F.K. SALAKO¹, G. TIAN² and B.T. KANG³

¹Department of Soil Science and Agricultural Mechanisation, University of Agriculture, Abeokuta, Nigeria
²Institute of Ecology, University of Georgia, Athens, USA
³Haverhill, USA

Abstract

This study was carried out in 1994 in the derived (DS) and northern Guinean savanna (NGS) zones of Nigeria at about 17–18 weeks after planting forage legumes. The forage legumes, which included Cajanus cajan, Crotalaria ochroleuca, Stylosanthes hamata, Centrosema pascuorum, Lablab purpureus, Mucuna pruriens (black and white) and Pueraria phaseoloides, were treatments in the randomised complete block design. Root morphology and length were evaluated for the 2 zones, but light received under canopies, soil temperature and soil water potential were measured only in the DS. Roots on soil profile walls were traced, and in the laboratory, computer software was used to evaluate root length after scanning. Canopy coverage was evaluated using a light meter. In the DS, the presence of gravel in the subsoil modified the morphology and distribution of roots even for the same species compared with the root architecture in the NGS zone. A hardpan in the shallow soil layers in the NGS caused a proliferation of roots in the topsoil, and a tendency for increased diameter (visual observation) of the primary roots. Total root length ranged from 834–1861 mm in the DS, and 939–1277 mm in the NGS, and was similar in both locations. Legume canopies intercepted 81–99% of sunlight, suggesting that they had the potential to maximise sunlight for their growth under adequate soil moisture and nutrient in the moist savanna. Legumes such as C. cajan, L. purpureus, M. pruriens and P. phaseoloides with potential to grow roots beyond 60 cm depth and which intercepted ≥ 95% of incident sunlight were recommended for sustained growth and rapid regeneration after cutting or browsing. Furthermore, the legumes can be combined with shallow-rooted crops for enhanced crop production in a crop-livestock farming system or in pasture, based on the differences in their root and canopy growth.