Resumo

SALAS, Ana; RIVERO, Carmen  e  CASANOVA, Eduardo. Source of absorbed phosphorus and relative agronomic effectiveness in maize growing on an ultisol of Cojedes state, Venezuela. Agronomía Trop. [online]. 2006, vol.56, n.1, pp.43-60. ISSN 0002-192X.

Partially soluble phosphate fertilizers manufactured from Riecito phosphate rock (RFR) constitute an alternative to meet P demands of annual crops in Venezuela. The presence of soluble and unsoluble forms of P in such fertilizers has suggested a positive interaction that could increase P utilization of the RFR by the crops. To evaluate this interaction, a greenhouse experiment was carried out with the aid of isotopic techniques (³²P) in order to determine the relative agronomic effectiveness (EAR) and the source of P taken up by maize, Zea mays L., in mixtures of RFR and triple superphosphate (SFT). The amount of P taken up by the plant derived from the soil (Pdfs), the phosphate rock (Pdfr) and the triple super phosphate (Pdff) in the mixture RFR+SFT was compared with the amount of P coming from the P sources applied alone. Results indicated a significantly higher EAR (P<0.01) for the mixture RFR+SFT (72%) as compared with the value given by the RFR (53%) which was due to an increase in the amount of P taken up from the mixture. Relatively constant values of Pdfs for the RFR and RFR+SFT treatments suggested that the presence of soluble P in the mixture did not increase the P utilization from the soil. On the contrary, the presence of a soluble form in the mixture significantly decreased the P utilization of the RFR as indicated by the Pdfr results. Thus, there was no evidence of a higher solubilization and/or P utilization of the RFR resulting from its combination with the SFT. These results suggest that the effectiveness of the mixture RFR+SFT, in comparison with the RFR applied alone, might be better associated with an improvement in the utilization of the soluble P form and/or an increase in the soil's Ca availability.

Palavras-chave : Partially soluble phosphate fertilizers; riecito rock phosphate (RR); triple superphosphate (TSP); mixture of RR and TSP; isotopic techniques (³²P).

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