Nitrogen fertiliser ready to be applied to a sugarcane field in far north Queensland.

Towards sustainable fertiliser use in the sugarcane industry

CSIRO is developing an approach to reduce fertiliser use in the sugarcane industry and minimise environmental impacts on the Great Barrier Reef, without compromising yields.

• 22 August 2007 | Updated 14 October 2011

• Introduction
• What is N-Replacement? 
• On-farm field trials
• Initial results
• Cane yield and sugar levels
• Environmental run-off
• Long-term soil productivity
• Farm profits
• Next steps
• Funding

Introduction

Nitrogen fertilisers are used by the sugarcane industry to promote plant growth and high sugar yields. Whilst much of the nitrogen applied to fields is taken up by the sugarcane crop, a percentage leaches through the soil into groundwater tables or nearby waterways.

Nitrogen (N) fertilisers have been identified as a significant source of diffuse pollution in catchments adjacent to the Great Barrier Reef.

To support long-term sustainability, the sugar industry is seeking new ways to maintain productivity while minimising the environmental impacts of nitrogen fertiliser use.

CSIRO and sugar industry partners have been trialling a new approach to the use of nitrogen fertilisers in sugarcane farming known as ‘N-Replacement’. Data gathered since 2004 provides the first field evidence that N-Replacement has the potential to considerably lower the risk of environmental N losses, while maintaining sugar yields and farm profits.

What is N-Replacement?

N-Replacement is a method for calculating how much fertiliser to apply to a sugarcane crop each year.

N-Replacement aims to maintain a stable balance of nitrogen in the farming system, by putting back only what is taken out. It involves balancing the amount of N fertiliser applied to a crop against the amount of N removed from the field by the previous crop, plus the amount of N lost to the environment during the last season.

On-farm field trials

Since 2004 CSIRO has conducted field trials in eight regions from the Wet Tropics in far north Queensland to northern New South Wales. 

At all sites, the N-Replacement concept was compared with the farmer’s conventional N management, and at some sites, with a very low rate of N fertiliser.

With the N-Replacement concept, fertiliser is applied at the rate of 1.0 kg of N per tonne of cane harvested in the previous crop (for green cane trash blanketing systems) or 1.3 kg of N per harvested tonne in burnt systems (the extra allowing for N losses in burnt trash).

Initial results

So far data has been analysed from 12 field trials run since 2003.

Measurements were taken for two or three seasons at each site with a total of 25 crops being analysed.

Overall, the N-Replacement approach used an average of 60 kg less nitrogen per hectare than conventional application rates, an average saving of 30 per cent.

Cane yield and sugar levels

Sugarcane yields differed little between different N treatments at each site.

Overall yields with N-Replacement were slightly (0.3 tonnes of cane per hectare) lower than the conventional N treatment.

Where there were more marked differences in yield, the difference was still not statistically significant.

The level of commercial cane sugar (CCS) in the crop was, overall, slightly higher (0.1 unit) using the N-Replacement approach compared to conventional N treatment.

Environmental run-off

The amount of nitrogen applied as fertiliser minus the amount removed in harvested cane (and burnt trash where appropriate) is referred to as the ‘N surplus’.

The N-surplus is a basic indicator of environmental losses, since nitrogen applied in excess of crop removal is available to be lost to the environment.

Estimates from the field trials are that the average N surplus was 80 percent lower using N-Replacement than for conventional N management.

Long-term soil productivity

Nitrogen is stored in the soil in both organic and mineral forms.

It is possible that sugarcane plants may compensate for a reduction in nitrogen fertiliser by 'mining' nitrogen from the soil.  This could lead to a decline in soil N stores, which would gradually reduce the productivity of the soil.

To check whether N- Replacement risks long term soil degradation, the level of soil mineral nitrogen (SMN) was measured immediately after each harvest. There was no evidence that N-Replacement was causing SMN to decline.

Farm profits

Farm profitability was generally maintained with N-Replacement. 

Sugarcane yields and CCS levels using N-Replacement were generally similar to conventional N treatments, while nitrogen fertiliser use (and therefore cost) was usually lower.

Next steps

If widely adopted, the N-Replacement method could significantly reduce nitrogen losses to the environment, and reduce farmers’ fertiliser costs without compromising sugarcane yields.

While these results are promising, there are still uncertainties associated with the N-Replacement approach, and further research is needed. For example:

• testing needs to continue for a longer time to ensure the method is sustainable
• assumptions regarding cane N concentrations and environmental losses need to be further investigated
• practical tools must be developed to enable cane growers to easily and effectively implement the N-Replacement approach when managing fertiliser on their own properties
• easy means of monitoring for N stress in sugarcane would help growers maintain their productivity

Research is underway into innovative means of monitoring for N stress using Near Infrared Reflectance (NIR) instruments within sugar mills.

CSIRO will continue working with the sugarcane industry and the Sugar Research and Development Corporation to further develop and test the N-Replacement concept.

Funding

This project is funded by the Sugar Research and Development Corporation in partnership with CSIRO. The researchers also wish to thank the growers and producer discussion groups who participated in the field trials.

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