Curbing your N-thusiasm

Losses of Nitrogen continue to be a major environmental issue across agriculture. Can controlled-release fertilisers help banana farmers achieve best-practice targets?

By Jeff Daniells, John Armour, Christina Mortimore and Stewart Lindsay

Targets set in the Reef Plan 2050 to improve water quality in the Great Barrier Reef include a cut of at least 50 per cent to dissolved nitrogen loads in priority areas by 2018 compared with a 2009 baseline figure.
Is a reduction in nitrogen losses of this magnitude possible for bananas in the wet tropics?
In the last edition of Australian Bananas (Issue 44, 2015) we looked at the need to better manage naturally-occurring nitrogen found in the top 60 centimetres of soil in the plant crop.
Our research outlined three ways to minimise nitrogen leaching following planting – later applications of nitrogen fertiliser, scheduled planting and strategies such as permanent beds.
This article looks at minimizing nitrogen losses in the ratoon crop. The data sets from this trial into leaching of nitrate-N and the Paddock to Reef Program show that the required reduction in losses can be achieved in ratoons by implementing nitrogen best management practice as we understand it.
Good alignment
Leaching of nitrate-N below the roots was low at one to seven kilograms per hectare annual (kg/ha/year) in the ratoon crops.
We attribute this to good alignment of fertiliser N and plant demand, as well as the presumably more extensive root system for N uptake in ratoon crops. The moderate N targets (250 kg N/ha/crop cycle) we set are an important component of efficient N management. The higher N rate of 350 kg did not increase yield and greatly increased leaching losses to 19 kg/ha/year.
In particular, losses can be reduced by setting a usage target of about 300kg N/ha/year and applying nitrogen at least every two to four weeks via fertigation. Broadcasting at these regular intervals on the rows (not the inter-rows) with banana trash cover present also gave comparable results.
At the low N application rates used in our trial there was little evidence of a benefit from controlled-release fertilisers in reducing loss of N in deep drainage.
Nevertheless, these fertilisers would have other benefits including:
  • ensuring the crop receives adequate nutrients during the extended wet season
  • having the potential to reduce the number of actual fertiliser applications required per year .
In relation to the first point, during the wet season, standard forms/methods of fertilising, such as fertigated or broadcast fertiliser, can be very difficult to apply on most plantations. Controlled-release fertilisers would be applied as one application prior to the commencement of the wet season.
On the second point, the reduction in applications would have the benefit of reducing labour costs and machinery movement and enhancing convenience.
Future R&D
On-farm development and extension of these and previous project findings is required to promote industry adoption.
Further development and release of decision-support software would help growers to further Improve nutrition management.
Controlled release N fertilisers also have the potential to reduce losses of nitrous oxide (a major greenhouse gas) by minimising denitrification – a common occurrence in the wet season. Research is underway at East Palmerston to measure gaseous losses.