Nitrogen Fertilisers
Application of nitrogen fertilisers can help accelerate feed recovery after a drought, either on pasture or crops.
But how much should you use? In what form and when? What other considerations should be taken into account?
For a start, let's look at some background.
Most nitrogen in the soil occurs in organic matter, while the plant-available nitrogen occurs in the inorganic form (ammonium or nitrate) and is usually less than 1% of total soil nitrogen. Low levels of plant-available nitrogen generally limit grass growth in autumn, winter and early spring, when mineralisation (breakdown) of organic matter is reduced with cold conditions. This coincides with the period of the best responses to nitrogen fertiliser, but is determined by temperature and moisture.
The response in New Zealand to nitrogen fertiliser under cool moist conditions depends on the growing conditions, not the form of fertiliser, so choose the cheapest form for the situation. Urea is the cheapest form of nitrogen, but if phosphorous or sulphur is also needed, other fertilisers such as DAP or sulphate of ammonia or mixes may be used. Water-insoluble forms (blood and bone, fishmeal etc) are inferior on pastures but may be comparable on crops. These have to be broken down to plant-available forms before plants can take them up, so are best incorporated into the soil.
During a drought, there is slow breakdown of organic matter and addition of urine to the soil, but no leaching in rainfall so nitrogen levels will be quite high. Nitrogen fertilisers can be used to increase those levels so growth is higher during the autumn and spring in cold areas, and winter in mild areas.
Losses from nitrogen fertilisers can occur by volatilisation, or leaching below the root depth. Volatilisation is highest from urea and negligible from sulphate of ammonia or calcium ammonium nitrate. Gaseous losses are worst at high levels of application and under hot windy conditions. Losses of less than 15% are likely when nitrogen is applied at less than 50 kg/ha, there is pasture cover, and rain falls within 24 hours of application.
- Application timing depends on when the additional feed is required and the ability of the pastures to respond. Nitrogen used in autumn can increase the amount of feed to carry over the winter and benefit early spring growth. In late winter/early spring the potential for growth exceeds the rate of mineralisation, so short-term responses are high.
- The best responses occur on dense grass dominant pastures with a reasonable leaf cover, which is about 50 mm high, or ryegrass or cereal greenfeed.
- Losses are less with soil incorporation so it should be drilled with greenfeed if possible.
- The ideal rate of application is 25-30 kg N, depending on application methods (50-65 kg urea/ha) but rates of up to 60 kg N/ha can be used.
- Urea is the cheapest form of nitrogen, but sulphur or phosphorous may be needed as well.
- The nitrogen-boosted feed is best fitted into the grazing rotation, rather than being spelled for a long time.
- Soil temperatures should be above 7ºC (10 cm).
- Ideally, rain should follow application.
- Nitrogen fertilisers have a small effect on the pH when used at normal rates.
- Nitrogen may increase nitrate levels in brassicas when grown on high fertility soils and the crop would need testing for nitrate poisoning.
- Nitrogen can be used on hay or silage crops but should be applied when there is adequate soil moisture.
- Nitrogen with new grass can help with pasture establishment. Normally 30 kg N/ha is sufficient but good responses can be obtained up to 60 kg N/ha.
- As long as temperature and moisture conditions are adequate, you can expect a response of between 5 and 15 kg DM/kg N. Budget on 10 kg DM/kg N/ha. The cost per kg DM produced will be between 7-10 cents/kg, usually cheaper than the alternatives.
- Another benefit to N application is that low rates (15 kg/ha urea) can increase feed quality both metabolisable energy and protein. Drought pastures have been measured as having very low ME (7.0), which is worse than hay.
Ken Muscroft-Taylor
Agricultural Consultant
Agriculture New Zealand
Darfield
Contact for Enquiries
Manager
North Island Regions
Sector Performance Policy
MAF Policy
Hamilton
NEW ZEALAND
Phone: +64 7 957 8313
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