Free nitrogen – everyone wants some of it but determining how much nitrogen will be mineralised in soil is easier said than done.
With extensive summer rainfall early in 2015, many growers want to know what is happening with nitrogen stores below the soil surface.
CSIRO Research Fellow Dr John Angus has spent his career trying to better understand the factors influencing mineralisation. He answers some key questions …
What are the key facts about mineralisation?
CSIRO Research Fellow Dr John Angus has spent his career trying to better understand the factors influencing mineralisation. He says increasing soil organic matter with legume residues can achieve more rapid mineralisation.
- Mineralisation is the main process by which soil organic matter is converted by microorganisms into plant-available forms of N.
- Soil organic matter consists of four main parts – plant residue, particulate organic matter, humus organic matter and resistant organic matter, which is inert material, mostly charcoal. These different forms of organic matter have different ratios of key nutrients – carbon, nitrogen, phosphorus and sulphur. Variations in these ratios can impact the speed at which decomposition occurs which alters supply of nutrients from mineralisation.
- Mineralisation is a continual process where plant-available N is produced before and after seeding. The rate of mineralisation varies with soil temperature, water content and, depending on the soil type, it can vary with the amount of organic matter and residues, and pH – mineralisation is slower in acidic soils.
- In crops in southern Australia, peak nitrogen demand usually occurs several weeks earlier than when mineralisation is at its peak. This is partly because low temperatures early in the growing season limit mineralisation. Peak mineralisation occurs when soils start to warm up in spring and its rate can vary between seasons.
- In mid spring – when crop growth is greatest – a plant’s daily nitrogen demand can be four to five times the rate of mineralisation. Peak mineralisation is 1kg/ha/day for an average loam soil with 1 per cent organic carbon. The rate of mineralisation is lower for sandier soils. A fast-growing crop will need 4-5kg/ha/day of N at this time.
How much N is being mineralised?
The amount of N mineralisation depends on three factors – the soil’s total N content, its temperature and water content.
During the growing season, the amount of mineralisation is reasonably predictable and models are available that will estimate mineralisation over time periods ranging from days to years.
In the year after a legume crop or legume-based pasture, there is an additional burst of mineralisation from legume residues. In the first year after a legume, this can be one quarter of the legume’s biologically-fixed N.
Is the rate of mineralisation constant all year?
No, in summer and autumn in particular, the rate of N mineralisation is not constant. This is because soil water, an important driver of mineralisation, can be reduced by weed growth and poor infiltration.
What is an example of the rate of mineralisation?
A rule of thumb for mineralisation, based on measurements in southern NSW in a continuously-cropped chromosol, is where there is 0.1 percent N in the top 10cm, it mineralises about 0.3kg N/ha/mm of summer rain and 0.5kg N/ha/mm of summer rain after a legume.
Can cultivation and stubble impact mineralisation?
Mineralisation is not greatly increased by cultivation in Australia, in contrast to soils in western Europe and North America where disturbance by cultivation causes a large increase. Measurements in Australia show little effect, which can be attributed to our light cultivation techniques. Research shows that retaining stubble on the soil surface does not increase mineralisation.
Should mineralised N be measured for fertiliser budgeting?
Growers can include an estimate of in-crop mineralisation in a nitrogen budget when calculating fertiliser requirement for the 2015 growing season.
The amount of N mineralised between harvest and sowing of the next crop can be measured by soil sampling before sowing. But after sowing, the amount of N mineralised is more difficult to measure because there is simultaneous mineralisation of nitrogen in the soil and uptake by the growing crop. Models provide the best estimates of in-crop mineralisation.
Is there anything growers can do to increase mineralisation?
Increasing soil organic matter with legume residues can achieve more rapid mineralisation. The rate of mineralisation in summer and autumn may be so high that legume residues are depleted. This means that in the following crop, the rate of mineralisation is the same or less than soil with no legume residues. Growers need to keep this in mind when budgeting fertiliser application rates.
Weather conditions that affect mineralisation can also affect N losses, such as leaching which is greatest in wet winters on sandy soil, and denitrification, which is the loss of soil nitrate as nitrogen gas, mostly on saturated heavy soils in summer. The best strategy for growers is to concentrate on increasing nitrogen use efficiency by determining the optimum N application rates and timings rather than concentrating on specific losses. The sooner N is taken up by the crop then it reduces the potential for losses.
Dr John Angus, CSIRO, firstname.lastname@example.org
Plant Available Nitrogen GRDC Fact Sheet
International Plant Nutrition Institute
Tools for growers - Nitrogen calculators
Generic Yield and N Calculator – a general tool based on user inputs of growing season rainfall.
Your Soil’s Water Limited Potential Calculator – an automated calculator that estimates water limited potential grain yields and N fertiliser needs for farms with rainfall less than 500 millimetres.
N Budget Calculator – a simple-to-use calculator designed to help farmers and their advisers in Australia’s northern grains region, but it does have application in the southern region.
GRDC Project Code
CSO198, CSO209, CSO212, MF001, DAV00095