In addition to enhancing water-holding capacity, high organic matter (carbon) performs many other functions in soil, including the maintenance of soil structure allowing water, air and nutrient availability.
Why is life in the soil important?
If you imagine the 4,500 million years of Earth’s history compressed into a normal earthly day, then life begins very early, about 4am. From here until 8:30pm there is no life other than this restless skin of microbes. Then comes sea plants, the odd jellyfish, and by 10pm a few land plants and animals. Then, thanks to ten minutes or so of balmy weather, by 10:24 the Earth is covered in the great Carboniferous forests whose residues give us all our coal. Dinosaurs turn up about 11pm and stay for 45 minutes. Humans arrive 77 seconds before midnight and recorded human history totals about 4 seconds.
Plants evolved about 400 million years ago. Plants take carbon dioxide (CO2), water and sunlight energy (ie. photosynthesis to produce simple sugars) that converts sugars to carbohydrates/ proteins and gives 60% of this energy as exudates in roots as microbe food (a symbiotic relationship). This production system is fundamental to soil, plant, animal and human health.
The key point is that there may be billions of bacterial cells in a gram of healthy, biologically active soil that comprises microbes, microscopic animals and larger animals, such as termites and earthworms. Without this abundant and thriving population in the soil food web the soil is dead and unable to sustain plant growth.
The soil biota (microbes) may comprise a million species or genetically variable ecotypes. However, all of these microbes are fundamentally dependent on the available substrate (eg. grassland, woodland, forest, agricultural crop, etc.) to sustain their population levels. It follows that most of the billions of soil biota will be species that are well adapted to the dominant substrate, at that time and place. Most of the others will exist as very low residual and dormant populations as they are starved of substrate. As conventional methods of describing a microbe population normally detect only high dominant populations the dormant species in the population are often missed.
However, we know they exist and if we add a new substrate, for example a toxin, then we can progressively build up the populations of those bacteria or mixtures of bacteria able to detoxify that soil by using it as a substrate. These bacteria are generally not new species but simply dormant species whose populations have increased due to the selection advantage provided by a suitable substrate, even if that substrate is a toxin. We can use such enrichment culture strategies for the bio-remediation of toxic soils and breaking down even the man-made toxins such as DDT.
The capability of microbes to adapt to disease suppression and detoxification is a critical and important role that is only gaining recognition, and will be vital in an ever-increasing threatened environment.
The management of microbes in our soils is also important to restore food nutrition in food and therefore the health of all land life, and the sustainability of farming.
It is for these reasons, that soil health is the most critical environmental issue for industry and governments to address, and it starts with the soil microbes or biota: the soil food web.
Where do soil biota live?
The majority of soil organisms are found in the top 10cm of soil, and comprise the soil food web. Agricultural practices which change the environment in the top 10cm of soil will impact on the type and number of soil organisms present. Many soil organisms are smaller than soil particles. If the top-soil is lost by erosion or compaction then soil organisms are also lost.
The soil food web
Nature’s soil food web is the basis for all life within the soil, and from there the basis for life outside the soil. The top 30 cm of living soil abounds with organisms that eat and are eaten, digest, excrete, multiply and die. All plants, grass, trees, shrubs and agricultural crops thrive or suffer depending on the health of this dynamic network of living organisms.
A teaspoon of healthy living soil contains:
• Over 600 million individual bacteria
• Several kilometres of fungal hyphae
• Several thousand metres of mycrorrhizal filament
• Up to 100,000 protozoa
• Up to 500 beneficial nemotodes
• Micro-arthropods (mites and springtails)
The understanding of this dynamic ecosystem in our soil is a fairly recent discovery, a benefit of scientific technology. This multitude of organisms has an inter-dependent relationship and perform vital functions for each other and for plants to survive. They decompose organic matter, create humus (made from the dead bodies of soil organisms), help mineral assimilation, bind rock particles together for soil structure, enhance soil permeability, produce hormones for plant growth, promote deeper root growth, protect plants from disease and parasites, convert or bind pollutants that get into the soil, and control soil organism populations and crop pests.
The food source for these micro-organisms is organic matter (OM). In the paddocks and forests, the changing seasons offer the dying grasses and other organic sources (eg. manures) that provide the organic matter for soil life. But in our developed areas these natural resources are rarely available, and as a result this beneficial soil life is decreased, and in cases of continued chemical saturation, virtually eliminated. As soil life is decreased, the availability of nutritional materials needed by plants is decreased. Therefore, in these situations (and on most conventional farming lands) OM has to be imported because the export of OM from farms in the form of food, fibre, fire, etc. exceeds the natural inputs.
Living soils are nature’s normal process, one that has been in motion for billions of years, while land plants have only existed for about 400 million years. The numerous problems we are witnessing in our environment result from our interference with nature’s normal functions and particularly the inter-dependence of plants and microbes that has evolved over millions of years. Our methods of development and input of numerous pollutants into our air, soils and water are continually assaulting organisms, of all life on Earth. The key to improving our soil, plants, our habitat and wellbeing is through nature itself. Enhancing this natural soil ecosystem produces healthier plants and a healthier environment for all living organisms.
Organic based techniques for improving depleted soils offer nutrient-rich living soils, teeming with beneficial microbial life and a balanced nutrient basis to provide for a strong sustainable soil food web.
Note: these notes have been compiled from publicly available sources, including web sites that are listed at the end of this paper, ie. www.bettersoils.com.au.