There was a correlation between the isolated factors Doses of cassava water and Sampling times and the levels of exchangeable magnesium and sodium (Figures 5(a) and 5(b)). The data shown in Figures 5(a) and 5(b) indicate that the interaction between the two factors alone was significant only at 63 days after the application of wastewater, with maximum concentrations of exchangeable magnesium and sodium estimated by the quadratic polynomial regression equations, equal to 3.08 and 0.73 cmolc L−1 , respectively, corresponding to an estimated dose of 108 m3 ha−1 wastewater
and 60 m3 ha−1 for exchangeable magnesium.