Potassium fertilisation strategies for rotational grain-legume crops — implications for the subsequent sugarcane crop

Sugarcane is regarded as a luxury accumulator of potassium (K). Soil testing at plough-out of older ratoons prior to establishing grain-legume crops (soybean and peanuts) sometimes highlights low soil reserves of the element. Agronomists have been seeking guidance on fertilization strategies to maximize break-crop grain yields in sugarcane soils that have depleted K levels. A replicated field trial was established to determine the effect of a range of K fertilizer additions on the productivity of a soybean and then a peanut crop in a long fallow from sugarcane. The residual value of the K supplied in the legume phase was also assessed in the subsequent sugarcane crop. Despite the additions of K from 0 to 250 kg K/ha in 50 kg increments there was no productivity response in either legume crop. The subsequent plant- cane crop (Q208 A ) also demonstrated no productivity response to K fertilizer that was applied to the soil prior to the establishment of the soybean crop. Analysis of plant-cane K concentrations showed no difference in K returned to the site as trash. However, there were significant differences in the quantities of K exported off-farm in millable stalk. The lack of fertilizer K responses, despite low soil-test K concentrations at the end of the sugarcane crop cycle, suggest considerable uncertainty in the interpretation of current soil-testing strategies remains. However, it is unsustainable to mine soil nutrients. Growers have two options; either to supply enough K to meet expected legume crop removal or alternatively recognize that if K is over-supplied to the legume phase, then there is the opportunity to ‘discount’ that in the plant- cane fertilization strategy. Key words Potassium, grain legume rotations, farming systems