The K is important for sugar concentration in sugarcane, but the salt effect on K fertilizer may reduce the productivity. The objective of this study was to evaluate productivity of sugarcane, to establish soil and plant K critical levels, and to determine a saline limit of the soil with maximum agronomic efficiency. The sugarcane variety RB992506 was cultivated in the field in Oxisol (Kandic) dystrophic with very sandy and low K content, mainly in subsurface. Six doses of K2O were used: 0; 25; 50; 100; 200; 250 kg ha-1. We evaluated soil and leaf K content, electrical conductivity of the saturated soil paste, productivity and technological quality of the sugarcane. The application of K promoted increase in soil and leaf K levels, increasing also both productivity and the sugar concentration. The dose of maximum agronomic efficiency was 150 kg ha-1 of K2O and production of 116 Mg ha-1. High doses of K (> 200 kg ha-1 of K2O) decreased the productivity due to elevated soil salinity by the application of K fertilizer. The critical levels of K were 0.18 cmolc dm-3 in the soil and 9.53 g kg-1 in the plant and were associated with low doses of K (< 25 kg ha-1 of K2O). The salinity limits that reduced productivity were 1.38 dS m-1 in the surface layer and 1.19 dS m-1 in the subsurface, suggesting that these critical levels and limits can be used to evaluate the fertility of sandy soils cultivated with sugarcane in tropical regions.