Based on original article by Petra Marschner, Fertile ground: what you need to know about soil to keep your garden healthy, The Conversation, 28th September, 2016 and video on Gardening Australia, “Old Friends”, 10th May, 2019
Soil is the basis of a garden or landscape. One of the least obvious aspects of soil is that it is teeming with microbes – if it’s healthy soil, anyway! There are many different types of microbes in soil. Some of them form symbiotic or interdependent relationships with the roots of particular plants. There are microbes that live in the roots of legumes (e.g. wattles and peas) and obtain food for their growth from the plant, while they themselves produce nitrogen in a form which the plant then takes up and uses for growth. Other microbes break down organic matter in the soil. Organic matter is one of the main components in soil that bind soil particles together. It gives soil a good structure so that air, water and dissolved nutrients can move through the soil effectively. For the organic matter to do this properly, it needs to be broken down by microbes first.
Garden soil (or soil in virtually any environment) needs to have a healthy and diverse microbial population for good plant growth. How can gardeners restore or increase soil microbial diversity? Because different plant types interact with a variety of different microbes, one way to increase microbial diversity is to plant a range of plant types.
Another way to maintain and improve soil microbial diversity and health is to regularly add organic matter to the soil. While organic matter helps to bind soil particles together and hold water and plant nutrients, it is also the main food for soil microbes. As these microbes constantly consume organic matter, it needs to be regularly added to soil.
Organic matter added as compost is useful for keeping a good structure in soil but does not have a high level of plant nutrients. Digging in fresh young plant matter provides more nutrients for plants but this fresh organic matter is decomposed very quickly. To maintain both good soil structure and sufficient plant nutrients, a combination of composted organic matter and inorganic fertiliser might give the best results for plant growth.
Soil is a complex ecosystem in itself and needs to be given plenty of attention so it remains healthy and productive and supports the best possible plant growth.