The plain is indented with shallow, slow moving rivers that sometimes stop at depressions, to make seasonal swamps and lakes. The main indentation is the Bori river system that runs West to East. Despite its flatness and the occurrence of clay deposits, plain is well drained.
17.3 THE SOILS
The natural soils of the Ogoni plain are Oxisols, the typical soils of the tropical rainforest, described in detail in Chapter 5.5.5. They develop in hot, rainy climates where there is regular downward movement of water, and where there are large amounts of added biomass. Because of leaching, Oxisols tend to be sandy and rather more so on the Ogoni Plain which is formed by sand deposits. Under rainforest, in the natural ecosystem, this sandiness is offset by a high surface concentration of humus resulting in high productivity and efficient recycling of nutrients.
Soil profiles observed in the Botem-Tai district show deep, reddish-brown, well drained, loosely structured, uniform profiles of sandy soils, the clay increasing with depth. In addition, the seasonal swamps gave evidence of isolated areas of sub-surface clay layers. Also there are the humus rich riverine soils created by seasonal waterlogging and a low level of leaching because of locally high water tables.
The sandy nature of the Ogoni soils is why clearing the forest for agriculture results in a very rapid loss of the humus that is essential to the structure and nutrient status of sandy soils; regular burning ensures that very little humus is returned to the soil. Typically, early high yields crash and it is very difficult to restore soil fertility. Impoverished soils, and low and declining yields are the recurrent complaint of the local people.
Soils of the terrace plateau range from dark greyish-brown to very dark brown in colour. They are deep, easy to till, and well-drained except in valley bottoms and low-lying depressions. These soils, known as Ogoni Sands, have a loamy sand surface layer underlain by a sandy-clay-loam sub-surface layer. They contain over 70% sand with little silt and clay. The clay content, about 10% in the surface layer increases with depth to about 30% at 60 cm depth. ...The soils are strongly acidic, ...and except in low-lying depressions their organic matter content and nitrogen contents are low, often decreasing rapidly below the top layer. With the exception of a fairly high available phosphate, plant nutrients are generally low in these soils.
Okonny, Braide and Isirimah in Land and People of Nigeria: Rivers State.
17.4 THE NATURAL ECOSYSTEM
The natural ecosystem of the Botem-Tai district, that is before the major impact of human population upon it, will have been Lowland Tropical Rainforest. It would have been similar, but by no means identical, to forests still seen today in isolated parts of the Niger Delta that are not subject to seasonal inundation and where the soils are more than one metre deep. During the long wet season, the flatness of the country would have allowed the rivers to spread out to form swamps and lakes; some of the lakes may have remained wet all year.
