This paper reports the results of four experiments with sweet potatoes (Ipomoea batatas Lam.) to study the effects of liming, two different NPK-fertilizers, manure and green-manuring on different areas of two localities of the State of São Paulo. Liming alone did not increase the yields. In the presence of manure, its effects corresponded to +5, +12, +20, and +48%, but in the presence of the NPK-fertilizers they varied from satisfactory to negative. The responses to manure were always very high, principally in the presence of lime. Although not sa large, the effects the NPK-fertilizers were also very good, bul the presence of lime, which enhanced them in some cases, in other coses depressed them considerably. The results of green-manuring were almost equal to those of manure. In the dosage used, manure supplied the soil with much larger quantities of nutrients than the NPK-fertilizers. The method of application of the latter - at planting time and in contact with the cuttings - contributed also to reduce their effectiveness. Regarding the yield reduction caused by liming, they are attributed to the so-called overliming injury. The study of the soils of two trials showed that in the limed the pH averaged only 6.7, ut the quantities of available phosphorus were appreciably reduced, especially in the treatments with the NPK-fertilizers.