PHOSPHORUS REQUIREMENT AND NODULATION OF HERBACEOUS AND SHRUB LEGUMESIN LOW P SOILS OF A GUINEAN SAVANNA IN NIGERIA

There is increasing interest to improve the N-2 fixation and P use eff iciency of herbaceous and shrub legumes currently being introduced in cereal-based cropping systems in the moist savanna zones of West Afric a. Knowledge about N and P nutrition of these legumes can assist in ad apting them to new areas where fertilizer use is not feasible by the s mallholder farmers. Pot and field experiments were carried out to exam ine variation among potential herbaceous and shrub legumes for their a bility to nodulate and to use P efficiently. These legumes were grown at two P levels (0 and 7 kg P ha(-1)) in soils collected from two fiel ds with different cropping histories (compound and degraded fields) at Yamrat in the northern Guinea savanna of Nigeria. In the compound fie ld, animal manure and household residues are applied annually whereas in the degraded field, cereals are grown continuously with minimal org anic inputs. Significant differences in growth, P content and mycorrhi zal infection rate occurred among the different legumes as affected by P and cropping history. Legumes grown in soil from the degraded field responded more to P application than those grown in soil from the com pound field. Phosphorus responsive legume species e.g. Mucuna pruriens var. utilis (black seed) and Crotolaria ochroleuca, had a low mycorrh izal infection rate compared to lower P responsive species such as Lab lab purpureus and Cajanus cajan. A significant relationship was found between mycorrhizal infection rate and plant growth but not with nodul ation of these legumes. All legumes nodulated naturally and the propor tion of N derived from atmospheric N, ranged from 38% for Centrosema b rasilianum to 90% for L. purpureus. Application of P increased the wei ght of nodules but did not improve the proportion of N-2 fixation. Thi s study showed that species such as L. purpureus has a high N-2 fixing capability and can also adapt to low P soils conditions in the northe rn Guinea savanna of West Africa.