Assessment of structural stability of a degraded sandy clay loam soil under cover crops in South-Eastern Nigeria

The effects of bare, two legumes and four grasses cover treatments on the structural stability of a sandy clay loam Ultisol were studied within a two year period. The experiment was of a randomised complete block design with seven treatments. The legume treatments were Centrosema pubescens (Ce) and Pueraria phaseoloides (Ps); while the grass treatments were comprised of Panicum maximum (Pm), Cynodon plectostachyus (Cp), Axonopus compressus (Ac) and Pennisetum purpureum (Pp). The Organic Matter (OM) content in plots under vegetative cover was significantly(P < 0.05) higher than that in the bare plots, with plots under Panicum maximum having the highest organic matter (OM) content (1.8%) at first sampling compared to the bare treatment plots which had the least (0.4%). After twenty four months it was observed that on the average the percentage of the aggregates in the size range exceeding 1.00 mm was lowest (20.5%) for soils under the bare treatment compared to those under grass (48.4%) and legume (35.5%), indicating that more than a half (54%) of the aggregates of the bare plots were small sized (< 0.50 mm diameter) whereas about half of the aggregates of grass treated plots were in the macro-aggregate range. Soils under vegetative cover generally showed significantly higher mean weight diameter (1.04 – 1.66 mm) in relation to bare treatment plots (0.84 mm), with soils under Panicum maximum showing consistently higher (1.66 mm) mean weight diameter compared to the other vegetative cover treated soils. Soil aggregates under grass treatment had the lowest range of Potential Structural Deformation Index (PSDI) values (14.63 - 20.13 %) compared to aggregates under legume treatment (24.07 - 27.17 %).The PSDI values indicated that soils under vegetative cover were on the average twice more stable to rain drop impact deformation compared to soils in bare plots.Keywords: cover crops, Ultisol, soil structural stability.