[External and internal phosphorus requirements in Ischaemum indicum and Brachiaria brizantha]

The external and internal phosphorus (P) requirements for Ischaemum indicum (syn. I. ciliare, I. aristatum, Phleum indicum) and Brachiaria brizantha cv. Marandu (CIAT 6780) were determined at CATIE's screenhouse facilities in Turrialba, Costa Rica, but using a low fertility acid soil collected from an area where I. indicum is the dominant species. The experimental soil was dried, grinded through a 4 mm screen, sterilized and homogenized, and 3.0 kg of this were used per pot. All pots were fertilized with a basal macro- and microminerals formula, but also were inoculated with mycorrhizae, and irrigated to field capacity. To determine the P levels included in the study, a P-sorption assay was performed in the experimental soil. A randomized complete block design, with 16 treatments and three replications was used. The treatments were defined by the factorial arrangement of the two grasses x eight levels of phosphorus fertilization (0, 75, 150, 300, 600, 1200, 1800, and 2400 kg P/ha). Several regression models were adjusted to leaf, stem, root and total biomass yields obtained 10 week after planting. The response to phosphorus was better described by the model Yi = A + B (1 - e(-CP)). From this, the maximum yields (A+B) were estimated for each species, which served as a basis for the determination of the P concentrations in the soil solution and plant tissue associated with 95 percent of the maximum yield, the parameters used to define the external and internal P requirements, respectively. Both species did not differ in their external P requirements (0.0139 and 0.0131 micro g P/ml of soil solution, for I. indicum and B. brizantha cv. Marandu, respectively), but internal P requirements were higher for B. brizantha (0.19 percent vs. 0.15 percent). These levels are lower than those determined for commonly used grasses (e.g., Cynodon nlemfuensis, Panicum maximum, Pennisetum purpureum); therefore, when pastures degrade as a consequence of soil fertility decline, the invading ability that characterizes I. indicum could be enhanced by its lower P requirements.