Phosphorus fractionation and crop performance on an alfisol amended with phosphate rock combined with or without plant residues

Abstract

The effects of Sokoto phosphate rock (PR) and plant residues on soil phosphorus (P) fractions and crop performance was studied in the field on an alfisol in the derived savanna of southwestern Nigeria.
The plant residues studied were leaves of Dactyladenia barteri, Flemingia macrophylla, Gliricidia sepium, Leucaena leucocephala, maize (Zea mays) stover and rice (Oryza sativa) straw. They were applied alone or in combination with phosphate rock. The PR was applied at a rate of 60 kg P ha-1 and the plant residues at 5 ton dry matter ha-1 during the onset of the major rainy season. Plots without PR and plant residues application were included as control. After four weeks incubation period of PR and plant residues, Crotalaria ochroleuca (shrub legume) was planted in 1999 and maize (Zea mays) was planted in 2001 as test crops. When PR was not combined with plant residues, incubation of Dactyladenia residues increased resin P from 0.13 to 0.79 mg P kg-1 at 4 weeks after incubation (WAI). Other plant residues did not cause appreciable changes in resin P. Application of Leucaena residues increased NaOHextractable inorganic P (Pi) from 14.36 to 20.47 mg P kg-1 at 4 WAI. Total extractable P increased under
all the plant residues at 4 WAI but decreased at 8 WAI. When PR was combined with plant residues, there were increases in almost all the P fractions under the plant residues at 4 WAI followed by decreases at 8 WAI. Averaged across P levels, addition of Leucaena residues resulted in higher resin P, NaOH-Pi, residual P and total extractable P compared with the other plant residue treatments. Averaged across residue treatments, resin P increased from 0.12 mg P kg-1 to 0.75 mg P kg-1 at 4 WAI and decreased to 0.08 mg P kg-1 at 8 WAI. All other P pools showed similar trends but with less pronounced decreases at 8 WAI. When PR was not applied with plant residues, Crotalaria dry matter correlate positively with NaHCO3-Pi (R2 = 0.59), NaOH-Pi (R2 = 0.53) and total extractable P (R2 = 0.50), while Crotalaria P uptake correlate positively with NaHCO3-Pi (R2 = 0.50) at 4 WAI. Maize grain yield and P uptake did not correlate with any of the P fractions. However, when PR was applied with plant residues, maize P uptake was positively correlated with resin P (R2 = 0.53) while Crotalaria dry matter (R2 = 0.60) and P uptake (R2 = 0.49) correlate positively with NaOH-Po at 4 WAI. Application of PR alone did not affect yields and P uptakes of Crotalaria and maize crops. Compared with the control without residues, Crotalaria dry matter production and P uptake increased significantly when residues were applied. Interaction effects between PR and Leucaena and rice straw residues on Crotalaria dry matter production and P uptake was significant. Interaction effects between PR and plant residues on maize yield and P uptake were not significant. Our
results suggest that combined application of PR with selected plant residues may enhance P dissolution from PR and improve crop performance in some cases.