New PMN Test Makes It Easier to Predict Soil's Nitrogen Supply
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RURAL CONTRACTOR & LARGE SCALE FARMER
GroundBreaker 2022 Issue 210
It is an uncertain world and never more so than now with disruptions from the global pandemic, climactic change and volatile economies. It makes sense, therefore, to focus on the aspects of living where we can take control. For arable farmers, one way to do this is to be more precise in every aspect of the production system. Using the tools and technologies available with advice from experienced and trusted advisors can help to provide some certainty in efforts to improve both production and the environment.
It is all about getting things right. This includes where, what or when farmers and contractors do everything from cultivation to planting, weed control, fertilising and irrigation. New technologies now allow precision management using data and information, and this is vital for accurate environmental and freshwater management planning.
One new piece of technology is a soil test that gives us confident predictions of the nitrogen mineralisation potential in cropping soils. Nitrogen management in cropping farms is more important than ever, to produce the targeted yield with ‘just enough’ additional N fertiliser input to be profitable while mitigating losses to the environment. The new test is called Potentially Mineralisable N (PMN) and provides a more robust prediction of the potential soil supply of N than previous tests.
The supply of nitrogen from the soil, is in two forms:
• Mineral N is the readily available nitrate-N and ammonium-N in the soil.
• Mineralisable N is the N that may be released (mineralised) from the soil organic matter through microbial activity over the growing season.
Figure 1 depicts the source and uptake of nitrogen for crops over a growing season. Nitrogen must be in a mineral form to be taken up by plants, whether that comes from the soil or from N fertiliser.
FIGURE 1: THE SOURCE AND UPTAKE OF N FOR CROPS OVER A SEASON. (FROM M BEARE, MINERALISABLE N TO IMPROVE ON-FARM N MANAGEMENT.)
The mineral N in soil can be measured using existing laboratory tests, and this provides a point-in-time measure. It is often done at the beginning of the main growing season, and sometimes just prior to any side-dressing. Tests are usually done on soil samples taken at 0-30 cm in depth (or 0-60 cm), and the reported lab results can be easily converted to kgN/ha by using an appropriate field bulk density factor.
The new PMN test is calculated from a laboratory measure of Hot Water Extractable Organic N (HWEON). The PMN test has been calibrated from more extensive laboratory and field research work, and has been under evaluation for the last three years under the SFF Project-Mineralisable N to improve on-farm N management. It is more accurate than the traditional Anaerobic Mineralisable N (AMN) test and may, in time, take the place of that test. For now, however, the AMN test will be retained as it has widespread use, including in state of the environment monitoring carried out by regional councils.
As the name of the test implies, the PMN test can only provide a ‘potential’ measure of the N that may be released from the soil organic matter under ideal conditions. The actual amount mineralized, will depend on the moisture and temperature conditions over the whole growing period. It may range from 20 to 45 percent of the PMN (kgN/ha). Where the climate is warmer and wetter, a higher amount of N will be mineralised than would be the case in a cooler and drier climate.
The range of potentially mineralisable N in cropping soil is very wide – from 40 to 300 kg N/ha per year depending on a range of factors. Measuring the potential supply for each crop paddock is, therefore, very useful to determine additional inputs needed for a successful crop and to modify management practices towards soil organic matter retention.
An SFF project currently underway will provide guidelines on how to use the PMN test for different conditions. Autumn 2022 is the target for this to be available. Arable farmers who already use soil moisture and temperature monitoring technologies, will be well placed to make use of the new test.
Hill Laboratories website (hill-laboratories.com) has information, including Services Offered and Crop Guides, to assist with recommended sampling protocols and key tests. Soil nitrogen test results provide the numbers to manage available resources more precisely, for both production and environmental benefit. Using experienced advisors to interpret test results and plan a ‘precisioN’ programme will help reduce uncertainty and enable growers to meet yield targets and protect the environment.
By Fiona Calvert, Hill Laboratories