No abstract

Plants have developed different mechanisms to absorb and solubilize phosphorus (P) in the soil, especially in environments with low P availability. This study evaluated the effects of different winter cover crops on soil P availability in a clayey subtropical (Hapludox) soil receiving soluble P fertilizer and a rock phosphate applied to the summer crop, under no-tillage. The experiment was carried out over 3 yrs (2009)(2010)(2011) with five different cover crop species: common vetch, fodder radish, ryegrass, black oat, white clover and fallow as control. The soil was sampled after the third year of cover crop cultivation and analysed for inorganic and organic P forms according to the well-established Hedley fractionation procedure. Phosphate fertilizers promoted accumulation of both labile and nonlabile P pools in soil in the near surface layer, especially under rock phosphate. Fertilizer applications were not able to change P fractions in deeper layers, emphasizing that the Brazilian clayey soils are a sink of P from fertilizer and its mobility is almost nil. Although the cover crops recycled a great amount of P in tissue, in a short-term evaluation (3 yrs) they only changed the content of moderately labile P in soil, indicating that long-term studies are needed for more conclusive results.

Use of cover crops in an integrated agricultural system can reduce demand of inorganic phosphorus (P) fertilizers, where the subsequent crops can take up P accumulated in cover crops biomass after the decomposition. In this research we hypothesized that some cover crops can take up P from less labile fractions and recycle it back to the soil through plant residues resulting in better P use efficiency of the system; cover crops are capable of P uptake from subsurface layers which leads to the accumulation of this P on the surface after the decomposition of their residues; and cover crop species respond differently to distinct inorganic P sources. To examine these hypotheses, a field experiment was conducted over nine successive years in South Brazil. The experimental treatments were established as a split-plot randomized block design, in a 3 × 6 factorial scheme, considering three P treatments [no-P, single superphosphate (SSP), and rock phosphate (RP)] as main plot and six cover crop treatments (common vetch, white lupin, fodder radish, ryegrass, black oat, and a fallow) as subplots. Soil samples were collected from the depths of 0-5, 5-10, and 10-15 cm and analyzed by Hedley P fractionation. In P-unfertilized cropping system, the amounts of labile and mod-labile P fractions were not modified by cover crops related to fallow. When inorganic P fertilizers were applied, the amount of labile and mod-labile P pools under fallow were higher than under cover crops in 5-10 cm depth. Black oat and common vetch cycled more labile P under SSP and RP, respectively. Common vetch and ryegrass resulted in the highest accumulation of organic P on the surface under SSP and RP, respectively. Black oat was capable to change P extracted by 1.0 M HCl to more labile forms. Fodder radish showed the highest P uptake in comparison with other cover crops. The higher P balance efficiency of the system was achieved under SSP in comparison with RP application but it seems that cover crops are more effective at improving the efficiency under RP compared to SSP.

No abstract

Soluble phosphates are the most common sources currently used in crop production in tropical soils; however, they present low efficiency and are more expensive than natural rock phosphates. The objective was to develop new phosphate fertilizers with slow solubility through the partial acidification of rock phosphates (RPs), incorporating materials with adsorption characteristics to favor slow dissolution and prevent phosphorus (P) fixation in the soil. Three rock phosphates, Araxá (ARP), Bayovar (BRP) and Morocco (MRP), were evaluated at two acidulation levels (25 and 50% Ac.) and two additives; pillared clays (PILC) and zeolites (Zeo), plus triple superphosphate (TSP) and a control (nil-P). The soil diffusion was evaluated in concentric rings in Petri dishes. Solubility was evaluated in leaching columns and sampled in layers from surface for P forms in the soil profile. The relative agronomic efficiency (RAE) was evaluated in maize. Greater diffusion was provided by TSP, followed by BRP and MRP both with 50% Ac. + Zeo, and MRP with 50% Ac. + PILC. Percolated P was more pronounced under TSP, followed by RPs (BRP and MRP) with 50% Ac. + Zeo. BRP and MRP + 50% Ac. were the most promising sources with RAE above 74% compared to TSP.