How are living organisms important to the soil?

Answer 1

Living organisms in the soil break down dead organic matter and convert it into forms that can be used by plants. They also attack or compete with disease-causing organisms.

Soil that hasn't been treated with chemicals is alive, with billions--trillions, in a rich organic soil--of microorganisms in a teaspoon of soil. In the absence of chemical fertilizers, plants have to work with the Soil Food Web to get nutrients from the soil. They pump a steady stream of sugars, starches, and sloughed-off root cells into the soil, to encourage a population bloom of microorganisms around their roots.

This is why the Rhizosphere, the soil zone within 1-2mm of roots, is one of richest, most biodiverse ecosystems on the planet. Roots are literally encrusted with bacteria and fungi feeding at the trough. Fungi are creatures of the soil. Their hyphae can reach hundreds of feet, and they can transport soil nutrients anywhere in the network. Plants provide food for the fungi, and fungi gather widely-disbursed soil nutrients (like phosphorous) from beyond the reach of the plant's roots, and transport them to the plant.

Predatory protozoa and nematodes by the millions swim around the perimeter of the roots, picking off bacteria and attacking fungi. Earthworms and red compost worms also join the feeding frenzy. The wastes they pass into the environment after digesting them act as a "micro-manure", providing nutrients in plant-available form right at the root hairs of plants, where they can easily be taken up.

Disease-causing organisms like root-knot nematodes and fungal pathogens have to fight their way through the crowd and brave predators to attack plants.