An improved method for preparing nucleic acid extracts from environmental samples contaminated by polymerase inhibitors such as humic and fulvic acids is provided. The methods of the invention utilize chemical compositions capable of precipitating humic and/or fulvic acids from organic samples. The methods may be used in connection with the preparation of DNA and RNA extracts, thereby providing purified preparations suitable for amplification using PCR, RT-PCR, and other nucleic acid amplification technologies. Reagent kits for preparing a purified nucleic acid-containing extract from an environmental sample of soil, fluid, or organic particles, using the chemical compositions of the invention are also provided.