Influence of Variable Chloride/Sulfur Doses as Part of Potassium Fertilization on Nitrogen Use Efficiency by Coffee

Chloride (Cl^-) is applied in coffee at rates as a "macronutrient" in the form of muriate of potash (MOP). Potassium (K⁺) is one of the most demanded nutrients by the coffee plant, and MOP is one of the most used fertilizers in coffee production. No scientific evidence shows how Cl^- applied with MOP influences coffee growth, nutrient uptake, and nitrogen use efficiency (NUE). In order to address these questions, a greenhouse trial over two years and a field trial over four years were conducted. The trials were designed to test the influence of variable Cl^-/S ratios on biomass accumulation, nutrient uptake, and NUE. A significant effect of the Cl^- rates on growth was observed under greenhouse conditions but a non-significant effect on yield under field conditions. Cl^- and S significantly influenced the NUE in coffee. The results allow us to conclude that Cl^- rates need to be balanced with S rates, and that Cl^- applied at macronutrient rates can improve the NUE in coffee between 7 and 21% in greenhouse conditions and between 9% and 14% in field conditions, as long as the rates do not exceed 180 mg L^-1 Cl^- and 80 mg·L^-1 S in the greenhouse and 150 kg·ha^-1·year^-1 Cl^- and 50 kg ha^-1·year^-1 S in the field. With the aim to improve the NUE in coffee, the Cl^- content in leaves in coffee should be lower than 0.33% of dry matter, and in soil lower than 30 mg·L^-1. In practical terms, coffee farmers need to balance K-based fertilizers to avoid the excessive Cl^- applications that reduce the nutrient use efficiency, especially the NUE.