The invention provides methods and compositions, e.g., kits, for removing contaminants from nucleic acids in a sample, e.g., environmental or biological samples such as soil, food, plant, animal, microorganism or water samples. The invention provides methods and compositions for isolating nucleic acids from environmental and biological samples in a scaleable process free of contaminating substances that inhibit PCR and other downstream applications. Exemplary sample types include soil, water, plant and food. The methods and compositions of the invention can be used for isolating and/or detecting nucleic acids from prokaryotic and eukaryotic organisms and for detecting multiple types of organisms in a sample. Thus, the methods and compositions of the invention are useful for detecting organisms pertaining to agriculture, forensics biology and/or combating bioterrorism.