The objective was to define critical levels of foliar P influencing foliar nutrient concentration and use efficiency by pastures in Southern Amazonia fertilized with P rates. Four experiments were conducted in a randomized complete block design, with five doses and four replications. Doses were established based on maximum phosphorus adsorption capacity (0, 3, 6, 12 and 24%). In the Ultisol the treatments were: 0, 44, 88, 176 and 352 kg ha-1 of P2O5. In the Oxisol the doses were: 0, 54, 108, 216 and 432 kg ha-1 P2O5. In each soil two experiments were carried out, one with Megathyrsus maximus cv. Zuri grass and the other with Urochloa decumbens cv Basilisk grass When the grass reached grazing height, chemical analysis and grass yield evaluation were carried out and P content was determined in the plant tissues. The highest value of P use efficiency was obtained by Zuri grass cultivated on an Oxisol. The critical levels of foliar phosphorus in Zuri grass were 1.9 and 1.8 g kg-1, grown on Ultisol and Oxisol, respectively. The critical levels of foliar phosphorus in Signal grass were 1.1 and 1.5 g kg-1, cultivated in Ultisol and Oxisol, respectively.
Keywords: Amazonian soils, fertilization, phosphorus adsorption, plant nutrition