An analysis of knowledge and knowledge gaps on K nutrition on non-sandy soils

Abstract

Potassium (K) deficiencies are becoming more prevalent in non-sandy soils throughout the Northern and Southern grain growing regions of eastern Australia, with the distribution of low K linked to combinations of soil type (mineralogy), years of cropping, intensity of K removal (crop rotations and incidence of forage/hay production) and tillage system. Critical soil test K concentrations are poorly defined on non-sandy soils but appear to be much higher than critical concentrations in sandy soils. Crops accumulate as much, or more, K than nitrogen (N) in crop biomass, so overcoming K deficiencies requires substantial increases in crop K uptake (i.e. 10’s of kgs, rather than kgs (phosphorus, P) or hundreds of mg (trace elements)). Ensuring large amounts of additional K uptake is challenging. Cotton and grain legumes remove much more K/t of harvested product than cereals and coarse grains, with lower K harvest indices in grain crops resulting in residue K enriching topsoil layers. Leaching does not redistribute topsoil K into deeper soil layers, except in sands. Fertiliser K application strategies need to enrich profile layers where there is a concurrence of high root densities and frequent/extended periods of available moisture during the growing season. Most plant available K is held on the negatively charged surfaces of clay and organic matter that are measured by the soil cation exchange capacity (CEC), with the size of this K sink increasing as CEC increases. In some soils, plant K uptake is limited by the rate of K release, rather than the amount of K that is potentially available. To overcome a K deficiency, fertiliser K can be mixed through large soil volumes to maximize root contact or applied in concentrated bands to create localised zones of high solution K concentrations. The former strategy maximizes the interaction with the soil CEC and can limit responses to K fertiliser applications in soils with high CEC, while the latter requires roots to be concentrated around the fertiliser band. Unknowns that are being addressed in the current project include determining the critical soil test K concentration that indicates a likely response to applied K fertiliser in soils with varying mineralogy and CEC, and optimising the K fertiliser application strategy to ensure good fertiliser K recovery in cropping systems with varying reliance on stored soil water.