The levels of exchangeable potassium in the soil above 0.2 cmolc dm−3 are considered high or adequate [18], indicating that cassava waste water used in this experiment served as a source of potassium and supporting the hypothesis that the potassium content in such effluents was sufficient to increase the levels of this cation in the soil and meet the demand of the cultivated sunflower during the experiment. This outcome was expected since cassava waste water is rich in nutrients (Table 2) especially in potassium, which could explain the increase in exchangeable potassium in the soil due to the application of increasing doses of cassava waste water. On the other hand, during plant growth throughout the cultivation, as observed for the available phosphorus in soil (Figure 2), the plants extracted available nutrients from the soil solution as needed, thus lowering the concentration of adsorbed potassium in the sorptive soil complex. This decrease can be attributed also to the susceptibility to leaching of potassium ions to the lower soil layers, as mentioned in Duarte et al. [3].