Loss of nitrogen (N) fertiliser off-farm is considered to be a serious threat to reef water quality. In addition to surface water runoff, deep drainage or leaching of N through the soil profile is an important loss pathway.
Approximately 90 percent of the Australian banana industry is located on Queensland's wet tropical coast bordering the Great Barrier Reef. The average industry N application rates have decreased since the mid-1990s by as much as 40 per cent and as a result a substantial reduction in N loss in deep drainage has been measured. Controlled release (CR) fertilisers have been considered the next step in improving nutrient management as the release of N may be better aligned with plant demand. However, there is an increasing range of CR products available with different methods of controlling the rate of release of N, and the impact on N in plant uptake and deep drainage has not been previously qualified.
The objective of this trial was to determine the effect of controlled release fertiliser products on N loss rates in deep drainage and plant performance in the north Queensland banana industry. The study commenced in 2011 at the South Johnstone Research Station (SJRS), and evaluated the fertilisers: 1) urea (an industry standard), 2) porous bags containing a commercial blend of NPK fertiliser and 0.4 percent humic acid, 3) Novatec liquid (nitrification inhibitor, BASF), 4) Multicote (polymer coated urea, Haifa) and 5) CoRoN (polymethylene urea, Barmac).
The project monitored loss of N in deep drainage, soil fertility and crop performance in a plant crop and first ratoon.
There were high concentrations of nitrate-N in deep drainage during the first few months of monitoring. This is consistent with high concentrations of mineral N in the soil at planting (refer to Soil nitrogen below). By February 2012 when bunches were emerging, concentrations had decreased to generally low concentrations, where they remained until the harvest of the plant crop. Loads of N in deep drainage were 23-48 kg N/ha for the plant crop with little difference evident between the different forms of fertiliser and rates of N applied. Loads of N in deep drainage were much lower in the ratoon crop. This is consistent with lower soil nitrate concentrations at the start of the ratoon crop cycle and a mature root system capable of taking up more N. N losses were ~1 kg N/ha.
There were high concentrations of nitrate-N and ammonium-N in the soil profile at planting as a result of soil disturbance during cultivation. The load of soil mineral N in the surface 0.6 m was 165 kg N/ha. Similar high mineral N loads at planting have been reported previously. At harvest of the plant crop 9 months later, concentrations were low in the urea treatment, reflecting loss from the soil in plant uptake, deep drainage and possibly denitrification. The high load at planting has masked differences in rates of N loss in deep drainage from the N fertiliser treatments during the plant crop.
In both the plant and the first ratoon crops, there were no significant differences in the plant parameters measured, notably bunch yield and height of the following sucker. This lack of difference was partly by design as we sought to investigate differences in losses of N in deep drainage from the treatments when plant growth was not limited by nutrient supply.
There were no differences among treatments in leaf N concentrations following intensive monitoring of the plant and ratoon crops. All leaf N concentrations were within the optimal range. Thus all the N fertiliser treatments have provided an adequate supply of N to the plant.
Benefits and Application
Low rates of N loss in deep drainage are encouraging, demonstrating that moderate N fertiliser targets (150 kg N/ha for plant crop and 250 kg N/ha for ratoons) are an important component of efficient N management.
Although there is no evidence of a benefit from controlled release fertilisers in reducing loss of N in deep drainage, these fertilisers may have other benefits. These would include the ability to ensure the crop is receiving adequate nutrients during the extended wet season when standard forms/methods of fertilising such as fertigated urea treatment or broadcast fertiliser can be very difficult to apply on most plantations. In contrast, controlled release fertilisers can be conveniently applied prior to the commencement of the wet season.
There is a need to consider the high loads of readily available mineral N from ground preparation before planting to manage N losses. Incitec Pivot Ltd. have indicated a desire to incorporate this result into routine N fertiliser recommendations.
The release of mineral N by cultivation for planting may provide more N than can be used by the crop. The use of permanent bed systems, with only a fraction of the paddock cultivated, should reduce the loss of N to the environment. Currently permanent bed systems account for less than 10 per cent of the overall area under bananas in north Queensland.