Removal of humic substances from soil DNA using aluminium sulfate

Direct extraction of soil DNA has become essential for the study of soil microorganisms. Humic substances co-extracted during DNA retrieval is a big problem because it greatly inhibits the enzymes involved in manipulating DNA. Popular commercial kits available for soil DNA extraction are unable to overcome this problem. Here we report an effective protocol for the removal of humic substance from soil DNA. The protocol involves flocculation of the humic substance by excessive Al3+, then removal of superfluous Al3+ via pH adjustment and finally release of soil microbial DNA by SDS lysis. This technique is superior to that employed by the UltraClean Soil DNA Kit and can be applied to a wide variety of soils.