Effective deep application of phosphorus (P) and potassium (K) fertiliser relies on critical factors. Understanding how to tune important variables in deep applications is emerging from trials in the northern cropping region (central and southern Queensland, and northern NSW).
The critical factors for successful deep applications include:
- product choice
- compatibility of compound products
- soil disturbance compared to volume enriched.
The seasons have been challenging since the trials began in 2015. Despite this, deep banding K and P in deficient soils increased yield by up to 30% (more than 1t/ha) in chickpeas last season. This was very profitable as chickpeas were worth around $1000 per tonne.
Testing before deep applications
In the trials, deficient soils had less than 8 mg/kg Colwell P and 78 mg/kg exchangeable K. We now have soil testing guidelines in the late stages of development to show where deep banding should be most effective.
Another test for predicting P availability is the BSES-P test, also known as the DGT-P test. Developed by the Queensland sugar industry, this test is especially good in calcareous, acid or high-iron soils. For more information go to DGT-P test Factsheet and GRDC P Management Factsheet.
Deep fertiliser availability
In highly alkaline, black and grey cracking clays, using triple superphosphate (TSP) as a source of phosphorus produced very variable results. Crop response to monoammonium phosphate (MAP) was more reliable in these soil conditions.
Glasshouse trials looked at deep banding K in low K vertosols. These soils have a high K buffering capacity. High fertiliser rates are usually needed to increase soil K concentrations because these soils tend to slow the rate of release of much of the K applied for plant uptake.
The trials show occasional high rate applications of deep banded K can be very effective, with long residual benefits due to the lack of tie up in soil. The data to date also suggests it will be even more effective to band subsoil K with other nutrients such as N and P, to encourage root growth in and around the fertilizer band.
M Bell, D Lester, G Schwenke, T Weaver and P Moody.