Soil contains large numbers of microorganisms - measured in hundreds of thousands per gram of soil. We can estimate the number of viable units in soil by making a dilution in water and plating out the dilution on a Petri dish with nutrient agar. In this petri dish there is the equivalent of 1/1000 of a gram of soil. The large, glassy, translucent colonies are bacteria. The fuzzy colonies are filamentous fungi. The small white opalescent colonies are yeasts (unicellular fungi). The red pigment in the agar is a substance called rose bengal. It helps to restrict the colony growth of fungi so that we can get a better estimate of the numbers. If we did not use rose bengal then the faster growing fungi would overgrow the plate in a few days and obscure or suppress the later or slower growing colonies. SEE PHOTO BELOW -
Soil Dilution at 10 -4 on Rose Bengal Agar
Note how easy it is to count 50-60 colonies per plate. Also most colonies are so well separated it is relatively easy to isolate them in pure culture for further study. There are four Trchoderma colonies on this plate. On other media these colonies would have spread across the plate and cover and suppress or kill the other colonies on the plate.