Nitrogen Management In 2020
After consecutive poor seasons and tight margins, growers will need to carefully consider the need for additional expenditure on Nitrogen in 2020. As always, targeting the most economic yield rather than maximum yield will be less risky and more profitable. Following are the main factors to be considered when making Nitrogen application decisions.
Crop Removal
2019 saw a wide range in crop performance with many crops being grazed out, while others were cut for hay or harvested. Nitrogen removal from crops grazed out will be negligible, while approximate Nitrogen removal from grain is 25 kg N/tonne while hay is 50 kg N/tonne.
Paddocks with a long cropping history which were harvested or cut for hay, are likely to require higher rates of Nitrogen than those with a more recent legume history, or those which were grazed out.
Paddock History and N usage
Results from Deep Soil Nitrogen testing, suggest that legume based pastures and legume crops continue to provide soil Nitrogen benefits for up to five years down the crop rotation. Such paddocks generally have the ability to respond to favourable seasonal conditions, being less dependant on applied Nitrogen. Application rates on these paddocks can be more conservative, as they have the ability to mineralise Nitrogen in spring if there is adequate moisture.
Nitrogen application rates in 2019 were variable, with some growers applying nil Nitrogen while others applied their standard rates to all paddocks. Consideration of these rates as well as crop removal and grain protein, should be factored in when determining application rates for 2020
Mineralisation
Depending on soil characteristics, Nitrogen can be mineralised in warm moist soil at a rate of 1kg N/ha/day. With good rainfall received throughout autumn, 50-60 kg N/ha could have been mineralised since summer, the equivalent of 110-130 kg/ha Urea.
This mineralisation will continue throughout the growing season, albeit at a slower rate once soil moisture or temperature decreases.
Paddocks with a history of legume pasture and to a lesser extent legume crops, will have a higher potential to mineralise Nitrogen throughout the soil profile. In this case, the crop is able to access Nitrogen from the same point in the profile from which it is accessing moisture, thus decreasing reliance on applied Nitrogen.
Realistic Yield Targets
Nitrogen applications should be matched with expected crop requirements. However, determining potential crop yields at this very early stage of the year is almost impossible. Growers should look at their own long term yield results, to determine what their most likely yield outcome will be. Some growers apply excessive levels of Nitrogen to make sure that there is no yield penalty in above average years. However this has the potential to increase Nitrogen losses and will incur additional costs which translate into increased business risk. A more targeted approach may be to set the crop up for 10-15% above the long term average, then re-assess during August / September in the case of cereals.
Deep Soil Nitrogen Testing
The only accurate way of determining Nitrogen availability is through a Deep Soil Nitrogen (DSN) test. Many clients have been conducting DSN testing during the past four years, which has proven to be a very good investment when conducted early in the season.
Nitrogen application decisions will need to be made on a paddock by paddock basis. In general it is expected that most paddocks should have robust levels of soil N, however paddocks which were cut for hay in 2019 may require extra N applications to offset high removal.
The article in the following GRDC link provides some useful data: