Dissolve a bit of fresh soil in water and look at it with a microscope. It easy to see some members of the soil food web. Protozoa, nematodes and mycelium from the fungi are easy to see. The number and diversity of species are indicator of a nutrient cycling in the soil. (Ingham, E.)
Take in to account that microbial populations differ according to the season you are observing. This method gets more complicated when you want to assess specific groups and populations. For this you may want go get some training.
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