Timing of fertiliser application and phosphorus loss from sandy soil

Key messages

• Timing of phosphorus application to pastures can significantly influence phosphorus leached through soils with low phosphorus buffering index.
• 10% of applied phosphorus was leached when phosphorus was applied in spring in comparison to an autumn application, which leached 60%.
• Timing of application also needs to consider other factors such as paddock trafficability during late winter/early spring.
• It is best practice to rely on soil testing to determine phosphorus fertiliser needs.

Methods

• Soil type: Sandy soil
• Annual rainfall: 870 mm

A replicated phosphorus response trial was repurposed to measure phosphorus leaching. Trial plots were sampled to estimate soil phosphorus fertility, and plots with similar levels of phosphorus fertility were split to allow four different timings of phosphorus application. The timings of application tested were autumn, spring, split across autumn and spring, and monthly applications in winter and spring. Each of these timing treatments had a total of 20 kilograms of phosphorus per hectare applied for the growing season.

Prior to the application of fertiliser, lysimeters were installed in each treatment to allow the collection of leachate below 10 centimetres and the measurement of phosphorus concentrations in the leachate. The amount of phosphorus leached was calculated by multiplying leachate volumes by concentrations.

The trial excluded stock, and pasture growth was determined on each of the plots using mower cuts and determining dry matter. All pasture biomass was removed from the trial site at each pasture assessment, and dry matter weights were accumulated for the growing season.

Results

• There was no significant difference in the amount of dry matter measured in each of the treatments, with an average of 6.5 tonnes of dry matter per hahectare.
• Around 2 kilograms of phosphorus per hectare kgP/ha or 10% of the applied phosphorus 20 kgP/ha was leached from the spring treatment.
• Approximately 4 kilograms of phosphorus per hectare kgP/ha or 20% of the applied phosphorus 20 kgP/ha was leached from the monthly and autumn/spring split treatment.
• Sixty percent, or 12 kilograms of phosphorus per hectare kgP/ha of the applied phosphorus 20 kgP/ha was leached when the P phosphorus fertiliszer was applied in autumn.