Fertilizer recommendation based on effective calibrating soil tests against yield responses to applied nutrient has been little used in Ethiopia. A field experiment was conducted at Dera District, Northwest Ethiopia in 2018/19 with the objective of determining soil K critical level; optimum K/Mg ratio; and evaluating the effect of KCl on selected soil chemical properties and yield and yield components of maize. Maize variety named BH-540 was used as a test crop. The experiment was laid out in RCBD with 7 levels of K fertilizer (0, 50, 100, 150, 200, 250, and 300 kg ha-1 K2O) with four replications. DAP (200 Kg ha-1) fertilizer was added at the beginning of sowing while urea (200 Kg ha-1) was added in split half at planting and half at knee-height stage. These fertilizer types were used as a source of P and N, respectively. KCl was used as K source and applied two weeks before planting in broadcast method. SAS (9.0 version) software was used for analysis. Soil samples were taken two-weeks after planting and compared with critical levels from literature. Soil samples were also collected after harvest (0-20cm depth) to evaluate the effect of KCl on some soil chemical properties. Soils were highly acidic in pH (5.10) and with medium exchangeable acidity (1.06 Cmol(+) kg-1) value. The soils had sufficient amount of K (434.20) mg Kg-1, Mg (620.77) mg Kg-1 and optimum K/Mg ratio (0.71). The optimum K/Mg ratio was registered from the control plots. Soil K in the studied area from plots receiving 0 Kg ha-1K2O was found above the critical value (250 mg Kg-1). Post-harvest soil sample results revealed that it has a significant negative and positive correlation with Mg and K/Mg ratio, respectively and had no effect on pH and exchangeable acidity. The highest maize grain yield (7.028t ha-1) was obtained from application of 150 Kg ha-1 K2O. However, this rate was not statistically significant. So, K fertilizer did not show a significant effect on yield and yield components of maize. Currently, it was insignificant to incorporate K as a package for maize. Keywords: BH-540; fertilizer; soil test; exchangeable acidity