Localized application of P in the soil has great influence on its availability because it reduces de adsorption reactions, precipitation and fixation, due to the lowered soil contact surface area. This study, carried out in a greenhouse, aimed to calibrate P soil concentrations for corn growth as affected by fertilizer placement. Samples of two Oxisols (LVAd and LVAm) were used. The treatment consisted of 14 combinations of 7 P rates, which were placed in holes in the upper mid part of the pot, with 7 P doses applied in the soil around the holes. Soil samples from both localizations were incubated separately with limestone for 20 days and with P for 40 days before being placed in the pots. Corn seeds were sown in the same holes as the fertilizer. The corn plants were grown for 32 days and the shoot dry matter determined. Soil samples were collected from both positions of fertilizer placement before sowing and after harvesting the plant, plus a composite sample of both soil positions after harvest. The soil P content was determined by Mehlich-1. Dry matter yield was not closely related with the applied P per pot, but it was related with the doses applied in both positions, which demonstrated the effect of localization. Plant growth depended essentially on the dose applied in the "hole". The high critical P level determined for this placement does not imply the use of a high amount of P per pot. The P content determined in the upper soil part after the harvest represented well the mean P availability, but did not show a close relationship with plant growth, indicating the difficulty of establishing a critical level that would represent the mean fertility of the whole soil volume. We suggest adjustments in the calibration method for P that would take into account the localized P applications.
sampling; available P; calibration; recommended doses
