In this paper are reported the results of tow experiments located on poor soils, derived from the Botucatu sandstone, to study the effects of liming and some fertilizers on the yields of sweet potatoes (Ipomoea batatas Lam.) and yams (Dioscorea alata L.). In one of the trials, conducted on an acid sandy soil high in organic matter, the responses to lime and a NPK-fertilizer were relatively very high. However, even with the best treatment (lime + NPK) the yields of sweet potatoes were very poor, apparently due to deficient drainage of the experimental area; notwithstanding, yams yielded fairly well, so that the effect of that treatment was also good in absolute values. The soil of the other experiment was likewise sandy and acid, but well drained and low in organic matter. With the better treatments, which included two different NPK-fertilizers, manure and green-manuring with leguminous, the yields of sweet potatoes and principally those of yams were very good. The responses to the mentioned treatments were also good in the absence of lime, but decreased considerably in its presence. Accordingly, liming, that wos fairly effective in the absence of the fertilizers, did not increase the yields in their presence. The study of the yields and of the soil indicated that, under the conditions of this experiment, the dose of 3 tons of lime per hectare per year was excessive, inducing the so-called overtiming injury.