The right answer is false, they have great impact on soil health.
In general, microorganisms play a fundamental role in the fertility of the soil (production of the minerals necessary for the plant: N, P, K, etc.) and the study of their activity is therefore important. In addition, certain bacteria participate in the release and transformation of metals in the soil and thus contribute to increasing or reducing the toxicity of heavy metals (nickel, cobalt, etc.).
Humus is essential in the function of soil production.
Humus is used primarily as a "pantry" for plants, and also plays a vital role in structuring the soil.