Bioremediation is an advantageous approach to decontamination of many soils. This thesis focuses on enhancement of pentachlorophenol (PCP) biodegradation in soil through the use of comminuted plant materials as soil amendments. The influence of a variety of soil amendments on biodegradation of PCP was examined. Amendment of soil with inorganic nitrogen, inorganic phosphorus, soluble carbon, or combinations of the three, did not increase biodegradation of PCP; however, its biodegradation was significantly enhanced when the same soil was amended with comminuted plant materials. Amendment of soil containing ca. 150 mg PCP/kg with red clover or alfalfa at 1% (w/w) increased biodegradation of PCP by introduced Sphingomonas chlorophenolicus cells (formerly Flavobacterium sp. ATCC 39723), and reduced soil PCP concentrations to $<$1 mg/kg. Comminuted wheat straw also enhanced biodegradation of PCP, although a higher rate of application (5%, w/w) was required. Amendment of heavily contaminated soils (ca. 1,000 mg PCP/kg) with red clover or alfalfa did not enhance biodegradation of PCP by Sphingomonas cells; however, when the same soils were amended with comminuted wheat straw (5% and 10%, w/w) Sphingomonas cells survived and reduced PCP concentrations to ca. 5 mg/kg. Amendment particle-size significantly influenced biodegradation of PCP. Rapid and extensive biodegradation of PCP was observed in soil amended with wheat straw (2.5%, w/w) of particle size $<$300 μm; however, PCP was not degraded when the same soil received coarser wheat straw particles ($>$300 μm) at the same application rate. Oxygen consumption and redox potential studies indicated that leguminous amendments promoted anoxia in soil and, thereby, inhibited the activity of Sphingomonas cells. Amendment of soil with wheat straw did not result in anoxia. Batch adsorption studies, utilizing \sp14C-PCP, indicated that PCP was adsorbed more extensively by leguminous materials than by non-leguminous materials. Amendment of soil with a rhamnolipid biosurfactant, physical disruption of soil aggregates, and removal of chloride did not reduce residual soil PCP concentrations. Studies conducted at an industrial wood-preserving site, on soil containing PCP (ca. 700 mg/kg) and PAHs (ca. 1,500 mg/kg), indicated that the use of comminuted plant materials as soil amendments enhanced large-scale bioremediation.