Soil amoebae roam the film of water coating each soil particle. This water layer is so thin that an amoeba can survive in very dry soils. An amoeba uses its blobby tentacles both to move and to feed. Amoebae eat so many bacteria that they compete with nematodes for food, controlling nematode populations.
Amoebae belong to the soil recycling crew. They can't use all of the nutrients in the bacteria they eat. So a lot of the nutrients go back into the soil. Amoebae and other protozoa must breathe oxygen, so they mostly live in the most aerated layers of soils.
Flagellates are a type of aerobic protozoan that consume bacteria within the soil. Flagellates do the job of immobilizing and mobilizing nutrients, making them available for plants to use.
Once bacteria are consumed, their nutrients are held within the Flagellates' structure until excretion, manipulation, or consumption of the flagellate occurs. If Flagellates weren't there to process nutrients, a lot of what our plants need to grow would never become available in the soil!
These protists are typically seen quite easily through a standard light microscope because of there size and fluid movement patterns. Their voracious appetites dine on detritus material like minerals, plant material, etc. that can be then solubilized through the ciliates digestion into a plant available food. They have a multitude of sizes, shapes, and behaviors. Some have tails that are used to attach onto objects while others have giant(at a magnification of 400x) spikes sticking out of different parts of their body.
Typically, when we see ciliates we know that we are not getting adequate oxygen for our roots to thrive. If ciliate numbers are too high, then they are most likely to be outcompeting many of the other organisms necessary for healthy soil ecology.