This study was carried out with the objective of relating the size of the particles of two natural rock phosphates with their relative agronomic efficiency (RAE), for corn in two successive growth, without revolving the soil, in greenhouse, in an oxisol, corrected for pH 5.5. The sources of natural phosphates were the Gafsa and the Gantour Black rock phosphates, with the following sizes of particle: a) <0.074mm, b) of 0.074 to 0.149mm, c) of 0.149mm to 0.297mm, d) of 0.297 to 0.71mm, e) of 0.71 to 1.41mm, f) >1.41 mm and g) as received, in which more than 80% of the particles presented the size between 0.074 and 0.5mm. The triple superphosphate with granules between 2 and 4mm was used as standard. For the first crop, the finely grounded (<0.074mm) presented a higher RAE than all the other particle sizes, including, as received form. While the particle size increased, the RAE of the rock phosphates decreased, however, particles up to 0.297mm reacted. For the second crop, the RAE increased with the size of the particles between 0.149 and 0.71mm. Particles with the size more than 0.7mm did not react during the two crops. For the second crop, the RAE of the rock phosphate as received form, was not strongly reduced in comparison to the finely grounded (0.074mm) phosphates. The rock phosphates as received form maintained the RAE around 40% in the two corn crops. The Gafsa rock phosphate was more efficient than the Gantour Black to supply phosphorus in initial growth of corn plants.
phosphorus; index of relative agronomic efficiency; phosphorus sources
