Seven methods for bacterial DNA extraction and purification from soil sampl
es were compared. Holben's direct lysis method recovered significantly grea
ter amounts of DNA than the other methods tested, while CsCl-ethidium bromi
de density gradient ultracentrifugation was better than gel filtration at r
emoving humic acid from crude DNA isolated from soil. When both these metho
ds were combined, 5.94 mu g of DNA (A(260/280) ratio around 1.754) was yiel
ded g(-1) oven-dried sandstone shale alluvial soil; similarly satisfactory
yields were obtained from Taiwan clay, and sandstone shale and slate alluvi
al soil managed under different farming practices. DNA obtained by these me
thods was readily digested by EcoR I and Hind III. When soil samples were s
tored for 3 weeks at 4 degrees C, the fraction of high-molecular-weight DNA
was reduced significantly. Thus, DNA extraction should be carried out as s
oon as possible after a soil sample has been collected from the field. When
hyphae of Pythium aphanidermatum and Fusarium solani were subjected to the
above lysis method, DNA could not be detected in the extract.