RESEARCH is currently assessing coated fertilisers, to measure and determine the pattern and duration of nutrient release. In contrast to uncoated fast release fertilisers, the coating on slow and controlled release fertilisers protects the fertiliser granule from immediately dissolving and releasing all the content shortly after application, by releasing the nutrients over several months.

Preliminary data has shown higher rates of release within the first month after application, with decreased release for the following months, depending on the expected product release duration (i.e., 2–3 months, 5–6 months). The various coating technologies rely on different release mechanisms, for example, the nutrient release through a polymer coating will be dependent on the type and thickness of the coating, as well as soil temperature while other coatings rely on microbial activity to degrade the coating and release nutrients.

The use of controlled release fertilisers may have a role in sandy soils to reduce nutrient leaching of conventional fertilisers. Increased nutrient use efficiency and banding of fertiliser in the root zone at planting may lead to a reduction in the rate of nutrient applied, fewer applications during the crops life and therefore reduced labour costs, when compared to fast release fertilisers. Initial trials of various slow and controlled release fertilisers showed great variability in nutrient release pattern.

Further research will focus on analysing the nutrient release pattern for various commercially available slow and controlled release fertilisers in Western Australian sandy soils.

Weekly measurements of nitrate concentrations in the soil solution, sampled using soil solution extractors, is one of the methods employed to monitor nutrient release and availability to plants, which combined with plant growth measurements can indicate the performance of fertilisers and their suitability to sustain vegetable production.

The research aims to identify application strategies that reduce fertiliser use and expenditure for vegetable producers, through the integration of slow release fertilisers for reduced application rates, the positioning of fertiliser at application, and a combination of their use with fertigation. The application of controlled release fertilisers at transplanting can potentially reduce labour costs from weekly fertigation applications, as well as minimising the likelihood of nitrogen leaching, particularly in the early crop growth stages.

Information and guidelines on the use of slow and controlled release fertiliser to potentially optimise fertiliser use efficiency in vegetable production in sandy soils will be developed once the project has been completed.