A variety of air-dried soils from 7 different
parts of the world (Table 3) were diluted (1 : 100)
and heat-treated (55°C, 6 min) to eliminate most
bacteria [7]. The soil suspensions were spread over
0.45-/~m Milipore membranes placed on the surface
of the previously described semi-defined agar but
containing glucose (2 g/l) in place of fructose. The
agar was supplemented with 50 /~g/ml of cycloheximide
and 50 mg/ml of nystatin to control the
fungal growth. Oxytetracycline was either absent
(control plates) or added to give a final concentration
of 25/~g/ml