Abstract

Sugarcane (Saccharum spp.) is an important industrial agricultural crop in Vietnam. Average sugarcane productivity in Vietnam is 64 Mg ha^-1, and the average commercial cane sugar (CCS) content is 10%, significantly lower than in leading sugarcane producing countries that achieve 75 Mg ha^-1 and 14-15% CCS content. Under rain-fed conditions in Vietnam, low sugarcane performance may be the result of poor nutrition management. In farmers’ practices (FP), nitrogen (N) is generously applied, while phosphorus (P) and potassium (K) are generally ignored. The objectives of this 3-year study were to examine and demonstrate the contribution of increased K application to sugarcane yield, quality, and economic benefit under commercial conditions. Field experiments took place from 2012 to 2015 in a parallel design in Gia Lai (Central Highlands) and in Khanh Hoa (Central Coast) provinces. Treatments included six K (KCl) rates (0, 200, 300, 350, 400, 450 kg K₂O ha^-1) together with 250 kg N and 150 kg P₂O₅ ha^-1. A local FP was also included as a control. Potassium application significantly improved sugarcane yields by 18.5- 31.5% in Khanh Hoa, and 9.2-26.8% in Gia Lai, compared to FP. CCS content increased from 8% at 0 kg K₂O ha^-1 to 11-12% at 200 kg K₂O ha^-1, reaching about 12.5% at the highest doses. In spite of the impressive response to K, the K agronomic efficiency (KAE) was very low, at 67 and 40 kg of cane per kg K₂O, in Gia Lai and Khanh Hoa, respectively. The low KAE may be attributed to water deficit problems and rapid nutrient leaching from the root zone. Thus, although the apparent economic optimal K dose is 350-400 kg K₂O ha^-1, use of additional approaches - such as soil enrichment with organic matter, more frequent K applications, and irrigation - are expected to improve K uptake efficiency, further increase sugarcane yield and quality, and reduce the required annual K dose.