Northern group finds nitrogen may be banked

Positive signs for soil nitrogen levels despite dry conditions


Farm Business
A sorghum crop grown over the 2018-19 summer at NGA's trial site at Macalister on the Darling Downs. PHOTO Northern Grower Alliance

A sorghum crop grown over the 2018-19 summer at NGA's trial site at Macalister on the Darling Downs. PHOTO Northern Grower Alliance

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Northern Grower Alliance trials indicate winter crops may have access to soil nitrogen.

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Results from ongoing GRDC-invested research by the Northern Grower Alliance (NGA) into crop nutrition are reinforcing confidence in the fact that losses of previously applied nitrogen are unlikely in the dry conditions being experienced in most of the northern region.

NGA communications manager Rachel Norton says results from three ongoing NGA trials established to evaluate nitrogen management in wheat showed high levels of additional nitrogen remaining after fertiliser treatments in the wheat phase.

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"These sites have continued to be monitored to evaluate the impact of this carryover nitrogen on subsequent crops," Ms Norton says.

The sites were soil tested following the 2017 winter-crop harvest and again in late spring 2018, prior to planting of commercial sorghum crops.

"The key message is that the levels of additional carryover nitrogen from fertiliser application were equivalent at both samplings," Ms Norton says

She says the nitrogen applied for cropping in 2018, but largely unused due to ongoing dry conditions, is a little bit like money in the bank.

"Once in the soil, it is generally safe, and remains there until withdrawn, but it doesn't generate much value until it is withdrawn and utilised."

The NGA's findings show that although losses of nitrogen from the system are unlikely following the dry conditions, it is important to determine the depth at which nitrogen is located.

"Soil moisture levels are currently very low and many crops will be primarily grown on in-crop rainfall in winter 2019," Ms Norton says.

"If most of the nitrogen is located deep in the profile in dry soil bands, it may be inaccessible to crops unless soil moisture can be replenished to similar depths."

Slow movement

The NGA project has found the movement of applied nitrogen in the soil profile has been slower than expected at all sites, which have been predominantly high-clay-content vertosols on the Darling Downs and in northern NSW.

Once in the soil, it is generally safe, and remains there until withdrawn, but it doesn't generate much value until it is withdrawn and utilised. - Northern Grower Alliance communications manager Rachel Norton

"In trials conducted from 2014 to 2017, we deliberately applied nitrogen to dry soil profiles in an attempt to maximise the distribution of nitrogen as the soil moisture was recharged," Ms Norton says.

"Despite this approach, the majority of fertiliser nitrogen applied was still found in the top 15 or 30 centimetres shortly after harvest at all sites.

"Trials results have shown us that moving the nitrogen deep enough in the profile in the first season after application can be a big challenge."

Wheat growing in an NGA trial site at Mullaley. PHOTO Northern Grower Alliance

Wheat growing in an NGA trial site at Mullaley. PHOTO Northern Grower Alliance

Ms Norton says trial results drive home the fact that although nitrogen is mobile, it can take a lot of moisture and time to move to the soil depths where it is most accessible later in crop growth.

"There are two key messages from this work for 2019 crops," she says.

"The major one is that the losses of unused fertiliser nitrogen applied in 2018 are likely to be low following the dry conditions, and the second is that although nitrogen is mobile, it can take considerable time and rainfall to move in our high-clay-content soils."

GRDC Research Code: NGA0004

More information: Rachel Norton, 0447 924 917, rachel.norton@nga.org.au

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