It is necessary speaking in term of relative centrifuge FORCE, the correlation between RPM and RCF acording to radius squarre... So, for one rpm in a centrifuge, the relative centrifuge force may be different for the same rpm in another centrifuge. For extraction of dna from environmental samples, i suggest you to use kits. You may have problem in purity and presence of PCR inhibitors (humic and fumic acids) in certain types of environmental samples (soils, muds, faeces). For extraction of DNA from soils, i used the MO-BIO Power Soil DNA Extraction Kit, wich shows good results.