The concept of developing and retaining soil organic matter is by no means a new idea, but in recent years it has gained more interest in the face of our changing climate.
How does soil organic matter help?
“Soil organic matter provides the foundation for microbial activity, which allows for the turnover of nutrients to make them available for plant use,” Barenbrug research agronomist Damien Adcock said.
“Of the 17 plant nutrient requirements, 14 are derived from the soil,” he said.
“The supply of soil nitrogen is a microbially mediated process; fixation of atmospheric nitrogen by rhizobia and the conversion of inorganic forms of nitrogen to plant available forms by nitrosomas and nitrobacter.
“The storage and supply of these plant essential nutrients determines how fertile the soil is, and how available it is for plant growth.”
Legumes, the natural fertiliser
Most pasture grasses will contribute to the development of organic matter within the soil through the introduction of biomass above and below the ground through leaves, stems and roots.
To really boost the levels of nitrogen in the soil, use legumes that actively capture — or ‘fix’ — the nitrogen that is in the air within the soil to make it available for plant use.
Legumes have the ability to fix nitrogen and have been used since before the introduction of modern fertilisers.
The legume family includes lucerne and clovers, commonly used in most temperate pasture systems, but also extends to other legume crops such as chickpeas, faba beans and the acacia family, which includes leucaena.
“Legumes form a symbiotic relationship with rhizobia and fix atmospheric nitrogen, add to that livestock urine and faeces and plant decay and it all gets released into the pasture to promote plant growth,” Barenbrug research and innovation manager Tom Dickson said.
“Plants need nitrogen to grow — it is basically a linear relationship between dry matter growth and nitrogen fixation if the conditions are right,” he said.
“Legumes produce around 17 to 25 units of nitrogen per tonne of dry matter produced — often in the vicinity of 150 units per hectare, per year. At $2 per unit for urea, that equates to $300 that you don’t need to spend for the same productivity.”
Ultimately, the aim of a farm is to be profitable and with increasing prices of urea, interest has turned to greater use of legumes.
A mixture of legumes and grasses will not only provide yield for grazing and hay or silage, but will also enrich the soil organic matter with vital nutrients such as nitrogen.
The flip side is that the same field may need to be harvested for farm income and not be devoted just to soil health.
In this instance, pasture mixes comprised of grasses and legumes can still give the benefit of soil health and organic matter, while providing a grazing option and hay or silage.
“Ideally, you want about 20 to 30 per cent legumes in your grass-clover pasture mix, which offer about 30 per cent dry matter yield, but also will provide 60 to 100kg of nitrogen per hectare per year to the system,” Barenbrug territory manager and agronomist Rob Winter said.
“It is important to remember that the nitrogen will accumulate in the soil over time, so in year one, we will have about 80 to 90kg — some is lost to grazing livestock and off-farm animal products as well as leaching and to the atmosphere,” he said.
“In year two, another 80kg is produced if the paddock is only grazed and remains healthy.
“In three to four years, you will have around 300 to 400kg of nitrogen in the system, at which point, fertiliser nitrogen may need to be topped up for specific seasonal targets, or not required at all.”
The right legume for the job
The key message for building organic matter and health is to choose the right legume species and variety based on desired outcomes, but more importantly what will be productive in your area.
For many years, extensive research has been undertaken to develop legumes that meet the productivity objectives such as dry matter, metabolisable energy and protein as well as nitrogen fixing capabilities.
Given the relationship between dry matter yield and organic matter accumulation, it is safe to assume high yielding species will be a positive contributor to organic matter levels in the soil.
“Collaborations with organisations such as SARDI has seen the development of lucerne varieties that we know have strong nitrogen fixing properties, but also adaptation to a range of climatic environments,” Mr Dickson said.
“For instance, our lucerne variety SARDI 7 Series 2 is more tolerant of acidic conditions and when combined a co-developed acid tolerant rhizobia strain can fix more atmospheric nitrogen in acidic soils.
“We also know through research that some sub-clovers are better at fixing nitrogen than others, so breeding programs aim to carry nitrogen efficiency and nodulation traits through to new varieties.
“For other species such as vetch, we know that it not only provides exceptional nitrogen fixation but superior quality hay and silage.”
For more information on improving your soil organic matter, contact your Barenbrug territory manager.