Accounting for potassium reserves and fixation in developing sugarcane fertiliser requirements

Knowledge gaps exist in the literature regarding inclusion of reserve-potassium (K) and K fixation when formulating K fertiliser requirements for sugarcane. This study investigated the levels of reserve-K and K fixation in the soils of the South African sugar industry, as well as their potential impacts on K requirements for crop growth. The investigation involved 113 topsoil (0-20 cm) samples, which included Acrisols, Arenosols, Ferralsols, Fluvisols, Leptosols, Luvisols, Nitisols, Plinthisols, and Vertisols. The release of K from the non-exchangeable reserves was measured using 1.0M boiling HNO3 minus exchangeable K, and was termed reserve-K. The capacity of the soil to fix added K was estimated through medium-term incubations and termed the K requirement factor (KRF, the amount of fertiliser K required to raise exchangeable K by a single unit). Both reserve-K and KRF varied widely across all soils and within soil types. Furthermore, there were also wide variations in the relationship between reserve-K and K fixation capacity. Soils were grouped according to their varying levels of reserve-K and K fixation, with a view to improve the reliability of fertiliser K recommendations. Soils with a combination of high to very high reserve-K and low K fixation capacity are of particular concern because of the risk of luxury uptake of K resulting in reduced sucrose recovery, whereas soils with low to medium reserve-K and high K fixation may maintain inadequate supplies of K for crop growth. In this study, the introduction of reserve-K and KRF in fertiliser K recommendations resulted in significant reductions in average K requirements across all soils. These findings point to a need to re-evaluate the basis upon which K soil testing is conducted, as well as current approaches to the development of fertiliser K recommendations.