Phosphorus critical level is influenced by soil physical and chemical attributes and characteristics of each plant species. The aim of this study was to determine the critical levels of phosphorus in soil and plants of Bracharia brizantha cv. Marandu and Panicum maximum cv. Tanzânia, using different doses and sources of P. The work was carried out under green house conditions using soil samples with different clay contents. In the experiment with the cultivar Marandu were applied 20 treatments in factorial scheme (5 x 4), corresponding to five P doses (0, 100, 200, 400 and 800 mg dm-3) and four P sources (monoammonium phosphate, triple superphosphate, simple superphosphate and Gafsa reactive rock phosphate). In the experiment with the cultivar Tanzânia was also applied magnesium termophosphate. The treatments were distributed in a complete block design with four replicates. No differences were observed among the effects of monoammonium phosphate, triple superphosphate, simple superphosphate, Gafsa reactive rock phosphate and magnesium termophosphate on the production of dry matter of grasses Marandu and Tanzânia. The triple superphosphate provided the lowest values of P critical level in soil, the Gafsa reactive rock phosphate provided the lowest values of P critical levels in grasses shoot.