Asian Journal of Soil Science and Plant Nutrition

Slope gradient by approaches which described organic matter dynamic and aggregate size distribution are important in guiding management practices of the soils of coastal plain sand. Study was conducted to evaluate effect of slope gradient on soil physical properties and organic matter quality in aggregates size fraction on coastal plain sand southern eastern Nigeria. Five soil samples were collected at 20m interval from each of the twelve selected slope, based on gradient (<10%, 11-20%and >20%) A total of Sixty samples were collected and analyses in the laboratory for physical and chemical properties. Each of the Sixty soil sample were separated with the used of dry sieving methods, into four aggregate Size fractions: 4.0-2.0, -2.0 -0.25, 0.25-0.05 and <0.05mm. The separation of the 60 soil samples gave a total of 240 aggregate. Size fractions. The date generated were analyzed in terms of slope gradient, as well as nitrogenous and non-nitrogenous organic compound in aggregate Seize fractions. The result indicated that slope gradient affect nitrogen and non-nitrogenous organic compound. Humin substance with aggregate size fraction of <0.05mm of <10% slope had high mean value of 0.68%. <10% and 11-20% slope high mean variability, while >20% slope had moderate variability in humin substance with aggregate size fraction of 4.0-2.0mm with <10% slope. Humic substance of 2.0-0.25mm showed that <10% slope had low mean value, while 11-20% had high mean value of 0.31 and 0.39%, respectively. Effect of slope gradient on carbohydrate content with aggregate size fraction of 2.0-0.25mm, indicated that 11-20% slope had high of 0.96%, while >20% slope had low mean value of 0.86%. Carbohydrate substance had high mean percentage among all organic substances even with high values, they are rapid disintegrated, while humin, humic and fulvic acid have long term effect in soil aggregate stabilization.