New protocols for the extraction of nucleic acids from soil

New protocols for the extraction of nucleic acids from soil doi:10.1111/j.1365-2672.2009.04536.x Sir, The publication by Wang et al. (2009), presenting ‘An improved method to extract RNA from soil with efficient removal of humic acids’, is of great interest. The applica-tion of the improved protocol proposed there resulted in the recovery of highly pure RNA from soil samples and increased the amount of extracted RNA tenfold compared to the original method. Therefore, the study may certainly be considered another step towards reproducible results in molecular biological soil analysis. However, an adjustment of the amount of applied Al2(SO4)3, as recommended by Peršoh et al. (2008) to quantitatively remove the humic substances from the analysed soil samples prior to cell lysis, was renounced. The proposed method may therefore be indeed more time-efficient, but may only provide comparable results in an experimental design, in which parallel sub-samples of one and the same soil sample are processed. It has to be considered that all types of nucleic acid extraction protocols, which are based on the separation of humic substances from nucleic acids subsequent to cell disrup-tion, share a common shortcoming. As demonstrated by Crecchio and Stotzky (1998), nucleic acids in aqueous solution irreversibly bind to humic acids. This results in a substantial loss of nucleic acids during such a posterior purification step. Furthermore, the amount of loss corre-lates with the quantity and depends on the, rarely known, quality of the humic substances in a given soil sample. Therefore, an entire removal of humic substances from soil samples prior to cell disruption is highly recom-mended for any kind of quantitative molecular biological studies, in which different soil types or horizons are com-paratively analysed for their nucleic acid content and composition