Coastal Plain Soils of Southeastern Nigeria: II. Forms of Extractable Iron, Aluminum, and Phosphorus

Abstract

AbstractThe distribution of citrate dithionite and ammonium‐oxalate extractable Fe2O3 and Al2O3 were determined in nine Coastal Plain pedons of southeastern Nigeria. Distribution of available, organic, and active inorganic forms of P, plus the P fixation and available water holding capacities of these soils were also studied. The dithionite extractable Fe2O3 and Al2O3 generally increased with depth and clay contents in each of the six, loamy, well drained pedons studied. The active Fe ratio usually decreased with depth, suggesting that higher proportions of Fe2O3 were generally present in more crystalline forms in the lower horizons of the well drained soils. The active Al ratio usually increased with depth, but it was high throughout in the more poorly drained, loamy aquic pedons. The well drained ustic pedons contained larger amounts of Fe2O3‐d than did the well drained udic pedons. They also contained more Fe2O3‐d than Al2O3‐d, and the active Fe ratios were higher (≧ 3). The loamy udic pedons and the loamy aquic pedons contained larger amounts of Al2O3‐d than Fe2O3‐d, particularly in the latter. The sandy Aquod was very low in dithionite and oxalate Fe2O3 and Al2O3, except that the spodic horizon had maxima of dithionite and oxalate Fe2O3. Available and total active P contents of the studied soils were generally low. The relative abundance of the extractable inorganic P forms was generally in the order of Fe‐P > Al‐P > Ca‐P in the well‐drained loamy pedons. Organic P in all the pedons was relatively high in the surface horizons and constitutes the most abundant form of P measured in these soils. Phosphorus fixation increased with depth and showed close positive correlations with free iron oxide and clay contents.