- A Nitrogen (N) budget that estimates plant demand for N and the soil’s capacity to supply N is useful when developing fertiliser N recommendations.
- Nitrogen use will be more efficient if applied in smaller split applications rather than in one large amount.
- Soil nitrate and plant tissue testing are useful management tool for accurately meeting a crops’ nitrogen requirements.
- Leaching losses of fertiliser applied N can be significant in Tasmania.
for Farming in Tasmania - Bill Cotching
Nitrogen (N) is essential for all plant growth processes and plants need it in relatively large quantities. In plants, it is a key element in amino acids, proteins, enzymes and chlorophyll. In pasture systems where N fertilisers are not applied, such as in extensive sheep and cattle enterprises, the clover component of pastures is the main supplier of N. N is also supplied to the soil through the symbiotic relationship that legumes (clovers, beans, pulses) form with a host-specific strain of bacteria known as rhizobia. Free living nitrogen fixing bacteria (cyanobacteria and genera such as Azotobacter, Beijerinckia, and Clostridium) supply some nitrogen to plants where N is not oversupplied via fertilisers.
On a soil test result there may be three measurements for N. Total N measures the total amount of N in the soil. Most of this N is contained in organic matter (plant residues, insects, fungi, bacteria, etc.) and is unavailable to plants in the short term. This bank of organic N is decomposed by soil microbes to produce plant available nitrate-N (NO3-) and ammonium-N (NH4+). Soil temperatures over 10oC are needed for effective mineralisation and uptake of N by plants. Nitrate-N is highly mobile in the soil solution so that it is readily available for plant uptake, but it is also readily leached in high rainfall/irrigation situations. Soil nitrate levels tend to fluctuate widely, depending on the season and rainfall. The application of synthetic nitrogen fertilisers has led to off-site environmental concerns resulting in regulations of nitrogen fertiliser application in Europe and New Zealand, and only in catchments that flow to the Great Barrier Reef in Queensland, Australia. In these catchments, a nutrient management plan must be developed for the whole of farm, block, or management zone prior to applying fertiliser on the agricultural property and maximum annual nitrogen and/or phosphorus application rates are prescribed (Queensland Government 2022).
Soil nitrogen testing
Estimating a target range for soil N and potential crop requirements of N during the growing season requires local knowledge and some experience with the local soils, crops and climate that is best obtained from your local agronomist.
Under perennial pastures when soils are dry, usually in January–February, samples for soil testing can be collected to indicate whether a response can be expected in the autumn. This is because some of the soil N is in the organic matter and is normally broken down to plant-available forms in response to increasing temperatures. If a paddock has not had an actively growing pasture or crop during summer, the available N is normally accumulated in the soil profile and autumn rainfall results in a typical flush of grass growth with responses to N fertiliser application in the autumn often being lower than expected. Soil testing in those situations can save money in N fertiliser applications. Even though pasture growth responses are lower in autumn than in spring, autumn N applications can build a feed wedge going into winter and be profitable due to the expensive feed the resulting pasture can replace.
In Tasmania, many grass-legume pastures are grown for several years before being ploughed, and so should accumulate more N than a cover crop does. An immediate history of grass/clover (usually Lolium spp. / Trifolium spp.) pasture was associated with a lower N requirement (48 kg/ha) compared to that of continuously cropped paddocks (193 kg/ha) (Sparrow and Chapman 2003a), because pasture residues provided more mineral N than did residues of crops (Sparrow and Chapman 2003b). Potato crops grown after pasture may therefore need only modest rates of N, say 50-100 kg/ha Sparrow et al. 2012).
Soil nitrate testing is a useful management tool for accurately meeting a crops’ nitrogen requirements and not wasting fertiliser (and money) by applying N when it is not required. Depending on the crop to be grown, a soil nitrate value of less than 25 kg N/ha may be low, and more than 100 kgN/ha may be high (Blaesing 2017). If sowing a crop such as poppies early, don’t do a soil nitrate test at sowing, as the crop won’t need much nitrogen at this time, and there will be a low efficiency of use of any fertiliser applied as the soil will be too cold and wet. If sowing later, then a soil nitrate test before sowing will be useful, as the plants will be growing more actively, and the soil will be warmer and drier and provide a more nitrogen friendly environment. If there is high soil nitrate at planting, don’t apply any N fertilisers until the crop commences growing vigorously. This management principle applies to all crops, but it still is common practice to apply relatively high amount of N at planting. This can be a waste of money and an environmental issue as a large proportion of the N applied at planting may be lost via leaching or as gaseous losses.
For poppies, a soil test should always be done at early running up, before fertilisers are applied (if you only do one test this should be it), and the soil nitrate at this time should be about 100 kg/ha. Another test a few weeks later (at hook) to make sure there is enough N to see the crop through, can also be useful. A nitrogen buffer in the soil of between 50 –80 kg/ha, at the end of the season is recommended. Similar approaches can be taken for other crops, i.e. test a soil sample just before the crop is entering a major growth phase and top up nitrogen depending on the results. A soil sample can be taken before planting to determine pre-planting or at-planting N rates, especially for crops planted in summer and autumn/winter, to determine residual N left over from previous crops. This approach is taken in countries and regions where N fertiliser use is regulated.
Management of nitrogen
One method to minimise N leaching losses under mixed cropping systems in Tasmania is for farmers to monitor residual soil N before crop planting and to use a nitrogen budget when determining fertiliser rates for following crops in the rotation. Practicable management options, such as deficit-based irrigation and reduced N fertiliser rates, as well as split applications of N fertilisers, were found to have the potential to generate significant financial savings via reduced input costs as well as reducing potential offsite N loss, while maintaining current levels of productivity (Lisson and Cotching 2011).
