[Photo: Charlie Walker is Production Inovation Manger, Incitec Pivot Ltd]
Split applications are a success. Charlie Walker looks at options when they are unsuitable.
The success of split nitrogen applications in winter rainfall and irrigated cropping systems is testament to the fact that crops do not need or use all of their nitrogen in one hit.
The move to two, and in some cases three, nitrogen applications a season has improved the efficiency of nitrogen fertiliser use, improved production outcomes and reduced risks in unpredictable seasons. We also know that in carefully managed drip-fed horticultural systems, frequent small applications of nutrient can provide even better results for high-value crops.
Obviously, this approach is not practical or economical for broadacre farmers. Indeed, in some broadacre systems and certainly in the summer rainfall winter cropping systems, even splitting applications using topdressing is risky at best.
So, in those areas where splitting applications is too high-risk to contemplate or where it is impractical, are there any options for tactical nitrogen management?
The main issue with urea, the most commonly used nitrogen fertiliser in broadacre cropping, is that it rapidly nitrifies in soils with healthy populations of nitrifying bacteria (within three to nine days) to provide a "pulse" of plant-available nitrate nitrogen once in the soil.
In response to this "pulse" of nitrate nitrogen, crops grow more rapidly and in doing so respiration or moisture- use escalates. This is fine when a crop is rapidly growing in the reproductive phase, but may not be best in terms of managing a canopy that optimises the use of its finite moisture resource. In recent years, we have seen that canopy growth can be managed by:
While proven to work in certain situations, topdressing presents both logistic and agronomic risks, such as spreader availability and the risk that follow-up rain does not occur. Mid-row banding requires specialist equipment.
In response, Incitec Pivot has been investigating a nitrogen stabiliser 3,4-Dimethylpyrazole phosphate (DMPP) developed by BASF. DMPP acts as an ammonium stabiliser by reducing the activity of the soil bacteria Nitrosommonas (responsible for the first step of nitrification) without affecting the activity of Nitrobacter (responsible for the second step of nitrification) or any other soil bacteria. It is applied as a light coating to conventional fertilisers, with no special handling equipment required.
Nitrifying bacteria is driven by moisture and temperature - early in the winter cropping phase when the break occurs and soil temperatures are still moderately warm. This activity may lead to the rapid evolution of a concentrated nitrate nitrogen pool in the developing root zone derived from nitrogen fertiliser and the breakdown of organic matter.
DMPP delays the availability of nitrogen fertiliser to the crop for up to six weeks. In theory, this will regulate the development of the crop canopy, reducing the chances of too much vegetative growth that cannot be supported during the reproductive phase.
DMPP shows promise in situations where it is desirable to apply all of the crop"s nitrogen fertiliser at, or close to, sowing; or in situations where topdress opportunities are limited.
Incitec Pivot"s trials across south-eastern Australia in 2004 highlighted the potential for DMPP as an ammonium stabiliser. Urea with DMPP improved yields and economic responses across the three sites (see Table 1). It was also noted that in drought situations where haying off was likely, using DMPP mitigated the downside of poor early nitrogen decisions by reducing screenings and yield losses associated with urea use.
Table 1: Effect of DMPP on combined wheat yield, protein and nitrogen (N) fertiliser recovery at Grenfell, Dookie, Mitre and Cummins in 2004.
Using an ammonium stabiliser such as DMPP can result in better nitrogen-use efficiency for a number of reasons:
Incitec Pivot will trial DMPP for one more season before commercial release.
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