Recent research into organic soil carbon and its effect on soil nutrient levels suggests care must be taken to avoid mining nutrients from our agricultural soils. CSIRO research scientist Jeff Baldock outlined his concerns in a recent video discussion with GRDC Communities Crop Nutrition. Nitrogen (N) is being mined from the soil in grain production systems, increasing the reliance on synthetic N fertilisers. This is particularly apparent in continuous cropping systems with no pasture component to rebuild soil organic matter.
Soil organic matter
It is important to match the crop’s N demand with the ability to supply that N from the soil and additional fertiliser. But it can be difficult to determine how much N is being provided from soil organic matter and therefore how much N fertiliser is needed.
Research shows we have already lost 20-70% of the original soil organic carbon from cropping systems Australia-wide. Every percent of soil organic carbon lost from the surface 10 cm of soil corresponds to a reduction in the N ‘bank’ in that soil of approximately 1 tonne per hectare. This loss will significantly affect the soil’s ability to supply N for future crops. If the demand for N exceeds natural supply, there is an increased reliance on fertilisers to provide N to growing crops. This can lead to other problems such as acidification and reduced profit maximisation.
A smart solution
We need to use N fertilisers more smartly and strategically. Applied N must be used efficiently and not lost from the soil to groundwater, streams or the atmosphere. Systems need to be developed where if N demand exceeds supply, management practices in that year or following years will ensure soil N stocks are maintained or rebuilt. This requires a greater understanding of how management practices impact the different components of soil organic matter and rates of N mineralised from soil organic matter