ABSTRACT
Soil microbial diversity is influenced by both organic and inorganic matter. This study was aimed at determining the effect of organic manure and inorganic fertilizer on the microbial and physicochemical properties of agricultural soil. Top soil (up to 5 cm depth) samples were collected from uncultivated area from the Faculty of Soil Science, University of Benin, Benin City, Edo State, Nigeria with no prior organic manure or inorganic fertilizer treatment. Microbial analysis was done on the collected soil samples treated with herbicide [Glyophosphate], organic [chicken droppings] and inorganic fertilizer [NPK; 20:10:10]. Soil parameters assayed included pH, electrical conductivity, moisture level, organic carbon, total nitrogen, phosphate and sulphate, potassium and heavy metal contents. The highest bacterial and fungal load in organic manure treated soil was 68.33 ± 4.41 x 102 cfu/g (week 6) and 78.33±0.88 x 102 cfu/g (week 4) respectively. The least bacterial load in inorganic and organic fertilizer treated soils were 22.67 ± 4.81 x 102 cfu/g (week 8) and 24.33 ± 0.67 x 102 cfu/g (week 2) respectively. Bacterial isolates detected in soil samples were Enterobacter aerogenes, Pseudomonas aeruginosa, Pseudomonas fluorescens, Staphylococcus aureus, Micrococcus luteus, Bacillus cereus, Bacillus firmis and Escherichia coli, while the fungal isolates were Penicillium sp, Candida sp, Aspergillus sp, Fusarium sp and Trichoderma sp. The highest occurring bacteria and fungi were P. aeruginosa (27.54 %) and Candida sp (32.69 %) respectively. The least pH occurred in fertilizer treated soil (6.87 ± 0.05); least temperature was recorded in organic manure treated soil (27.63 ± 0.27), while control soil recorded the least conductivity (47.47 ± 1.84 µ/cm), moisture content (6.58 ± 0.22 %), organic carbon (5.20 ± 0.02 %) and total nitrogen (5.80 ± 0.04 %). Also, at the end of the study, organic manure soil treated soils was observed to show the least sulphate content (5.89 ± 0.33 mg/g), while the control soil recorded the least phosphate (4.6 ± 0.30 mg/g) and potassium content (5.64 ± 0.10 mg/g). Significant different was obtained in heavy metal analysis in the course of study with the least values recorded at the end of the study. At the end of the study, the least iron, cadmiun, lead and zinc content were recorded in the control soils with values at 4.69±0.19 mg/g, 0.17±0.03 mg/g, 1.54±0.05 mg/g and 2.51±0.26 mg/g respectively. The highest iron content (9.55±0.01 mg/g), cadmium (1.45±0.15 mg/g), lead (9.02±0.49 mg/g) and zinc (11.23±0.13 mg/g) were recorded in organic manure treated soil. Approved moderate use of fertilizers could go a long way in reducing the effect of these materials on microbial diversity and up the food chain to human life.
