Measurement and Simulation of Soluble, Exchangeable, and Non-Exchangeable Ammonium in Three Soils

Clay minerals can fix high amounts of ammonium in non-exchangeable form, which can be of management importance when some ammonium applied with fertilisers is sequestered and not available to the crop. Non-exchangeable ammonium can be released slowly and increase soil N availability. The percentage of ammonium fixed depends, among other factors, on the ammonium dose applied. We applied increasing amounts of ammonium to three soils and then measured (after 48h) the soil equilibrium concentration of non-exchangeable, exchangeable, and soluble ammonium. At a 70mg NH<inf>4</inf>-Nkg-1 dose, non-exchangeable ammonium measured 60, 24, and 17% on the three soils, while at 2800mg NH<inf>4</inf>-Nkg-1 the fixation percentage decreased non-linearly to 12, 4, and 5%. Ammonium equilibrium concentrations in non-exchangeable, exchangeable, and soluble fractions as a function of ammonium applied were simulated with two innovative analytically-solved models using two Langmuir equations, one for each pair of pools. Both models predicted well the three ammonium fractions in all soils, with average relative root mean squared errors ranging from 2 to 5%. Sensitivity analysis demonstrated the importance of all four model parameters; however, to extend the model to other soils requires measurement of two parameters - cation exchange capacity and maximum fixation of ammonium in non-exchangeable form. The other two parameters are equilibrium constants that vary little among soils, according to our preliminary results. Therefore, to simplify model use, averaged optimised constant values on multiple soils might suffice. Additional experiments on more soils are needed to verify this hypothesis.