Studies on a Nigerian Forest Soil

Summary Features of the profile of a mid‐slope forest soil typical of the red concretionary variant of the Iwo Series, which is common in Western Nigeria, are the well‐defined gravel horizon from 7 to 50 in., and the sandy horizon above it. The carbon content in the top 2 in. is 3.95 per cent. There is a very sharp fall to 0.38 per cent. below 2 in. The percentage carbon then decreases slowly down the profile. The C/N ratio is also high in the worm casts (15.7) and topsoil (13.6); below z in. it is much lower, and falls to 3.1 at 60 in. Both the root growth and worm activity are confined to the top few inches of the profile. The pH in the 0–2 in. horizon is 7.7, representing a considerable accumulation of bases in the surface soil. Below 2 in. the pH is fairly constant at about 6.7 down to 50 in., when it falls again to 5.6. Although contributing only 2.5 per cent. to the weight of the top 4 in. of soil, the plant residues comprise 17.3 per cent. of the organic matter in this layer. Most of the organic matter, however, is in the coarse‐silt aggregate fraction. After removal of plant residues, the C/N ratio of the remaining fine‐earth fractions falls as the particle size of the fraction decreases. Aggregation in the surface soil is caused by organic matter, but at 20–30 in. aggregation is due to the high free iron oxide. Extraction of the aggregate fractions with 0.5 M sodium hydroxide and 0.1 M sodium pyrophosphate (pH 7) indicates the varying nature of the organic matter in these fractions.