Two methods for the determination of the quantities of P to be added to samples of 10 g of soil are presented, so that after a 4 day incubation period, 30 ppm of P will remain soluble in a 100 ml of 0,05 N in HCl and 0,025 N in H2SO4 solution. The methods are as follows: a. based on the correlations between the quantities of P added to the samples and the ones that were extracted by the cited extractor. b. based on the similarity of the triangles obtained from the rectangular coordenates of the representative points of 30 ppm of extracted and added quantities and of the points immediatly inferior and superior to this value. It is concluded that both methods show equivalent results, being indifferent the use of either.