PeCAN asked Eddie Bailey, geologist and soil food web specialist, if we could reproduce an excellent article he wrote for The Soil Association. Eddie's approach to soil brings a whole new evolutionary understanding to the subject which will only result in greater respect for the brown stuff we all rely upon.
The soil food web
350 years ago, a Dutch draper called Van Leeuwenhoek (pronounced van-LAY-ven-hook) invented a lens to study the finer details of his woven fabrics. After soaking fabric in rainwater and clay, to clean it, he noticed through his lens something unusual, something incredibly small - and it was moving! His fascination grew. He produced ever better lenses and over time he described a wonderful array of micro-organisms that he called 'animacules'. Van Leeuwenhoek's work gained recognition from the most prominent scientists of the day and today he is widely referred to as the Father of Microbiology.
But what was he looking at?
Van Leeuwenhoek had stumbled across bacteria, archaea, protozoans, algae, microarthropods, nematodes and fungi. Without realising it, he had discovered the soil food web: the incredibly rich, diverse, integrated community that underpins life on earth, no less! He never published his research, preferring simply to write letters containing his observations to the Royal Society.
How many are there and where did they come from?
In a teaspoon of healthy soil, it is estimated there are up to five billion bacteria and archaea, over a million protozoans, thousands of nematodes, dozens of microarthropods and perhaps a kilometre of fungal hyphae. But where did they come from?
To answer this question, we need to travel back in time some four billion years and dive to the deepest parts of the ocean. It is thought that where the Earth's tectonic plates were being stretched to breaking point, hot fluids rich in hydrogen, carbon, sulphur and iron spilled out onto the seabed. These minerals combined to form the basis of the first simple organic molecules. As molecules combined and began to reorganise, eventually single-celled organisms we call bacteria and archaea arose.
These microbes invented photosynthesis 3.5 billion years ago which oxygenated the atmosphere. Some 1.3 billion years later, organisms with complex cells (protozoan-type eukaryotes) evolved from symbiosis between anaerobic bacteria; these creatures are the common ancestor of all animals and fungi.
A second union involving aerobic eukaryotes and cyanobacteria happened 1.6 billion years ago creating green algae and organisms that would eventually become the precursors to land plants. Finally, some 450 million years ago, green algae formed a union with fungi and the outcome was the embryophytes - the first true land plants such as lichens, liverworts, hornworts and mosses.
Modern genomic research is showing that the biological skills and tools required to exist as a land plant were provided by those incredible, tiny bioengineers and biochemists. It is more than symbiosis, it is 'ingeniobiosis' (ingenio = engineered; biosis = way of life).
What do soil micro-organisms do?
Bacteria and archaea have created a mind-boggling array of plant-growth promoting substances such as hormones, volatile organic compounds, vitamins, enzymes, antibiotics, and signalling compounds in reaction to harsh environmental conditions, or biotic stress such as insect attack. They fix and cycle carbon and nitrogen and they make nutrients bioavailable to plants. In partnership with fungi, they give soil structure.
Mycorrhizal fungi can also be thought of as miners shuttling water, micronutrients and essential trace elements to the roots they connect to.
The protozoans help to keep bacteria and archaea populations stable, and in turn nematodes are preyed on by microarthropods. I call it the Soil Serengeti as the predator/prey relationships are extremely important in balancing life in the soil, and the natural predation and pooping is critical in supplying plants with ammonium.
Going back to our roots
How can you tell if your soil food web is not working if you do not have a microscope? Well, the most obvious sign is stressed plants - they may show wilted or yellowing leaves, or insect nibbles, or infection. This results from lack of food and water for the bacteria and fungi. And digging! Soil is a habitat and all these micro-organisms need to eat and drink on a regular basis, and need a stable home in order to support all the rest of the soil food web in growing healthy, disease resistant and resilient plants.
The food I am talking about is carbon sugar from plant roots and a regular input of compost (humous). Continuously bare soil is poor soil. Weeds are nature's way of supplying the carbon sugars to the soil. We need weeds!
If you have a dark chocolate brown, moist soil with plenty of roots and earthworms, you can be assured that the soil ecosystem is intact. Feeding and watering, little and often, will maintain this precious asset.
Our thanks to Eddie Bailey for allowing us to publish his blog post on our website.