Two experiments were carried out simultaneously in a greenhouse to evaluate the effects of plant age and regrowth periods on soil and plant phosphorus critical levels for two tropical forage grasses. Treatments corresponded to the 2x5x5 factorial arrangement in the initial growth study, and the 2x2x5 factorial in the regrowth study.Those factorials consisted of two species (Brachiaria decumbens cv. Basilisk and Panicum maximum cv. Mombaça); five plant ages (14, 28, 42, 56, and 70days after emergence) in the initial growth and during two periods of regrowth (28 and 42days) after an initial growth of 42days; and five doses of phosphorus (0, 64, 128, 320, and 640mgdm-3). Treatments were distributed in a randomized block design with three replications. A Red-Yellow Latosol sample was used. Mehlich-1 extractant was used to determine soil phosphorus availability. Soil phosphorus critical levels were greater for B. decumbens than for P. maximum.Soil phosphorus critical levels decreased exponentially with plant ages from 95.1 and 75.6mgdm-3, at 14days and to 38.1 and 28.4mgdm-3 at 70days, for B. decumbens and P. maximum, respectively. Soil phosphorus critical levels were similar in both regrowth periods with values close to those estimated for 70day old plants. Phosphorus critical levels in plant shoot decreased exponentially with plant age from 0.57 and 0.60dagkg-1, at 14days, to 0.08 and 0.07dagkg-1, at 70days, for B. decumbens and P. maximum, respectively. B. decumbens had greater values of plant phosphorus critical levels except for 14-day-old plants. In the regrowth experiment, the phosphorus critical levels, in the shoots were greater for 28-day-old plants, but for 42-day-old plants the critical levels decreased to similar values of those estimated at 56 and 70days after emergence, in the initial growth experiment. Therefore, it is necessary to evaluate plant age after its establishment and at each growing cycle to determine its nutritional status and to recommend phosphorus fertilization.
Panicum maximum; Brachiaria decumbens; Mombaça; forages