Some important points relating to N management under crops (Sherriff 2005) are:
- Nitrogen and water must be in the soil when they’re needed, e.g. when crops begin growing vigorously.
- There are big losses of N over the winter (even organic N from green manure) – you need to try and reduce these losses through careful management. Make sure soil nitrate levels aren’t too high at the end of the season by using a nitrogen budget and monitoring soil N levels, and if possible, grow a cover crop to ‘catch’ the nitrogen over winter and reduce erosion losses.
- Nitrogen use will be more efficient if split into a few smaller applications according to needs at different growth stages (no more than 70 kg N/ha at any one time) and not one large application, especially during early crop growth stages.
- Think about the environmental conditions (soil temperature) as well as price before you decide which nitrogen fertiliser will be the most efficient and cost effective to use.
- Starting the season with a low soil nitrate level can be useful as then the nitrogen availability can be manipulated to suit the crop.
Only apply N when pasture is actively growing, i.e. soil moisture is adequate and not drought stressed but not waterlogged, and other major soil nutrients are non-limiting.
- Do not apply N less than 21 days apart for applications of 30 kg N/ha in spring or 28 days apart for applications closer to 50 kg N/ha.
- Apply 20 – 50 kg N/ha per application (1 – 1.5 kgN/ha/day depending on growth rate). The general recommendation is to apply urea fertiliser to perennial ryegrass pastures within a window from three days before grazing to 3–4 days after grazing.
- Apply a maximum of 180 - 200 kgN/ha/yr to optimise the consumption of home-grown feed.
- Continue to graze ryegrass pastures at the recommended 2.5 leaf stage. The N does not change the leaf emergence rate of ryegrass, it only increases the size and weight of each leaf. Grazing too early does not fully capture the dry matter growth response.
- If applying N fertiliser before grazing, grazing should not be delayed to a point where it could fall into the nitrate poisoning high-risk window (7 to 14 days after application. Excessive plant nitrate can cause nitrate poisoning in livestock. This can occur with brassicas, stressed annual ryegrass, short-rotation ryegrass or oats. Toxicity problems are normally related to situations when the cows are unusually hungry. Changes in other components of the diet, particularly reductions in energy supplements or lack of gradual adaptation to changes in the diet, predispose the animals to nitrate toxicity complications.
- Avoid high rates of N on wet soils.
- Urea is the cheapest source of N. Di-ammonium Phosphate (DAP) is a cost-effective source of N, if the P is needed at the same time.
- In Tasmania, apply a blend of urea/sulfate of ammonia in winter due to low available S.
Urea (46% N) is considered to be the cheapest form of N and can be applied by top dressing or directly with the seed when sowing a crop. When using urea, it is important to control volatilisation losses. If applied incorrectly, up to 40 percent of the nitrogen applied as urea can be volatilized (vaporized) and lost as a gas. If applied correctly, little nitrogen will be lost. The urea acidifying potential is relatively high. Microbial activity is required to make urea available to plants. Sulfate of ammonia (20% N, 24% S) is a more expensive source of N but can be used when sulfur is also required. SOA’s acidifying potential is also relatively high. Di-ammonium phosphate (DAP - 18% N) or Mono-ammonium phosphate (MAP - 10% N) are often applied when phosphorus is also required. In horticulture crops, nitrate fertilisers are often used, especially in split applications, because they provide better control over available N levels (and potentially less losses) than urea or ammonium-based fertilisers. Nitrate fertilisers commonly used include calcium nitrate (with or without boron), potassium nitrate and magnesium nitrate, to also supply a second nutrient. In crops that are fertigated, liquid urea ammonium nitrate (UAN) is a fertiliser that is often used because it contains different N sources which potentially make nitrogen available over a longer time frame.
Potential farm N losses
When applying N as fertiliser, there is the risk that excess nitrates are either lost as runoff into waterways or accumulate in the soil to be leached into the groundwater. The fate of nitrogen and water in mixed cropping/pasture systems in Tasmania is complex and influenced by a wide range of inter-dependent factors including seasonal climate, chemical and physical soil properties, crop management practices, crop sequence and the levels of residual nutrients and water carried over from previous crops via soil and surface residues.
N fertiliser rates on potatoes ranged from 279 to 480 kg N/ha across a study of six farms growing potatoes in northwest Tasmania (Lisson and Cotching 2001). These rates were the highest of any crop, and not surprisingly potatoes had the highest estimated N leaching (29 kg N/ha/year), four times more than estimates for the other crops (poppies, cereals and other vegetables, and pasture). Across all six farms and years there was an average excess of N supplied as fertiliser over N uptake in potatoes of 89 kg/ha/yr. The potential exists for substantial seasonal N leaching losses from all elements of crop rotations, but potatoes can be the ‘leakiest’ of all crops with up to 141 kgN/ha potentially lost via leaching in northwest Tasmania (Lisson and Cotching 2011). These potential N losses can be prevented by using a nutrient budget, applying split N applications and adopting scheduled irrigation.
Calculating the agronomic nitrogen use efficiency (NUE) can help to review whether a nitrogen program has been ‘leaky’ or matched crop/pasture demands. Agronomic nitrogen use efficiency (NUE) is a measure of the amount of nitrogen (N) taken up by a crop/pasture and removed via harvest or grazing compared to the amount applied. It is an important indicator of environmental sustainability and economic efficiency in crop production because it shows the relationship between N inputs and crop/pasture yield.
Sandy pasture soils used for milk production in high rainfall parts of Tasmania have been found to lose large amounts of fertiliser applied N in surface drainage (Broad and Corkrey 2011; Holz 2010). Rates of 27 kg N/ha/year were found, but this loss can be minimised by following the guidelines above. Elevated concentrations of nutrients in water draining agricultural catchments, particularly N and P, have been associated with environmental problems such as eutrophication and algal blooms in waterways.