A new method for determination of potassium in soils using diffusive gradients in thin films (DGT)

Potassium is an essential plant nutrient often limiting plant productivity. Ammonium acetate extraction is often used to predict the potassium status of soils. However, correlation between extracted K and plant uptake is often poor, especially over a range of different soil textures. Diffusive gradients in thin films (DGT), which determines the diffusive supply of elements, has been shown to accurately measure plant available elements in several cases. Up until now, however, the DGT devices available have not been suitable for measuring K. We set out to develop a DGT device suitable for the measurement of K in soil and test its ability to predict plant available K. The DGT device contained a binding layer based on Amberlite IRP-69 cation exchange resin. It proved suitable for the measurement of K under conditions similar to those usually found in soil if a 2-h deployment time was used and the labile K concentration was limited to 400 mu M. Prediction of plant K concentrations with DGT were similar to those with ammonium acetate extractions over a range of typical agricultural soils with sandy and sandy loam textures. The results indicate that this new type of DGT has the potential to improve the accuracy of predictions of the K status of soils, although more tests using a wider range of plant species and soils are necessary.