The Properties of Soil Under the Canopy of the Locust Bean Tree (Parkia biglobosa), in Relation to Tree Biomass, in Farmland in Oyo Area, South Western Nigeria

Abstract

The African locust bean (Parkia biglobosa) is a multi-purpose tree that is commonly retained on small-holder farms in the savanna region of West Africa. Studies that examined the effects of the tree on soils underneath its crown did not examine the biomass parameters of the tree while those that studied the biomass parameters of the tree species did not consider the soil. This study examines the properties of soil under the locust bean tree canopy in farmland in the derived savanna of Oyo area, south-western Nigeria. It also quantitatively characterized the girth, height and crown diameter of locust bean trees in the peasant farms and correlated them with the properties of soils underneath the trees. The soil was sampled at two depths of 0- 10 and 10- 20 cm under and outside Parkia canopies. The t-test was used to compare the means of soil properties under and outside Parkia canopy in order to ascertain whether statistically significant differences exist between them. Organic matter was slightly higher (18- 19%) under Parkia canopy. Exchangeable magnesium, calcium and cation exchange capacity were significantly higher in soil under the tree canopy. This is due to the slightly higher organic matter, reduced leaching under the tree canopy as a result of rainfall interception, the trapping of dust and aerosols by tree crowns and the greater concentration of earthworm casts under Parkia canopy. The properties of the 0- 10 cm layer were more strongly correlated with Parkia biomass parameters than those of the 10- 20 cm layer. Tree height was strongly and positively correlated with soil clay (0.70), organic matter (0.73), total nitrogen (0.69) but negatively correlated with sand content (-0.80) of the 0-10 cm layer. The growth of Parkia trees and seedlings will be enhanced in the more clayey sites but would be retarded in areas with predominantly sandy soils and this should be taken into consideration in afforestation, community forestry and shelterbelt projects involving the establishment of Parkia trees.

Keywords: Parkia biglobosa, tree canopy, tree biomass, soil physical properties, soil chemical properties, soil-plant correlation