The efficient use of fertilizers requires a knowledge of their effects, not only on shoot biomass, but also on below-ground biomass, to identify an adequate fertilization level from the point of view of a better partition between both. Dried root mass per soil volume has been used, especially in perennial crops, as a substitute for estimates of root biomass. With this objective, the root density of three progenies of the pejibaye palm (Bactris gasipaes Kunth) was studied under a combination of four nitrogen (0 to 400 kg/ha/year of N), phosphorus (0 to 200 kg/ha/year of P2O5) and potassium (0 to 200 kg/ha/year of K2O) doses, in a field experiment. The area was located at Ubatuba, SP, Brazil, in an allic alluvial soil, corrected by liming. The samples were collected during the first heart of palm harvest (October, 1993), when the plants (cultivated in a 2 x 1 m spacing) were two years old. Samples were collected, between and within planting rows, using a root-soil auger sampler, 50 cm from the main stem and at a 0-20 cm soil depth. There were differences among genotypes (GE), with progenies 2 and 3 showing, on average, higher root densities than progeny 1 (5.15 and 6.20, against 2.61 g/dm³, respectively). The interaction GE x PO (sample position) was significant: progenies 1 and 2 showed the highest root densities within planting rows (3.30 and 6.60 g/dm³ within row, against 1.92 and 4.70 g/dm³ between rows, respectively), while progeny 3 showed an inverse order (5.57 g/dm³ in the within row, and 6.84 g/dm³ between rows). There was a positive linear response to applied potassium (R² =0.93 to 0.97), with increments between minimum and maximum doses varying from 18 to 28%. There was no isolated significant effect of P, while increasing doses of N provided a positive and significant linear effect, more evident in progeny 1 (averages of 2.26, 2.39, 2.62 and 3.16 g/dm³ for the doses 0, 100, 200 and 400 kg/ha/year of N, respectively). There was no significant interaction between GE and fertilizers (NPK), except for GE x N x P. Although the responses varied with progeny, higher root density was always found at high nitrogen doses.
Bactris gasipaes; root-system; heart of palm; nutrient uptake
