Soil microorganisms (figure 1) are responsible for most of the nutrient release from organic matter. When microorganisms decompose organic matter, they use the carbon and nutrients in the organic matter for their own growth. They release excess nutrients into the soil where they can be taken up by plants. If the organic matter has a low nutrient content, micro-organisms will take nutrients from the soil to meet their requirements.
For example, applying organic matter with carbon to nitrogen ratios lower than 22:1 to soil generally increases mineral nitrogen in soil. In contrast, applying organic matter with carbon to nitrogen ratios higher than 22:1, generally results in microorganisms taking up mineral nitrogen from soil (Hoyle et al. 2011).
Figure 1: Colonies of bacteria shown in light blue in soil, each bacterium approximately 1 micron in size. (image: Karl Ritz)
Symbiotic nitrogen fixation is a significant source of nitrogen for Australian agriculture and may account for up to 80% of total nitrogen inputs (Unkovich 2003). In the symbiosis, rhizobia or bradyrhizobia fix nitrogen gas from the atmosphere and make it available to the legume. In exchange, they receive carbon from the legume. The symbiosis is highly specific and particular species of rhizobia and bradyrhizobia are required for each legume. For more information see fact sheet “Legumes and Nitrogen Fixation”.
Most agricultural plants (except lupins and canola) form a symbiosis with arbuscular mycorrhizal (AM) fungi (figure 2) that can increase phosphorus uptake by the plant. The hyphal strands of AM fungi extend from plant roots into soil and have access to phosphorus that plant roots cannot reach. The AM fungi can provide phosphorus to plants and in return they receive the carbon they need to grow.
Importantly, this symbiosis is only beneficial for plants when available phosphorus in soil is insufficient for the plant’s requirements. Increasing phosphorus availability may be especially beneficial on phosphorus fixing soils in Australia, which are widespread and can store 100 kilograms of phosphorus per hectare (Cornish 2009).
Figure 2: A mycorrhizal fungi growing into plant cells where it has formed tree-like structures (arbuscules) that allow phosphorus to be transferred from the fungi to the plant. (image: Lynette Abbott).
The degradation of agricultural pesticides in soil is primarily performed by microorganisms. Some microorganisms in soil produce enzymes that can break down agricultural pesticides or other toxic substances added to soil. The length of time these substances remain in soil is related to how easily they are degraded by microbial enzymes.
Some microorganisms and soil animals infect plants and decrease plant yield. However many organisms in the soil control the spread of pathogens. For example, the occurrence of some pathogenic fungi in soil is decreased by certain protozoa that consume the pathogenic fungi. The soil food web contains many relationships like this that decrease the abundance of plant pathogens.
Biological processes in soil can improve soil structure. Some bacteria and fungi produce substances during organic matter decomposition that chemically and physically bind soil particles into micro-aggregates. The hyphal strands of fungi can cross-link soil particles helping to form and maintain aggregates (figure 3). A single gram of soil can contain several kilometres of fungal hyphae (Young and Crawford 2007). In addition, soil animals increase pores by tunnelling through soil and increase aggregation by ingesting soil.
Figure 3: Fungal hyphae (shown in blue) extending through soil (image Karl Ritz).
We currently understand less about how management practices affect soil biological fertility than how they affect soil chemical and physical fertility. However, the management practices described below may help improve and maintain the biological fertility of soil.
Figure 4: A non-pest soil nematode (image Karl Ritz).
Cornish PS (2009) Phosphorus management on extensive organic and low-input farms. Crop & Pasture Science 60: 105 – 115.
Young IM and Crawford JW (2004) Interactions and self-organisation in the soil-microbe complex. Science 304: 1634 – 1637.
Hoyle FC, Baldock JA and Murphy DV (2011) ‘Soil organic carbon – Role in rainfed farming systems: with particular reference to Australian conditions’, in Rainfed farming systems, Springer Science-Business Media BV, Netherlands.
Unkovich (2003) ‘David and Goliath: Symbiotic nitrogen fixation and fertilisers in Australian agriculture’, Proceedings of the 12th Australian nitrogen fixation conference. Glenelg, SA Sep 2003.
Author: Jennifer Carson (The University of Western Australia).
This soilquality.org.au fact-sheet has been funded by the Healthy Soils for Sustainable Farms programme, an initiative of the Australian Government’s Natural Heritage Trust in partnership with the GRDC, and the WA NRM regions of Avon Catchment Council and South Coast NRM, through National Action Plan for Salinity and Water Quality and National Landcare Programme investments of the WA and Australian Governments.
The Chief Executive Officer of the Department of Agriculture and Food, The State of Western Australia and The University of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.