The Diffusive Gradient in Thin-films (DGT) Phosphorus is an alternative P soil test. Colwell-P can overestimate available P on calcareous soils, and acid gravel soils.
How does the DGT test work?
The DGT is a cylindrical plastic device that uses an iron oxide gel as a P-sink, which attracts available P through a membrane. To run the test a soil sample is moistened to 100% water holding capacity. The DGT device containing a clear membrane, followed by the P-sink, is placed on top of the soil. The test measures how much P diffuses through the soil and into the gel. The membrane controls P movement into the gel. After 16-24 hours the amount of P bound to the gel is measured.
The DGT test can be a better predictor of plant P requirements because it mimics the action of plant roots. It accounts for both:
- the initial soil P concentration, and
- the ability of the soil to resupply P as it moves into the gel.
When to use the DGT test?
Consider DGT-P if you have calcareous soil and/or your Colwell-P values are odd. Colwell P doesn’t always provide data that makes sense based on responses seen in the paddock. DGT-P might also offer greater accuracy on gravel, high PBI soils. Initial isotopic studies in WA suggests DGT-P outperforms Cowell on these soils.
Using DGT for the first time
If you haven’t used DGT before, test both DGT and Colwell P / PBI. Pay attention to areas where DGT and Colwell P recommendations are conflicting. Test strips will confirm which test method was more accurate for your site. When using P test strips:
- Make sure there is a zero P strip.
- If using MAP or DAP, make sure N is not limiting so any response is from P.
- If you can, use the recommended rate and a higher rate to check.
- Grain yield data is key as critical values are based on grain responses to P.
Interpreting DGT Values
DGT for other nutrients
The DGT method is being trialled on other nutrients. Research is underway on DGT for potassium and aluminium.