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 Cl and 80 mg·L S in the greenhouse and 150 kg·ha·year Cl and 50 kg ha·year 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. 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.