Clay minerals in eight soil profiles from southern Nigeria were investigated by means of X‐ray diffraction, infrared spectroscopy, differential thermal analysis, and electron microscopy. These soils are derived from basement complex rocks, arenaceous sedimentary rocks and basalts.Kaolinite is the most predominant clay mineral for all the soils studied. A pronounced variation in clay mineralogy was found in the soils derived from basement complex rocks sampled at various locations along a toposequence. Halloysite occurs in freshly weathered materials, while smectite is found in poorly drained, lower members of the toposequence.Gibbsite occurs in soil derived from cretaceous arenaceous rocks in the high rainfall area. Among all soils studied, the highest amounts of amorphous Al silicate and extractable Fe and Al oxides were found in those derived from basalts. In all cases, Fe oxides exist predominantly in crystalline forms.
Field observations were carried out on an Egbeda soil series in western Nigeria to study the degree of soil-nutrient variability within and between plots due to pre-clearing vegetation. The soil showed lower soil pH, organic C, exchangeable K and Ca contents following cassava grown by traditional methods than following secondary forest vegetation or thicket regrowth. Except for exchangeable K, the degree of variability of the above parameters was observed to be ill the following order: secondary forest vegetation > thicket regrowth > cassava plots. The available P status was generally low and showed little relation to pre-clearing vegetation. The implications of soil variability due to pre-clearing vegetation on soil sampling and in field experimentation are discussed.
Long-term land quality studies on a toposequellce, presenting a continuum of edaphic and hydrologic conditions from dry land to hydromorphic valley bottom opened interesting possibilities for sequential testing of drought tolerance of rice varieties.Variations in the soil nitrogen cycle according to the location in the toposequence may be used for further fundamental and applied research.The phenomena of iron deficiency in tile higher part of this specific toposequence and of iron and/or managanese toxicity in the lower part can be used in screening varieties resistant or tolerant to iron deficiency and to iron and/or manganese toxicity.A very close relationship was found between tile occurrence and severity of blast (Pyricularla oryzae) and the dynamics of the moisture status of the soil; tile rice on the upland part was severely affected after drought stress. Blast affected some varieties much more than others in the dry land part of the toposequence. The principles of sequential testing are also valid for other crops on which research is. now proceeding. Sequential testing of crops can be used for other conditions where one or several environmental parameters change over short distances.
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