Ground Cover Supplement : GC Supplement - More profit from crop nutrition II
7 Nitrogen 20 to 40 per cent of applied nitrogen is being lost into the atmosphere. FAST FACT commonplace because most paddocks examined yielded more than 80 per cent of their maximum yield without application of nitrogen fertiliser. He says this over-application is most likely 'insurance' to help meet the nitrogen demands of a crop and maximise its profitability in situations where yield potential increases because of better- than-expected seasonal conditions. Kingaroy-based Mr Crosthwaite typically recommends grower clients apply 25 to 40kg N/ha to winter cereals that are expected to yield between 1.5 and 2 tonnes/ha and that follow summer peanut or faba bean crops. Mr Crosthwaite estimates that legume crops trickle-feed 20 to 25kg N/ha to subsequent grain crops. His recommendation is consistent with the findings of Professor Bell's NANORP research, which showed growing winter legumes in rotation allowed for a 30 to 50 per cent reduction in applied nitrogen rates in the following sorghum crop. Using these reduced rates, sorghum yield was maintained and N2O emissions were halved. Further south at Orange, Mr Harbison helps growers to more accurately estimate likely yield: "Over- fertilising because the target yield has been overestimated can waste $80/ ha on nitrogen fertiliser," he says. "On the other hand, under-fertilising because yield has been underestimated can 'cost' 1 to 2t/ha. That missed grain yield could be worth up to $500/ha, money that would usually go straight towards an operating surplus." Mr Harbison says top-up fertiliser applications in August are generally 'make or break' in terms of achieving a return on total nitrogen investment in the Orange region of central NSW. However, he says the risk of this practice can be minimised by reassessing (visually and by tiller counts) the crop's yield potential just before applying the nitrogen top-up, combined with estimating the probability of that potential being realised based on soil moisture content and the seasonal outlook. If the soil is unlikely to supply enough nitrogen to meet the crop's needs, he then determines the most cost-effective way to apply the nitrogen fertiliser that is required. Because urea is the main form of nitrogen fertiliser used by grain growers, Mr Harbison recommends applying it before predicted rainfall events (measuring at least five millimetres) to help avoid gaseous nitrogen losses. Most soils have a pH (calcium chloride) less than 5.5, so the main alkalinity trigger for volatilisation losses is the initial hydrolysis of the urea; even then, that risk is low in Mr Harbison's grain-growing district. He says rain-washing the urea into the soil helps reduce nitrogen losses and enables it to become part of the soil nitrogen 'bucket' available for plant uptake. TIPS FOR GROWERS Mr Harbison does not recommend topdressing nitrogen where paddocks are waterlogged, or where they are wet and likely to become saturated with more rain: "It is a bad decision and an even worse investment. Nitrogen will be poorly utilised by plants. It will be very vulnerable to leaching and denitrification losses." This advice is aligned with Professor Bell's research: "Applying all nitrogen fertiliser in a single application, especially for sorghum, well in advance of the crop's demand for it, increases the likelihood of denitrification after heavy rain," he says. "However, given the lack of ability to apply in-crop nitrogen in northern regions, other management options are more important for reducing nitrogen losses. Because banding of nitrogen creates a zone of high soil nitrogen that is susceptible to denitrification, using narrower band spacings -- more bands per hectare -- results in lower in-band concentrations of mineral nitrogen, which may be a way to reduce the risk of potential nitrogen losses." Professor Bell says applied nitrogen losses can also be reduced by substituting some nitrogen fertiliser with slower- mineralising nitrogen derived from legumes. Legumes provide a more constant supply of nitrogen to subsequent crops than the feast/famine scenario resulting from nitrogen fertiliser applications. There is also generally less stubble after legumes compared with grain crops, and lower labile carbon content in the soil may also help reduce nitrogen losses. Professor Bell's NANORP work further tested the potential for several enhanced efficiency fertilisers to minimise nitrogen losses and improve NUE in cropping systems. Urea coated with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP), called ENTEC®, was found to be the most effective in achieving one of these objectives. Trials showed it reduced N2O emissions by 50 to 80 per cent, particularly at high nitrogen rates, but it did not significantly improve NUE. Any reductions in nitrogen losses or yield increases were not enough to justify the additional cost of using DMPP-coated urea compared with conventional urea. This is because plants were already 'well lubricated' with high rates of nitrogen fertiliser, so while DMPP-coated urea can lower emissions, any increase in the total amount of nitrogen available to plants is unlikely to affect yield because the crop already has an ample supply of soil nitrogen. □ G RDC Research Code DAF00004 More information: Ian Crosthwaite, 0409 622 319, firstname.lastname@example.org; David Harbison, 0408 820 467, email@example.com; Professor Mike Bell, 07 4160 0730, 0429 600 730, firstname.lastname@example.org; Dr Graeme Schwenke, 02 6763 1137, 0418 636 421, email@example.com PHOTO: BRAD COLLIS The University of Queensland’s Professor Mike Bell led nitrogen management trials in Queensland and New South Wales through NANORP.
GC Supplement - Spray application 2016
GC Supplement - Pulse breeding advances