Potassium Rates and Application Methods: Effects on Soil K Availability and Crop Response in Planosols and Ferralsols

Potassium (K) fertilization guidelines often oversimplify, overlooking factors like soil mineralogy and K application methods. This study aimed to evaluate the influence of K fertilizer rates and application methods on crop yields and the forms of K in soils with different textural and mineralogical features. In 2020, two field trials were conducted to evaluate the impact of varying K rates, added through band or broadcast methods, in Planosol and in Ferralsol. Soybean yield was evaluated in both trials, while flooded rice yield was evaluated only in the Planosol. Soil samples collected from the 0-10 and 10-20 cm layers and soil K content was determined using Mehlich-1, nitric acid (HNO3), and sodium tetraphenylborate. In the Ferralsol, K desorption by HNO3 in the 10-20 cm soil layer was 1.4 times higher the in band application (171 mg kg(-1)) compared to broadcast method (92 mg kg(-1)). The Mehlich-1 extractor proved insensitive to detecting K rates in the Planosol (91 and 43 mg kg(-1) in the 0-10 and 10-20 cm layers, respectively), while it exhibited responsiveness in the Ferralsol, demonstrating positive increases with K rates. Using a proper chemical extractor is crucial for accurate detection and recovery of applied K in the soil. Grain yields were 4.50 Mg ha(-1) for soybeans in the Ferralsol and 9.72 and 5.55 Mg ha(-1) for rice and soybeans, respectively, in the Planosol. The findings reveal that applied K distributes into preexisting pools, persisting in the applied location, thereby reducing the anthropogenic K gradient with band reapplication.