Comparison of potassium fertilizer recommendations for grain crops in Brazil and Paraguay

Abstract Fertilizer recommendations in Brazil and Paraguay are based on regional systems. This study systematically compared potassium (K) recommendations for soybean ( Glycine max (L.) Merr), maize ( Zea mays L.), and wheat ( Triticum aestivum L.), highlighting their similarities and differences. Eleven fertilizer‐K recommendation systems were analyzed, including 10 Brazilian and one Paraguayan. The classes of availability, K buffering capacity , and recommended K rates were compared according to Mehlich‐I extracted soil K. The critical soil‐test K level varied at 30–135 mg dm −3 of K, a 3.5‐fold variation. For most systems, K recommendation considers available K as a primary decision‐making parameter, with some systems also using the cation exchange capacity (CEC) to interpret available K concentration both directly (CEC classes) and indirectly (K saturation of the CEC). The maximum recommended K rate in the soil varied widely. To produce 3 Mg ha −1 of soybean, the rates varied by 58–375 kg K 2 O ha −1 ; for 10 Mg ha −1 of maize, by 97–375 kg K 2 O ha −1 , and for 4 Mg ha −1 of wheat, by 62–330 kg K 2 O ha −1 . Conversely, the available K concentration in the soil after zero fertilization ranged from 48 to 385 mg dm −3 for soybean, from 56 to 604 mg dm −3 for maize, and from 54 to 542 mg dm −3 for wheat. These variations represent differences of roughly 700%, 980%, and 900%, respectively. Such discrepancies between systems can hinder the adoption of recommendation systems by users. Therefore, there is an urgent need to create strategies to reduce these discrepancies and harmonize potassium fertilization recommendations in Brazil and Paraguay.