Potassium Dynamics in Vertisols Following Potassium Fertilizer Application.

We conducted glasshouse experiments to evaluate K dynamics in some K deficient Vertisols, which have not responded to K fertilization in the field. Surface (0-10 cm) and sub-surface (10-30 cm) soil samples were taken from four sites in northern NSW, Australia. Potassium was applied at 0, 100, 200 or 500 mg kg^-1 and three consecutive maize crops were grown for 7 weeks each time. Shoot dry matter yield and K concentration in the shoots were measured each time. Following the final harvest, exchangeable and non-exchangeable fractions of soil K were measured. There was no significant change in the shoot yield from the application of K. However site, depth and harvest effects were significant in relation to the shoot yield. We hypothesize that non-exchangeable K in the soils has been severely depleted by continuous cropping. Applied K was readily fixed at the selective sites created by the removal of non-exchangeable K in the soils, particularly surface samples.  The degree of K fixation and release from non-exchangeable sites varied for soils from different sites and for the same soil at different depths. Potassium dynamics in the soils will be discussed in relation to their clay mineral composition and K adsorption characteristics.