Boundary line analysis for establishing cationic macronutrient availability classes in cocoa soils of Bahia, Brazil

Abstract To ensure high cacao yields, assessing soil fertility is crucial for appropriate amendment and fertilizer use. This study aimed to determine cationic macronutrient availability classes for cacao at two soil depths (0–10 and 0–20 cm) using the Boundary Line (BL) approach, with the goal of contributing to fertilization recommendations for modern cacao cultivars. Specifically, it sought to establish novel classes for the 0–10 cm layer and refine/validate existing recommendations for the 0–20 cm layer. We analyzed a database from 15 experimental areas in southern Bahia, Brazil, collecting soil samples at 0–10 and 0–20 cm depths to measure K + , Ca 2+ , Mg 2+ , and their ratios (Ca/Mg, Ca/K, Mg/K, (Ca + Mg)/K). The boundary line analysis helped identify potential response curves for cacao productivity, establishing seven nutrient availability classes from very low to excess. For the 0–20 cm soil layer, the adequate ranges for Ca 2+ (23–47 mmol c dm −3 ) and Mg 2+ (7–16 mmol c dm −3 ) were aligned with literature, while the K + range (48–94 mg dm −3 ) was lower, potentially reducing potassium fertilizer use. In the 0–10 cm soil layer, the ranges were: K + (57–101 mg dm −3 ), Ca 2+ (32–62 mmol c dm −3 ), and Mg 2+ (8–20 mmol c dm −3 ). These ranges in the upper layer can serve as additional references. This study also provided the first specific adequate availability classes for Ca/K (10–20) and Mg/K (2–4) ratios. The research established reference values for "tendency to excess" and "excess" classes. These findings provide new reference values for the interpretation of soil analyses for cacao cultivation and support the fertilization process, complementing information from the 0 to 20 cm layer.