PHOSPHORUS FORMS, LATERITIC NODULES AND SOIL PROPERTIES ALONG A HILLSLOPE IN NORTHERN GHANA

Inceptisols and Alfisols formed from Volta shales in northern Ghana ar e moderately to strongly weathered, and contain varying amounts of lat eritic nodules and sesquioxides. The plant-available P and fertilizati on potential of these soils are affected by the presence of the lateri tic nodules, which act as P sinks. We examined the amounts and distrib ution of P and related them to the nodule content, to chemical propert ies of soil fines (< 2 mm) and nodules, and to soil development on gen tle 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 an d 900 mg kg(-1). Resin-plus bicarbonate-extractable labile P was betwe en 7 and 18 mg kg(-1) and mostly less than crop requirements. On the u pper slope, where the topsoil contained large amounts of nodules, smal l nodules contained more P than large ones. This suggests that the sma ll nodules are an efficient sink for P that cycles actively in the top soil. 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 f ines. 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 fi nes 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 waterlo gged lower slope. (C) 1998 Elsevier Science B.V. All rights reserved.