You can take 1 g of the 10 cm depth from the soil, and add into 10 ml of the sterilized distilled water tube, then be shaking, and 1 ml of it must transfer into another tube containing sterilized 9 ml, by which you do the serial dilutions. One ml of each tube can place on the plates of Czapex Dox Medium at 25 C for 7 days.