Healthy soil helps control rain water, preventing run off by absorbing the water. It sustains plant and animal life. It filters pollutants. Soil cycles nutrients and provides a support structure for plants.
When the Soil Food Web is in balance you have healthier plants. A healthy soil has anywhere from 100 million to 1 billion bacteria per teaspoonful. There are also several square meters of fungi, several thousand protozoa, about 2 to 3 dozen nematode bacterial feeders, a few fungi feeding nematodes and a few predatory nematodes. That’s quite the little zoo of critters.
Old forests have about 1000 fungi:100 bacteria in the soil under them
Deciduous trees 100 fungi: 5 bacteria in the soil under them
Shrubs and vines contain 5 fungi: 2 bacteria in the soil under them
Grass is about 1 fungi: 1 bacteria
Which means if you have lots of weeds you want to look at your soil. Weeds are nature’s way of re-balancing the bacteria in the soil. Add some organic matter to your soil if weeds are a big problem.
In a healthy soil, bad organisms that harm your plants will be eaten by the good organisms. Nutrients will be retained in a form and amount that can be used by your plants.
To keep healthy soil you need organic material and you need to avoid tilling, pesticides, herbicides and chemical fertilizers. If you apply a thin layer of compost over your soil it will prevent erosion. When it rains the earth worms will come up, and pull down some of that compost into the soil.
Natural Resources Conservation Service: Soil