Influence of Sterilization Methods on Selected Soil Microbiological, Physical, and Chemical Properties

Abstract Valid results from studies of adsorption of labile organic compounds in soil can be obtained only if microbial degradation of the compound is inhibited. This study investigated effectiveness of sterilization of three silt loams with dry heat, cobalt‐60 irradiation, propylene oxide, mercuric chloride, autoclaving, sodium azide, microwave, chloroform, and antibiotics. These treatments were begun after air‐dried, sieved soil was moistened to −30 kPa water potential and incubated for 48 h at 25 °C. Sterilization was effective and microorganisms were eliminated by cobalt‐60 irradiation, propylene oxide, mercuric chloride, and autoclaving 2 × or 3 × as evidenced by a lack of microbial growth on potato‐glucose agar, plate count agar, and nutrient broth. Soil physical and chemical properties were altered by most sterilization treatments. Measured surface areas of the three soils were significantly reduced by propylene oxide. Propylene oxide and sodium azide produced an average pH increase for the three soils of 0.7 and 0.3 units, respectively. All effective sterilization methods except mercuric chloride significantly increased extractable Mn levels in the three soils. None of the effective treatments significantly influenced cation exchange capacity or levels of extractable Ca, Mg, and K of the soils. Mercuric chloride resulted in effective sterilization with minimal changes in soil chemical and physical properties. Following proper safeguards, mercuric chloride could be used to prevent microbial degradation in studies to evaluate adsorption of labile organic chemicals by soil.