Five potatoes fertilizers trials were carried out. in diffeient soil types of the State of São Paulo with the purpose to study the effect of nitrogen, phosphoric acid and potash, on tuber production. It was found that nitrogen increased plant growth, intensified green color of foliage and retarded maturity. Nitrogen also increased susceptibility to fungus diseases. Phosphoric acid increased yield and shortened the vegetative period. Potassium induced a light -green color of the foliage, but otherwise had almost no influence on plant development. Phosphoric acid was the most important factor in increasing tuber production : nitrogen was second and potassium, third. A response to potassium was noticed only when this element was applied together with phosphorus and nitrogen.