How to balance soil nitrogen for top quality pasture

Ensuring good soil nitrogen levels for pasture grass is, however, more complicated than assessing crop soil nitrogen levels. In a permanent pasture, at any given time, much of the soil nitrogen is in various organic forms that are difficult to assess for availability. While soil testing can provide valuable information, more commonly than not, it underestimates the amount of nitrogen likely to be available to a pasture through a given growing period.

Research has supported that animal growth rates relate closely to pasture protein and digestible energy levels. For example, research at Tamworth conducted by DPI shows perennial grass Premier digit at 17 per cent protein when soil nitrogen fertility is corrected versus less than 11pc when not addressed. That difference would equate to steer growth rates of 1kg/head/day or better compared to probably 0.2 kg/head/hay at the deficient level.

In addition, the Tamworth research documented pasture growth rates at least three times greater when soil nitrogen deficiency was addressed. Other research has shown pasture nitrogen response rates are far better when other soil deficiencies like phosphorus and sulphur are also addressed. This is much the same situation in a cropping business.

Valuable research in southern NSW, has emphasised how cropping gross margins, over a combination of low and high rainfall years, were higher when crop legumes were part of the rotation. This research is led by Dr John Kirkegaard and involves GRDC funding, CSIRO, FarmLink, and a number of local advisors, and it is detailed in a recent FarmLink publication. The nitrogen contributed to the rotation via the legumes is part of the story. More details about this research will be in future Down To Earth articles.

In the pasture quality and quantity story, the role of legumes supplying at least some of the nitrogen is also very important. Legumes supply nitrogen for the grass component and provide high-quality feed in their own right.

Read more: Effective strategies to control St John's wort

Research has shown that pasture legumes with effective rhizobia are capable of building soil nitrogen by around 20kg/ha per 1t/ha legume drymatter growth. In a year like the recent three that many farmers have experienced, it is not unusual for legume drymatter production to have been between 2 and 6t/ha drymatter basis. That is equivalent to providing 40 to 120kg/ha nitrogen per year.

In contrast to a cropping business, not much nitrogen is being exported off the property via meat and wool. A typical off-farm export of nitrogen would be around 10kg/ha per year via animal product. However, losses can occur via other means, such as denitrification in waterlogging conditions, some volatilization from urine, and perhaps some runoff. In well-managed pastures (good groundcover) and for many soils, these losses are mainly low.

Hence, in a typical mixed perennial grass with a good legume component, there commonly is a good level of soil nitrogen over good and bad years. In our own property's case, in central west NSW east of Coonabarabran, with a combination of native grass plus winter legume or tropical grass plus winter legume, we add nitrogen some years but commonly not.

For example, in January 2020, after three years of drought and poor winter legume growth, we applied 50kg/ha nitrogen to tropical grass pasture. After the past three good winter/springs, with legume growth commonly 2 to 5t/ha, drymatter basis, we have not applied any nitrogen via fertiliser.

Clearly, the challenge is to have legume growth as close as possible to its full potential (very much seasonal related). This involves many aspects, such as the best species for the soil type, the best variety within the best species for the area, well nodulated with appropriate rhizobia, soil fertility corrections where needed and good management for the legume pasture component.

Next week: Lime requirements for acid soils.