What is mineralisation and when does it occur?
Mineralisation is the phrase used to describe the process whereby organic forms of Nitrogen (N) in the soil are converted to available forms of N. This process is driven by microorganisms present in the soil decomposing organic matter for their energy supply. These micro-organisms require N to build protein for their body tissues. As the organic matter is decomposed the micro-organisms use some of the energy released from the organic matter whilst the rest is returned to the soil. The amount of N returned to the soil depends on the carbon N ratio of the organic matter (C:N ratio). Organic matter with a high C:N ratio eg. 30:1 will mineralise slower than organic matter with a lower C:N ratio eg .14. If the organic matter has a higher C:N ratio then some of the mineral N that may already be present in the soil will be utilised by the micro-organisms thus resulting in N immobilisation.
The surplus N that is released by the microbial activity and not utilised by the micro-organisms is president in the soil as nitrate or ammonium N and can be utilised by the planned crop. Any N that is immobilised by the micro-organisms will be slowly released back into the mineral N pool as the micro-organisms die and their bodies decompose.
Once the organic matter breakdown has commenced the first product produced is ammonium N via the breakdown of proteins, amino acids and other compounds. This process is known as ammonification. In the presence of oxygen (aerobic conditions), moisture and the correct temperature range the ammonium N is quickly converted to nitrate N by specialist bacteria called nitrifying bacteria. Nitrate N is the form of N most readily available to plants.
For mineralisation to occur there needs to the correct balance of oxygen, temperature, moisture and organic matter in the soil. Mineralisation will occur when conditions in the soil are favourable for plant growth. Summer rainfall provides the moisture and temperature and the organic matter present is determined by the organic carbon (OC) levels in the soil.
Summer rain in terms of mineralisation of N is generally determined as anything more than 15mm in 1 rainfall event after the 1st of January. Rainfall events in excess of 15mm onto soil warmed by the summer heat provide the perfect conditions for increased soil microbial activity.
The rainfall events experienced by the majority of the state in January 2018 have provided perfect conditions for N mineralisation. However before a broad brush approach of assuming a certain amount of N has been mineralised and thus reducing N inputs is implemented just because summer rain has occurred a few important factors need to be taken into consideration:
- Organic Carbon levels (OC)
- Soil type- particularly bulk density
- Carbon N (C:N) ratio of the organic matter
- Rate of mineralisation
CSBP’s NUlogic program takes all of these factors as well as economics into account to give an accurate N recommendation in regards to individual circumstances.
How does this affect you and your N decisions for 2018?
Certain assumptions can be made using OC levels, soil bulk density and assuming the C:N ratio of organic matter. However making assumptions on the amount of N in the soil can be dangerous and can result in yield penalties if for example N at seeding time is reduced on the basis of these assumptions.
In the absence of all of the above information the only sure fire way to determine how much N has been mineralised by a summer rainfall event is to re-test the soil after the rain. By analysing the results from the retested soil through CSBPs NUlogic model CSBP area managers can help you to draw conclusions on how much N is available to the crop and N budgets can be adjusted according to crop yield potentials.
CSBP plant testing in season is also an extremely accurate way to determine exactly what the plant is accessing in the way of not only N but all major and minor nutrients. Even if levels of N have been measured in the soil via soil testing this does means that that N will be available for use by this year’s crop. Factors such as N leaching from rainfall and immobilisation of N from high stubble loads from previous years can render the measured N unavailable. By plant testing you will be able to determine exactly the levels of N present in the plant and in the soil and be able to better make key decisions regarding N inputs for the 2018 season to ensure maximum yield and nutrient use efficiency is maintained.
- Summer rainfall will result in N mineralisation. The amount of N mineralised depends on many factors.
- Don’t reduce N inputs at seeding time in the hope that you have N available in the soil. N can easily be reduced in season if needed. Reducing N at seeding time may limit crop yield potential and mean you may be playing catch-up all season.
- Don’t assume- measure. Measure by re-testing soil sites and back this information up by plant testing in season.
- Match your N inputs to realistic yield potentials.