I asked Logan Labs once and Susan told me they use the acetic acid extraction method as explained by Barbosa et al 2001.
http://www.tandfonline.com/doi/abs/10.1081/CSS-120000249
http://s3.amazonaws.com/udextension/lawngarden/files/2012/10/CHAP12.pdf
http://www.tandfonline.com/doi/abs/10.1081/CSS-120000249
http://s3.amazonaws.com/udextension/lawngarden/files/2012/10/CHAP12.pdf