Soybean YIELD As a Function of Potassium Fertilization in Soil UNDER High Fertility.

Potassium (K) is the second nutrient required in greatest quantities by soybean and plays a fundamental role in grain quality and consequently, on crop productivity. The study began in 2013/2014 season, to objective evaluate soybean yield as a function of potassium fertilization in soil under high fertility. The experiment was carried out under field conditions in dystroferric Red Nitosol, in Itambé, Paraná, Brazil, and K⁺ content in soil ranged between 3.6 and 3.9 mmol_c dm^-3. Were used 6 K rates (0, 40, 80, 120, 160 and 200 kg ha^-1 of K₂O), by potassium chloride, applied 15 days after planting, under experimental design to randomized block, with four replications. Plants were harvested at the R8 stage in the useful area of the plots, which consisted of 4 rows of soybeans with 7 m long and 0.45 m apart from each other (12.6 m²). Subsequently, has measured the mass of grain, corrected to 13% moisture and productivity kg ha^-1. After collecting field data and calculations of productivity, evaluated all assumptions of analysis of variance (ANOVA), and applied to multiple regression analysis for the quantitative factor (K doses). There were no significant increases in soybean (R² = 0.34, F = 1.12, Ŷ = Ȳ = 3017 kg ha^-1) as a function of the application K in the soil, once the K⁺ in soil were above the critical level considered for culture included in this production system. However, starting from the improvement soil quality, the demand of K by the culture can increase to meet higher productivity, and therefore, have response to application of higher rates of K in the soil.