Soil organic matter (SOM) is critical for indicating soil health, which ultimately influences the productivity of crop and pasture systems. Improving growers’ understanding of the value of SOM and its role in profitable and sustainable farming systems is the focus of research by Jayne Gentry, senior development extension officer with the Queensland Department of Agriculture and Fisheries (DAF).
Ms Gentry’s GRDC-funded research has shown that the key to improving SOM – and crop productivity – is to grow as much biomass as possible in your farming system.
SOM versus soil organic carbon
SOM is everything – dead or alive – in the soil system that is of biological origin. Understanding the relationship between SOM and soil organic carbon (SOC) will enable researchers and grain growers to learn how to improve soil health.
The composition of SOM varies from soil to soil and over time. However, all SOM (across all soils) comprises about 60 per cent carbon. As SOM is difficult to measure, SOC is instead measured as an indicator of changes in SOM levels.
“It has taken decades for our Australian soils to get the SOC levels they now have, so it will take five, 10 or 20 years to see a change in measured SOC under different farming systems,” Ms Gentry says. “The breakdown of SOM is a slow cycle of microbial decomposition, where soil microflora eat the carbon present in the SOM.
“The microbes utilise the carbon they need, and excrete waste in the form of plant-available nutrients, predominantly nitrogen, phosphorus and sulfur, while carbon dioxide is released into the atmosphere.”
The benefits of SOM include increased water-holding capacity and cation exchange capacity, which are more valuable for the sandy soil types more common in the southern and western grain-growing regions of Australia.
“The value of SOM in the northern grain regions is as a crop nutrient bank and so contributing to the native fertility of soils in northern region,” Ms Gentry says.
Biomass and SOM
Australian soils have low SOM levels relative to other soils in the world, and the levels are decreasing over time, Ms Gentry says.
“Clearing of native vegetation for crop and pasture farming systems has led to a decline in SOM levels in Australian soils.
“This is because the quantity of biomass produced is generally lower under our farming systems than the previous native vegetation.
“SOM is like a bucket: the more dry matter you put in the more SOM there is. Eventually SOM levels will hit an equilibrium where the amount of dry matter going into the soil through crop or pasture plant biomass will equal the release of SOM cycling within the soil.”
Monitoring SOM Levels
To identify SOM levels of different farming systems, Ms Gentry’s research has involved soil sampling 1000 paired sites across the northern grains region to measure and identify SOC levels under different farming systems, and how these have changed over time.
The greatest impact of lower levels of measured SOC has been the reduction of inherent soil fertility through loss of crop nutrients, primarily nitrogen and phosphorus, over time.
“The soil system no longer has the buffering capacity to release crop nutrients as phosphorus and nitrogen are drawn from the system through crop production,” she says.
“Grain growers subsequently need to use alternative nutrient sources, such as chemical fertilisers, green manure crops or organic amendments. “Growers need to be conscious of maximising biomass by optimising crop nutrition and water use efficiency.
“Maximising biomass will build SOM over time, so growers should aim as often as they can to grow the biggest, most water-use-efficient crops.”
Pastures have been widely recognised as the quickest way to build SOM; however, they have to be productive. To maximise production, adequate nutrition has to be provided.
This project has measured SOC under different pasture rotations: grass only; grass/legume mix; and grass with annual nitrogen fertiliser applications. The largest biomass production under these systems over the four-year period was through applying 100 kilograms per hectare of nitrogen to the grass-only plot, tripling biomass compared to grass only.
“The exciting part of this research is that these techniques are productive and have economic benefits to the grower, plus potentially increase SOM levels,” Ms Gentry says.
“The methodology and resulting scientific data provides invaluable information to agronomists and growers on changes in measured SOC levels under different farming systems and across soil types.”
Ms Gentry has also assessed the impacts of organic and synthetic crop nutrients on plant biomass and measured SOC levels in trials near Warra, in southern Queensland.
Results indicate there is no significant difference when applying manure at a commercial rate (in this case five tonnes of manure per hectare every three years). However, the research did reaffirm the need to provide adequate crop nutrition.
“The choice of product to optimise crop nutrition is up to the grower, based on costs and preferred farming systems,” she says.
“We did find that manure was a rich source of phosphorus but not nitrogen, so even though composting amendments and manure are fantastic, growers need to match crop nutrient requirements to maximise production, whether that involves increasing the rate of organic product applied and/or additional nitrogen.”
Growers are also cautioned that soil nitrogen and phosphorus levels are usually low when rotating out of a cropping phase into grass pastures, so understanding soil nutrient balance is important for productive pastures.
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