In Figure 4(a), one can observe that the data of exchangeable calcium correspond to a positive quadratic polynomial model as a function of the application of increasing doses of cassava to the ground and to a decreasing linear model as a function of the extension of the sampling times (Figure 4(b)). The largest estimated value of exchangeable calcium in the soil, 2.90 cmolc dm−3 , corresponds to the estimated dose of90.5 m3 ha−1 , with a decrease in concentration in the soil as higher doses were applied. As for the sampling times Figure ( 4(b)), it was found that the concentration of exchangeable calcium in the soil decreased by approximately 22%, from
2.97 to 2.43 cmolc dm−3 at 21 and 105 days of cultivation, respectively.