One size does not fit all for N use

By Charlie Walker, agronomist, Incitec Pivot Ltd

In the past it may have been simple to consider only two options for nitrogen application - pre drill and forget about it, or wait until the crop is growing and see what it needs.

Today, research indicates every crop, every paddock and every situation needs assessment.

Therefore, a tactical nitrogen management program that accepts “one size does not fit all” may provide dividends.

Tactics are determined by the whole season, not just what occurs post-sowing. In this context, nitrogen should be viewed as a resource that needs managing.

In winter dominant rainfall zones “effective” summer/autumn rain (events that wet soil below evaporation depth) may warrant delaying part or all of nitrogen requirements until post-sowing.

Where warm topsoil stays wet for extended periods mineralisation of nitrogen occurs at 1-2.5 kg/ha/day. The key is carbon-rich topsoil stays wet.

When there is little or no insulating barrier to aid this do not expect much nitrogen to mineralise.

Post-2002 season was a good example. In many areas light stubbles gave little protection against evaporation, which was compounded by heavy grazing or the cutting of hay. Where effective summer rainfall fell in the summer 2002-03, cultivation was often the best way to put a protective skin over the stored subsoil moisture. Where this did not occur, post-drought residual nitrogen was lower than expected.

This, of course, is not always the case - retention of moderate to heavy stubbles help to mitigate the effects of evaporation at the soil surface.

When the lead-up to cropping is dry with a later break, often the best approach is to apply significant nitrogen at or close to sowing, taking care with the application method. This is particularly so when the prior crop was low in protein, implying it ran out of nitrogen before water.

The objective is to supply starter nitrogen to stimulate early root growth in dry, cooling soils.

It is apparent that in longer season high-rainfall areas, nitrogen often performs best when applied later as the higher organic matter in the soils mineralise nitrogen faster; longer seasons provide more topdressing opportunities and heavier stubbles tend to aid in the mineralisation process.

In lower-rainfall regions the reverse may be true, with wide row banded nitrogen at sowing giving similar results to late or split application without taking top dress risks.

Nitrogen management requires having a feel for the supply of nitrate nitrogen at the root zone. This relies on selective soil measurements and some assumptions and, possibly, further measurements as the season progresses.

Growers must understand plant needs. While early growth is high in nitrogen, the small proportion of bulk means most nitrogen is required from fully tillered stage onwards. This is the period when cereals have an active root surface to intercept mobile nitrate-nitrogen. It then makes sense to apply nitrogen in this window to give the highest efficiency of uptake, especially in more reliable rainfall areas.

A key factor in getting nitrogen management right is plugging gaps with other nutrients. In our environment one of the greatest potential gaps is phosphorus, particularly when a crop is grown in a short season.

Recent research in Victoria, NSW and Queensland shows maximum benefits of high nitrogen status best exploited with adequate phosphorus supply.

Method & timing of N application

* This is an excerpt from a paper presented by Charlie Walker to the GRDC Grains Research Update, Southern Region, earlier this year. The full paper is available at www.grdc.com.au/growers/res_upd/south/04/walker.htm

For more information:
Charlie Walker, Incitec Pivot Ltd, 03 5279 4101, charlie.walker@incitecpivot.com.au

By Charlie Walker

The advent of durum wheat in South Australia and prime hard in southern NSW has raised the question, “Can I top dress late and increase my protein to cash in on these grades?” The answer is yes but guidelines govern the ability to lift protein.

The crop has to be set up for “critical protein” by boot stage. Critical protein is the point at which the addition of nitrogen will no longer increase yield, rather it will increase protein. Critical protein for wheat is usually in the order of 11 to 12 percent. In high-yield years, I have often conducted trials and seen commercial examples of where topdressing at flowering has resulted in yield increases (see table below left). Crops where this occurs have been destined for low protein (8.5 to 10.5 percent) before topdressing. The result with such crops is simply to increase yield by a smaller amount than if nitrogen was applied earlier, with negligible protein increases.

The next guideline relates to the water rules. Rarely will late topdressing work if there is not a significant supply of plant-available-water under the crop. It is more critical at flowering because crop water use has increased to 4 to 7 mm a day. Picking a rain event is also important, not only to rapidly move nitrogen into the root zone, but because volatilisation losses of nitrogen from urea increase under warmer conditions.

Topdressing late generally results in lower nitrogen-use efficiencies. Topdressing from 5-leaf to boot stage typically gives a 40 to 45 percent recovery of nitrogen fertiliser in grain.

At flowering, applications more commonly give recoveries of 30 to 35 percent. Given this, it is vital to ensure that there is a market for the higher protein grain and through diagnostics be confident that your nitrogen management has you on target. The main reason for lower recoveries is a regressing root system late in the crop, and nitrogen uptake under moisture stressed conditions. To grow a high protein crop by necessity, we need to run out of water before we run out of nitrogen.

Region North