Abstract Fertilizer recommendation systems in Brazil and Paraguay are based on soil analysis and correlation‐calibration experiments; however, they often recommend divergent phosphate fertilizer rates for the same crop and yield target. This study compares phosphate fertilizer recommendation systems for soybean Glycine max (L.) Merr., wheat Triticum aestivum (L.), and maize Zea mays (L.), using standardized yield targets of 3, 4, and 10 Mg ha −1 , respectively. Eleven regional recommendations were evaluated: ten from Brazil and one from the Eastern Region of Paraguay. Most systems use the Mehlich‐1 method to extract available P, except São Paulo, which uses an anion exchange resin. The widespread use of Mehlich‐1 allows comparison of critical soil test values (CSTV) of P across tropical and subtropical soils. In several systems, CSTV declines with increasing clay content, likely due to extractor limitations and P readsorption during decantation, yet few recommendations explicitly incorporate P sorption capacity (e.g., clay content) when defining rates. Differences among calibration experiments have resulted in a wide range of CSTV (4–56 mg dm −3 ) and recommended P 2 O 5 rates of 82–415 kg ha −1 for soybean (≈36–181 kg P ha −1 ), 87–329 kg ha −1 for wheat (≈38–144 kg P ha −1 ), and 118–362 kg ha −1 for maize (≈52–158 kg P ha −1 ). Mehlich‐1 thresholds above which phosphate fertilization becomes unnecessary also varied widely (5–35, 3–37, and 6–46 mg dm −3 for soybean, wheat, and maize, respectively). These results highlight the need for minimum guidelines to harmonize soil P test interpretation and phosphate fertilizer recommendations, improving P use efficiency in Brazil and Paraguay.
Duarte‐Monzón et al. (Sun,) studied this question.