Soil in Suitability Evaluation for Irrigated Lowland Rice Culture in Southwestern Nigeria: Management Implications for Sustainability

Rice grows in all agro-ecological zones in Nigeria and it is largely grown by small-scale farmers. An investigation was conducted to assess the representative pedons used for rice cultivation in the southwestern part of Nigeria. The aims were to assess how suitable these lands are for rice cultivation and what constraints to continuous and sustained rice production occurs on these soils. Data (climate, soil, water, etc.) were collected from rain-forest agro-ecological zones (AEZ) in which these pedons are located. After laboratory analyses, the land qualities/characteristics (LQ/LC) of these pedons were compared with the land-use requirements (LUR) for cultivating lowland rice using two methods of evaluation (conventional/FAO approach and the parametric method). Results showed that the two methods rated the aggregate suitability of these pedons for rice between marginal (S3) and unsuitable (N1). Major constraints identified in these pedons are poor soil texture, which translates to poor water management coupled with suboptimal nutrient contents [i.e., available phosphorus (P), exchangeable potassium (K), and cation exchange capacity (CEC)], which may predispose rice plants to excessive iron (Fe2+) uptake (or bronzing or yellowing symptoms). Grain yields (GYs) collected from the farmers' field (1994 and 1995 cropping seasons) showed that in the current state of two of the soil series (Apomu and Matako), the GY of the two rice cultivars ranged between 0.61 and 2.13 t/ha and declines progressively across years in the two cropping seasons. The result suggests that in spite of good climate for irrigated lowland rice cultivation in SW Nigeria and indeed all Nigeria, special attention must be paid to the soils on which this crop is grown in terms of the soil texture, appropriate water management, and optimum contents of soil nutrients, especially available P, exchangeable K, and CEC. Thus for a sustained and continuous rice production on these soils, it is important that nutrient contents of the soil be augmented with fertilizer (organic and inorganic) coupled with appropriate water management.