A healthy soil...
... contains billions of microscopic organisms per teaspoon. If the microbes are present in a high diversity, they can work with the plants to keep the soil healthy and the plants will grow better.
For soil fertility, the most important groups are the beneficial bacteria, fungi, protozoa, nematodes and micro arthropods. Further up in the food chain are the larger organisms who keep everything in balance. This is a web of feeding and being eaten in which a huge number of species of each group are present (soil food web). However, in many cases in our agricultural soils and gardens today, bacteria and pests are the only organisms present.
It is possible to wait for soil life to recover on its own through adjustments in tillage, grazing, tree planting. Inoculating the microbial life in the soil is a faster method that is also used in combination with the methods mentioned above.
Restore the soil food web?
It is possible to restore soil life with home-made compost or worm compost in which the important groups of organisms are present.
If you want to buy organism-rich compost and compost with all desirable organism groups is unavailable, it is probably best to combine fungal dominant compost and good worm compost to get a hold of as many organism groups as possible.
In all cases, it makes sense to look at both soil and compost with a microscope to check if organisms are present or not. 2 weeks after inoculating the soil, another check should be done, to see if the organisms are already present in the soil in the right proportions. In general, this process should be repeated several times until the situation in the soil stabilises.