DISPONIBILIDAD DE FoSFORO EN UN SUELO DE LAS SABA NAS BIEN DRENADAS VENEZOLANAS, BAJO DIFERENTES COBERTURAS Y TIPOS DE FERTILIZACIoN

In order to evaluate the effect of cover crops (grass or legume) and the fertilization type on phosphorus availability in a system no-tillage corn - cattle, a field experiment was conducted on a Typic Plinthustults soil in well drained savannas of Vernezuela. The design was a completely random factorial arrangement 24 (cover crop fertilization), with three replicates. The cover crops were Urochloa dictyoneura (UD), Centrosema macrocarpum (CM), and the natural savannah (SN) as control. The P fertilization types were Control (no fertilizer), BRF + M (low dose, 25% P as rock phosphate + mycorrhizal inoculation), ARF (high dose, 100% P as rock phosphate), and ARF + FD (high dose, 50% P as rock phosphate, and 50% P as diammonium phosphate). Soil samples were taken at three depths (0-5, 5-15 and 15-30cm), and at various times since the introduced cover crops establishment in 2002, till completion of three cycles of no-tillage corncattle (2008). Available phosphorus (PD-Olsen) and sequential phosphorus fractionation were evaluated. Results indicate statistical differences (pUD>SN, while the trend of fertilizer sources varied according to type of cover crops, being in CM: ARF+FD>ARF>Control=BRF+M, while in UD it was ARF>ARF+FD>Control=BRF+M. The introduction of cover crops increased all P fractions with respect to SN, mainly the moderately labile fractions, and the P-organic>P-inorganic.