Nitrogen at 40 to 100 kg/ha (and sometimes more) is standard practise for many cereal and canola crops, including dual purpose and grazing-only ones. Such use commonly also return big profits, despite fertiliser cost, if carefully assessed and tailored to likely crop needs.
Many agronomists use simple computer or apps programs to calculate crop nitrogen requirements. These base recommended nitrogen rates on several factors such as estimated likely yield, pasture and crop legumes as part of the rotation, top and subsoil test results and likely in-crop nitrogen mineralisation rates. Research backs the logic and generally sensible recommendations these programs provide although interpretation based on experience and local knowledge adds to their value.
Grain-only crops take far more nutrients, including nitrogen, out of the paddock via grain harvest than grazing only ones. For example a 5.0 t/ha 11 per cent protein wheat crop removes almost 100 kg/ha nitrogen via grain harvest. A 3t/ha wheat crop at the same protein removes almost 60 kg/ha nitrogen.
A grazing winter cereal crop sown early and providing for example 6.0 t/ha grazing (drymatter basis), while requiring around 200 kg/ha nitrogen to produce that feed only removes a fraction of that via meat and wool produce. The majority of the nitrogen is returned to the paddock via trampled material, dung and urine.
However for the current season it can take some time for the returned-to-paddock nitrogen (as part of protein or other nitrogen forms) to convert to nitrate nitrogen, the form most taken up by plants.
Therefore the reason why good grazing production from dual purpose or grazing only crops also requires high soil nitrogen.
What is interesting is that grazing-only crops grown consecutively on the same paddock commonly see a gradual building of soil nitrogen fertility because of the low nitrogen removal rates via product taken off the paddock.
Nitrogen being such a mobile nutrient however is also quite vulnerable to loss from a paddock in certain circumstances.
For example a very wet winter may result in parts of paddocks waterlogging for variable periods.
A lot of denitrification can occur with losses of nitrogen to the atmosphere. In well-drained soils in wet conditions also nitrogen can leach out of the root zone, or at least to lower root layers.
Hence nitrogen build up via legumes and fertiliser applied to previous crops and not fully used by them may not always be what one might have estimated it to be. Therefore the value of soil tests as part of the decision making tools.
Soil test limitations for nitrogen include its mobility of forms with nitrate being the main crop-available form and the most useful test information.
Soil testing close to sowing is a more reliable indicator than one taken months earlier (in contrast to phosphorus where timing is not so important).
Many factors affect nitrate test results.
Nitrogen mineralisation (conversion of various nitrogen forms to nitrate, the main crop-available form) is highest when soil is warm and moist, and lowest when dry and cold.
If weeds or self-sown crop are allowed to get away in the fallow a lot of nitrogen can be tied up and not available to the following crop.
Even after a lucerne pasture phase, where a lot of nitrogen has been “fixed” for future crop use, available nitrate form can be low for the first post-lucerne crop if mineralisation conditions have been poor. Clearly making sound nitrogen fertiliser decisions is complex.
Next week. Carefully select a range of annual legumes for paddock and seasonal variability.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
