Five experiments were conducted to study the efficiency, in the presence of PK, of 120 kilograms of nitrogen per hectare supplied as ammonium sulfate, Chilean nitrate, castor pomace or mixtures of these fertilizers for potatoes in the Paraíba Valley, where they are planted during the dry period and irrigated by the subsurface method. As an average of the different sources and of the five experiments, the yield increase in response to nitrogen was +3.93 metric tons to the hectare (+44%). While the effects of ammonium sulfate and Chilean nitrate were respectively +2.81 and +1.67 tons, that of the mixture of these sources (each supplying half of the nitrogen quantity) attained +3.86 tons. Either olone or in the mixtures costor pomace yielded better than the mineral sources; where applied alone, its effect reached +5.46 tons to the hectare. The soils studied were high in organic matter, and apparently no leaching of nitrogen occurred under the conditions of the experiments. The superior performance of castor pomace is attributed to two factors: 1) only its nitrogen content was considered in the calculation of the doses to be used, so that in the treatments with it the quantities of PK were higher than in those which received ammonium sulfate or Chilean nitrate; 2) on the other hand, while the entire dose of castor pomace was side-placed by planting time, only half of the ammonium and nitrate doses was applied in that way, the other halves being top-dressed one month after the emergence of the shoots, apparently too late for efficient use by the potato plants. Owing to the exaggerated price of castor pomace, the profits that it and its mixtures gave were relatively low. In this sense the mixture of ammonium sulfate and Chilean nitrate was much better. Such results emphasize the need for studying a suitable method of applying mineral nitrogen under the particular conditions of the potato-growing in the Paraíba Valley.