Phosphorus forms, lateritic nodules and soil properties along a hillslope in northern Ghana

Inceptisols and Alfisols formed from Volta shales in northern Ghana are moderately to strongly weathered, and contain varying amounts of lateritic nodules and sesquioxides. The plant-available P and fertilization potential of these soils are affected by the presence of the lateritic nodules, which act as P sinks. We examined the amounts and distribution of P and related them to the nodule content, to chemical properties of soil fines (<2 mm) and nodules, and to soil development on gentle hill slopes. Total P of soil fines in the surface horizons ranged from 80 to 280 mg kg−1, and total P of nodules ranged between 430 and 900 mg kg−1. Resin- plus bicarbonate-extractable labile P was between 7 and 18 mg kg−1 and mostly less than crop requirements. On the upper slope, where the topsoil contained large amounts of nodules, small nodules contained more P than large ones. This suggests that the small nodules are an efficient sink for P that cycles actively in the topsoil. Despite this, the labile P of soil fines in the surface horizon was greater on the upper slope (18 mg kg−1) than the lower slope (7 mg kg−1), suggesting that the sorption is partly reversible and that P-rich nodules can maintain elevated native P levels in surrounding fines. Organic P accounted on average for 35% of total P in the surface horizons, with highest proportions at mid-slope positions. Comparison of the amounts of Ca-bound P (extractable with dilute HCl) in soil fines and silstone fragments indicated (i) that there had been external Ca inputs to the upper slope soils (probably from fire and dust), (ii) that there was primary mineral P present on the eroded mid slope, and (iii) that pedogenic Ca–P had been formed on the periodically waterlogged lower slope.