The investigation in Potassium has received scarce attention in Uruguay because the natural level of exchangeable K in traditional agricultural soils was relatively high. However, recently symptoms of K deficiencies in crops have become frequently observed in situations where the agriculture is very intensive, and more marginal soils are cultivated. The objective of this study was to determine the amount of K₂O needed to increase soil test K in 0.1 cmolc kg^-1 in 36 Uruguayan soils representative of the extensive crop production. Approximately 800 g of dry soils treated with 0 and 13 mL of a KCl 0.08 M solution were incubated for two month at field capacity. Soils were analyzed for exchangeable K with NH₄OAc 1 M at pH 7 after 30 and 60 days of incubation. Potassium application increased the soil test K in almost all soils in an average of 0.15 cmol_c kg^-1, and the relative increase was negative correlated with clay content, soil organic matter, or Ca and Mg exchangeable. In the 19% of the soils the relative increase at 60 days was lower than that one determined at 30 days. The amount of K₂O needed to increase the soil test K ranged from 85 to 569kg ha^-1 of K₂O, with an average of 231 kg ha^-1. More investigation in field conditions and pools of K in soils are needed to elucidate the dynamic of K in Uruguayan soils.