A happy relationship between organic matter, yield and nitrogen

Legumes such as clovers provide excellent biomass, which in turn promotes soil organic matter, vital for plant nutrients such as nitrogen for productive pasture growth.


The concept of developing and retaining soil organic matter is by no means a new idea, but in recent years has gained more interest in the face of our changing climate. Improving the resilience and productivity of our farms as rainfall and temperatures change, has become increasingly more urgent.

So how does soil organic matter help?


First, it is important to understand what soil and soil organic matter is, and while it might seem obvious, what it provides. Soil is the basis from which the majority of our plants grow. Soil organic matter makes an essential contribution to soil health.  It’s health, or lack of, may negatively impact plant growth.

For pasture and crops, how we manage our soil and the plants we use on farm, will have a significant effect on the development and maintenance of soil health, of which organic matter is a vital component.


What is soil and soil organic matter?

Soil is essentially a mixture of sand, silt, clay, air, water, living and dead animals, plants, and microbes. Organic matter is described as the dark, rich brown layer known as humus and compost. It is created by the breakdown of decaying plant and animal material and home to the microbes and insect life that assist in that process. The ratios of the soil and organic matter will determine how much moisture can be retained and the level of nutrient cycling that can be achieved. The structure of the soil profile will also determine where the organic matter is situated, and therefore how well plants can access the nutrients within. A sandy soil, low in organic matter, will be more free draining and not hold as much moisture; low levels of organic matter will mean the soil hosts reduced microbial activity, and be slower to break down plant material.


“Soil organic matter provides the foundation for microbial activity,” says Damien Adcock, Research Agronomist, Barenbrug, “which allows for the turnover of nutrients to make them available for plant use.

“Of the 17 plant nutrient requirements, 14 are derived from the soil. 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.”


In essence, organic matter cannot be generated without plant growth above ground. For plant growth, the right temperatures and rainfall are needed, as well as the necessary nutrients. Where organic matter levels are low, the use of artificial fertilisers may be needed to give the system a kickstart. Once the system is underway, the use of artificial fertilisers may be reduced to maintenance levels as the soil organic matter retains the necessary nutrients and bugs to maintain nutrient cycling below ground. A soil test will verify whether this is the case.


Are your crops looking healthy? If not, it may be lacking a vital nutrient.


There could be a number of reasons why your crop is not performing at its best, and an agronomist is best placed to assess the underlying issues. Soil tests may also be warranted.  Yellowing of plant leaves can be an indication of insufficient nitrogen, and coupled with reduced yield, may indicate that the soil health and organic matter is not where it should be.


“The first thing I look at in a paddock is what the farmer is doing now, and what the desired outcome is to be, whether it is cropping, livestock or a combination of both,” says Rob Winter, Territory Manager and Southern Region Agronomist.

“Then consider are the plant species used in the pasture best suited to the environment – does it get enough rain, is the pH suitable for that plant. If the right species and variety is chosen for that farm, then you would expect the yield and reliability to be high.

“With high yield and reliability, coupled with rainfall, the crop or pasture should produce the biomass (or dry matter) needed to promote organic matter health and thus improvements for soil health.

“How the pasture is cultivated and managed is another factor in a healthy system. Adopting a ‘no till’ cultivation practice will mean that the soil is not excessively turned over, which in turn means that it is not exposed to as much heat and does not lose as much soil moisture.”


If the pasture is to be grazed, overgrazing can be detrimental to the plants’ health and its ability to bounce back. Under grazing, can be equally as problematic as livestock will preferentially eat preferred plants and leave the ones they don’t like – often the annual weeds. By leaving behind the weeds, the composition of the pasture will change over time and upset the balance of the system.


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, the use of 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, clovers and medics, 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,” says Tom Dickson, Research & Innovation Manager, Barenbrug.

“Plants need nitrogen to grow, it is basically a linear relationship between dry matter growth and nitrogen fixation if the conditions are right. Legumes produce around 17-25 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 nitrogen, 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 20-30% legumes in your grass-clover pasture mix, which offer about 30% dry matter yield, but also will provide 60-100kg of nitrogen per hectare per year to the system,” says Rob.


“It is important to remember that the nitrogen will accumulate in the soil over time, so in Year 1, we will have about 80-90kg – some is lost to grazing livestock and off-farm animal products as well as leaching and to the atmosphere. In Year 2, another 80kg is produced if the paddock is only grazed and remains healthy. In 3-4 years, you will have around 300-400kg 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. Will that medic suit your situation or is there a better option? Does your lucerne tolerate that level of acidity? And which sub clover gives yield and nitrogen fixing?


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. 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,’ says Tom.


"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.”

As research progresses, improvements in legumes occur, and with that older varieties are superseded. Current legume projects at Barenbrug include the development of sub-clovers which are particularly important in temperate pasture zones, for use in pasture mixes.


“We will be looking at how mixes that include sub-clovers, work together in a pasture environment to assess sustainable traits in terms of water and nitrogen use efficiency, but also on forage yield and nutritional value,” said Allen Newman, Southern Breeding Manager, Barenbrug.

“The questions we ask are: How well do those new varieties cope in different climate zones – how adaptable are they? And what happens to the composition of the mix over time – does one variety become more dominant, does another fall away? How do they perform over a number of years?”


Testing sites form an integral part of determining performance of legumes in pastures and are carried out around the country for different environmental and climatic challenges. The process can take many years to complete and fully assess the results to achieve the breeding objectives.


One thing is for certain, legumes will always be part of the mix for productivity and organic matter, with the added benefit of being a generator of nitrogen fertiliser.


For more information on improving your soil organic matter, contact your Barenbrug Territory Manager