Abstract

Coffee (Coffea robusta) is an important crop for Vietnam. Vietnam obtains the second highest yield of coffee in the world, just after Brazil, with around 1.2 million Mg per year. Exported coffee products contribute significant income to the Vietnamese economy, about US$3.62 billion in 2014 alone. Most coffee plantations in Vietnam are located in the Central Highland region on two main soil types: (i) Reddish brown soil derived from basic and intermediate magmatic rocks (basaltic soil); and (ii) Reddish yellow soil derived from acid magmatic rocks (granite soil). Current farmer custom regarding fertilizer application for coffee in the region is problematic; farmers tend to overuse fertilizers with no consideration of NPK ratio. Usually, too large amounts of nitrogen (N) and phosphorus (P) are applied, while potassium (K) is often neglected. In view of the important roles of K in plant performance, the objective of the present study was to evaluate the efficiency of K fertilizer on yield and product quality of commercial Robusta coffee in the Central Highlands of Vietnam. The goals of the project were to identify the optimum K application dosage for commercial Robusta coffee in the Highlands, and to demonstrate the positive effects of K fertilization on coffee yield and quality. Two parallel field experiments were conducted from 2012 to 2014, one in Dak Lak province, and the other in Kom Tum province. Six annual doses of K (MOP) application (0, 400, 500, 600, 700 and 800 kg MOP ha^-1) were tested on a uniform background of annual N and P doses. Coffee tree growth was sufficient and the yield was highest at 600 kg KCl ha^-1, with 3.99 and 3.55 Mg beans ha^-1, in basaltic and granite soil, which was 47.3% and 49.7% higher than with zero K application, respectively. Further increased K dosage failed to add any extra value. Potassium application improved vegetative growth, reduced fruit abortion, increased fruit and bean size, and reduced mealybug damage. Economic analysis also shows that profit is at maximum at annual K dosage of 600 kg KCl ha^-1. Apparently, this K dose should be recommended to the regions farmers. Nevertheless, it is more than twice the dose regularly recommended for coffee worldwide. Further analysis and research are required regarding the interactions among soil properties, precipitation regime, and coffee requirements before solid conclusions and instructions to farmers are issued.