential for soil aggregation which allow air and water to move through the soil. Organic matter retains moisture (humus holds up to 90% of its weight in water), and is able to absorb and store nutrients. Importantly, organic matter is the primary food source for microorganisms and other forms of soil life in dryland agriculture. Organic matter quality is important, as incorporating large amounts of high-carbon material (i.e. wheat stubble) can deprive plants of soil derived nitrogen in the short term.
Organic matter contributes to the development of the darker friable topsoil that retains moisture and cycles nutrients for plant growth. The value of organic matter in soil health is hard to overestimate; providing ground cover, increasing soil stability, cycling nutrients, habitat and erosion control. Farming practices that reduce soil organic matter such as burning, tillage, overgrazing and continuous cropping run the risk of contributing to a decline in soil condition which may not become evident for many years.
Soil Biology I. Beneficials
Soil organisms carry out a wide range of processes that are important for soil health and fertility. They decompose and recycle soil organic matter (SOM), improve nutrient availability and soil structure, transmit or prevent diseases, and degrade pollutants.
The total number of organisms, species diversity and their activity will fluctuate with changes in the soil environment. These living organisms can be classified by size into macro fauna (> 10 mm) such as earthworms, termites and other large insects; meso fauna (range in size from 200 µm to 10 mm) such as mites and collembola (or springtails); micro fauna (20 200 µm) such as protozoa and nematodes; and microflora such as fungi and bacteria. Collectively, these groups make up the soil biota and the total mass of organisms comprises the soil biomass. Bacteria and fungi are the most numerous, being several million organisms (producing up to 1 km of fungal hyphae) in each gram of soil.
The soil food web is a way of relating soil organisms to each other based on what they eat. The soil food web ‘starts’ with organic matter (e.g. animal wastes, crop residues, pasture or any plant material including active and dead roots) in the soil which is shredded, eaten and decomposed by insects, nematodes, bacteria, and fungi. These residues provide energy and nutrients for soil biota. The decomposers (saprophytes) of the soil food web have an important role in nutrient retention and account for 80 to 90% of soil biological activity in the soil. Beneficial nematodes, protozoa, earthworms, collembola and micro-arthropods such as mites consume bacteria and fungi, releasing plant available nutrients. Nematodes and protozoa are in turn consumed by mites; and mites and collembola by beetles and ants.
Soil Biology II. Non-beneficials
Organisms that attack living plant tissue and cause plant diseases are called ‘pathogens’. These undesirable soil organisms include a range of insects, parasitic nematodes, protozoa, viruses, bacteria and fungi. For a disease to develop there must be a host plant, a pathogen and an environment suited to its growth.
Pathogens negatively affect or inhibit plant growth and are usually a normal component of the soil population - naturally existing in relatively low numbers. Some pathogenic species usually only causes disease in one species of plant, but others can cause disease on or in plants that are not closely related to each other. Pathogens can exist in the soil for long periods of time without causing an outbreak of disease in plants. Growth and survival of pathogens is dependant on soil conditions (soil pH, water content, oxygen level, nutrient level, presence of other soil organisms) and are different for each species of pathogen.
Greenhouse Gases
Certain atmospheric gases capture longwave radiation emitted from the earth’s surface, and prevent a portion of the earth’s heat energy from returning to space. These ‘greenhouse gases’ blanket the earth, keeping it warm enough to sustain life. The primary greenhouse gases of importance in relation to Australian agriculture are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Agriculture currently accounts for approximately 18% of Australia's greenhouse gas emissions.