ABSTRACT

The study of the effects of legumes on soil fertility improvement is essential due to declining productivity of many tropical soils which are the major constraints limiting agricultural productivity. A field experiment was conducted at the Eastern farm of Michael Okpara University of Agriculture, Umudike to determine the effects of legumes (Centrosema brasilianum, Centrosema pascuorum and Centrosema pubescens) on the physicochemical properties of soil and their nitrogen fixing potential. The experiment was laid out in a randomized complete block design with three replications and terminated fourteen weeks after planting. Soil samples were collected before and after planting and from natural bush regrowth. Results indicate that the Centrosema species showed significantly (P<0.05) higher percentage increases in organic carbon and organic matter compared to the preplanting soil and natural bush regrowth. However, Centrosema pascuorum gave highest significant values of organic carbon (0.98 ± 0. 01%), organic matter (1.69 ± 0.01%), soil total nitrogen (0.11 ± 0.00%) and available phosphorus (11.60 ± 0.31 mg/kg). It also showed highest significant increase of exchangeable calcium (5.61 ± 0.01 cmol/kg) and magnesium (2.0 ± 0.06 cmol/kg). Centrosema brasilianum gave highest significant values of exchangeable sodium of 0.52±0.0 1 and exchangeable acidity 2.40. There was no significant difference in pH and potassium among the species studied and bush regrowth. Nonetheless, correlation results indicated significant (P<0.0 1) positive association between soil total nitrogen and exchangeable acidity in C. brasilianum and C. pascuorum but a negative correlation in C. pubescens. There was also significant correlation between organic carbon and organic matter in all the legumes except in C. pascuorum. Nodule distribution and plant total nitrogen also varied among the legumes studied. Centrosema pascuorum also gave highest significant number of nodules while Centrosema brasilianum gave higher amount of plant nitrogen 2.82%. This study indicates that among the legumes studied; incorporation of C. pascuorum in the soil may contribute more to improve the nutrient status that would lead to sustainable agriculture. It can be concluded that C. pascuorum should be used in improving soil nutrients.