Herbicides are chemicals used to control weeds’ growth. The improper application of
herbicides poses a challenge to agricultural sector as non-target organisms are directly affected and
residues are left in soil. This study was aimed at assessing the effect of paraquat and glyphosate on
soil microorganisms. Fifteen kilograms of composite soil from an Organic Vegetable Garden was
prepared into 15 microcosms; containing 1 kg soil each. Five different treatments were prepared in
triplicates. The treatments include recommended field rate (RFR) (Paraquat-3.62g/kg; Glyphosate-
5.63g/kg), twice the recommended field rate (TRFR) and a control without herbicide. Soils were
serially diluted and cultured on Nutrient Agar (NA) and Potato Dextrose Agar (PDA) for the isolation
and enumeration of both bacteria and fungi groups at 1, 5, 10, 15, 20, 25 days. Isolates were
characterised and identified. Herbicides residues were determined using Gas Chromatography (GC)
after 50 days of exposure. Data collected were subjected to analysis of variance at P = 0.05. The
results obtained indicate that herbicides significantly inhibited bacteria population at 10 and 15 days
with the highest inhibitions of 44.18% and 51.12% by paraquat and 46.00% and 56.92% by
glyphosate at the RFR and TRFR respectively. Fungi populations were significantly inhibited within 5
to 20 days with the highest inhibitions of 22.50% and 80.43% by paraquat and 53.93% and 63.57% by
glyphosate at the RFR and TRFR respectively. Eighty-four bacteria isolates were isolated and
identified to the genera of Bacillus (35), Klebsiella (30), Enterobacter (6), Micrococcus (3),
Pseudomonas (7), Proteus (1), Serratia (1) and Acinetobacter (1). Thirty-two fungal isolates obtained
were identified as: Aspergillus flavus (7), Aspergillus fumigatus (2), Aspergillus aculeatus (2),
Aspergillus tamarii (4), Aspergillus candidus (3) and Fusarium spp. (14). From the GC-Pulsed Flame
Photometric Detector analyses, no significant difference was observed between the herbicidetreatments
and the residues but there was significant difference between herbicide-treatments and
controls. This study showed exposure of soil to herbicides reduces its microbial population and the
isolates obtained, have potentials for use in herbicides biodegradation in the environment.
Keywords: Herbicides, paraquat, glyphosate, microcosm