THIS PROMOTIONAL FEATURE WAS PROVIDED BY BALLANCE AGRI-NUTRIENTS
Understanding nitrogen (N) levels in soil is important, as this nutrient is a key driver of growth and yield.
Nitrogen is vital for chlorophyll production, which enables plants to photosynthesise (convert sunlight into sugars). With sufficient nitrogen crops grow vigorously and produce abundant green foliage, further increasing their ability to photosynthesise and improve yield.
‘’Apply too little nitrogen and there is a risk of the crop not achieving its yield potential. Apply too much nitrogen and you not only waste money but you also increase the risk of nutrient loss to the environment’’ says Ballance Agri-Nutrients Science Extension Officer Grant Bickley.
There are a number of nitrogen tests available to help refine crop nitrogen fertiliser requirements, depending on what you are growing and what you are wanting to measure.
The mineral N test (sometimes called Deep Soil Mineral N test) is generally used for cereals and grass seed crops. The mineral N test measures the ammonium (NH4+) and nitrate (NO3-), fraction of soil nitrogen. ‘’This is the immediately available portion of nitrogen to the plants, at the time of sampling and is frequently used to determine how much nitrogen needs to be added as fertiliser, based on the potential yield of the crop’’ says Grant.
Mineral N tests do not measure organic nitrogen, also known as potentially mineralisable N (microbial process that involves the gradual breakdown of soil organic matter to release mineral N during the growing season).
Taking a Mineral N test
Mineral N tests are performed at a depth of 30cm or 60 cm for cereals and 30 cm for grass seed crops.
Mineral N testing is generally completed coming out of winter, as during winter drainage can occur resulting in nitrogen losses. ‘’The extent of these losses are dependent on such factors as soil type, rainfall, temperature as well as the previous crop and its management’’ says Grant.
Ideally, undertake the test as close to cereal nitrogen side dressing as possible, or before spring plantings. For grass seed, the test can be taken prior to closing, or as close to nitrogen application as possible.
In taking the test, collect 10 to 12 samples that are representative of the paddock.
Samples should be kept cold to prevent biological mineralisation occurring in the soil while in transit to the laboratory. Ideally, samples are chilled, then packed into a chilly bin with frozen icepacks and sent by courier to the laboratory.
Interpreting test results
Laboratory test results will provide separate figures for ammonium and nitrate, reported in mg/kg soil. These need to be converted to kg N/ha.
In order to do this, simply follow the mineral N calculation below.
Mineral N calculation:
*Depends on soil texture and depth of sample. Deeper the sample the greater the bulk density. Clay loams will have higher bulk density compared to silt loams.
For example, if a mineral N test result showed 7.0 (mg NH4+/kg) and 10.0 (mg NO3- /kg) in the soil and the sample depth was 30 (cm) and the bulk density of the soil at this depth was 1.1 (g/cm3), the calculation would be:
(7.0 mg/kg + 10 mg/kg) x 30 cm x 1.1 g/cm3 x 0.1 conversion factor = 56 kg N/ha
Once you know how much nitrogen (N/ha) is immediately available for plant uptake, you can then determine how much additional fertiliser nitrogen is required based on the potential yield of the crop.
A wide range of nitrogen products are available to meet the varying requirements of crops.
‘’Where urea is broadcast, the potential for loss is increased if conditions are not ideal for application,’’ says Grant. In these circumstances SustaiN can help reduce the loss of nitrogen by volatilisation, retaining more of the applied nitrogen in the soil to be utilised by the plant.
Where nitrogen and sulphur are required, especially in early spring, SustaiN Ammo 30N and SustaiN Ammo 36N provide both nitrogen and readily available sulphate sulphur, ideal for promoting growth in autumn and spring sown cereals.
For more support, talk to your Ballance Nutrient Specialist or Ruralco Representative.