New research has drawn attention to the need to better manage naturally-occurring nitrogen found in the top 60 centimetres of soil during the crop establishment phase. Jeff Daniells, John Armour and Stewart Lindsay report.

A Reef Rescue project investigating different slow release nitrogen fertilisers was confronted with major leaching losses in a plant crop which were unrelated to the different fertilisers being applied.

High levels of freely available nitrogen present in the soil at planting were the culprits.

Benefits for growers from better managing this free nitrogen at planting time will include fertiliser savings and reduction in the various losses of nitrogen to the environment.

The Reef Rescue research has been conducted at South Johnstone Research Station. Additional research has also been conducted at two North Queensland banana farms which have been looking at the effects of sown ground covers and new nitrogen management practices. The South Johnstone research found the main period of leaching of nitrate nitrogen below the banana root zone was in the few months after planting.

This is not the first time the phenomenon had been observed. During our research trials in 1998 we wrote in Bananatopics Vol 26 page 6 about how to make the most of the abundant freely available nitrogen at the time of planting.

At the time, the environmental significance of nitrogen losses was barely on the agenda. It is now a priority and projects such as Reef Rescue are assisting with solutions.

Free nitrogen

Banana soils in North Queensland have as much as five to 10 tonnes of nitrogen per hectare (N/ha) in the top 60 centimetres.

Most of this is not generally available to the banana plant because it is tied up in organic matter and microbes and is only released slowly.

However, cultivation and land preparation prior to planting causes rapid breakdown of organic matter and microbes and subsequent release as freely available nitrate-nitrogen (nitrate-N).

In our Reef Rescue trial, we had 130 kg of nitrate-N present in the top 60 cm of soil. However, by harvest of the plant crop, soil nitrogen in the unfertilised inter-row area had decreased to low levels.

Under some circumstances the 130 kg that was present at planting could be sufficient to grow a plant crop. But it is mostly a case of too much nitrogen too soon, because this nitrogen is very subject to losses associated with heavy rainfall events.

Less leaching

Three ways to minimise leaching of nitrogen following planting include – later applications of nitrogen fertiliser, scheduling planting at set periods well ahead of the wet season and using strategies such as permanent beds.

Fertiliser companies have already expressed interest in amending their recommendations on fertiliser applications to incorporate nitrate-N levels at planting.

This would assist with timing the applications of nitrogen fertilisers to minimise nitrogen leaching. For example, if there are high nitrate levels at planting it may allow nitrogen fertiliser application to be delayed in plant crops.

Scheduling planting times to MayJune and August-September should maximise the opportunity for plants to take up freely available nitrogen before the wet season is underway from January onwards and when leaching is greatest.

Any strategy reducing the area of land cultivated per hectare of bananas planted should lead to less loss of nitrogen to the environment.

One such approach is known as the permanent bed system  where only about half of the paddock area is cultivated. Currently permanent beds account for no more than 10 per cent of the area under bananas in north Queensland.

Acknowledgement

This project was funded by the Australian Government’s Reef Rescue R&D program.