Maize yield response as affected by phosphorus, sulfur and nitrogen as banded applications on a volcanic ash derived tropical soil
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Phosphorus and sulfur deficiencies have been observed on many of the volcanic ash derived soils in Central America. One exploratory experiment was initiated in 1987 to examine maize (Zea mays L.) grain yield response to phosphorus, sulfur and nitrogen applied together on a volcanic ash derived soil in the Pacific coastal plain of Guatemala. Four cycles of yield data were collected in 2 rainy and 2 dry seasons. Treatments consisted of rate combinations of N (0, 100 kg ha-1 as urea), P (0, 22 kg ha-1 as triple superphosphate) and S (0, 57 and 114 kg ha-1 as CaSO4.2H2O) applied together in continuous bands in a randomized complete block design. Residual P and S response was measured during the last two cycles, whereby only urea was applied at a constant rate to all plots. The combined analysis of the first two cycles demonstrated a linear response of yield to applied S when no P was applied (4.95, 5.75 and 5.95 Mg ha-1 at 0, 57 and 114 kg S ha-1 respectively), while yield response was quadratic when S as CaSO4.2H2O and P as triple superphosphate were applied together in a continuous band at 100 kg N ha-1 (5.38, 6.38 and 5.48 Mg ha-1 at the same S rates respectively). Response of yield to S was linear without and with P for the combined residual analysis of the last two cycles at the same N rate (4.65, 4.94, 5.26 and 4.68, 5.53, 5.56 Mg ha-1 respectively). Grain yields were maximized over the four cycle period using a joint N, P, S band (100 kg of N as urea, 22 kg P as triple superphosphate, and 57 kg S as CaSO4.2H2O ha-1). It is hypothesized that precipitation of dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate (DCP) took place within the joint N-P-S band subsequently reducing the amount of P fixed as Fe or Al hydroxides and/or amounts of P complexed with amorphous allophane. The precipitation and subsequent dissolution of DCPD and DCP within the band could have increased P availability with time. Alternatively, SO4 = blocking of adsorption sites could have increased P availability by reducing the amount of P fixed by the soil. It is possible that both of the previously mentioned mechanisms played a role in first cycle yield responses since increases were noted at the low S rate (57 kg S ha-1), while reductions were found when the S rate was doubled (114 kg S ha-1). Significant residual response for the last two cycles was observed for the joint triple superphosphate, CaSO4.2H2O, urea band, especially at the high S rate. This suggests that excess precipitation of DCP and DCPD could have occurred and that this provided delayed dissolution of the precipitated P reaction products. Chemical characterization of precipitated reaction products within the band is needed as well as further verification on similar soils in order to validate the observed response.
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