Secrets of soil
Soil or dirt? Often, we use these words interchangeably but in fact they are very different things. Healthy soil is the deep brown colour of 70% dark chocolate and crumbly; it contains a minimum of 3% organic matter (ideally 20%) and is full of living organisms. By comparison dirt is pale brown in colour, more like milk chocolate and, compacted with large clumps; it is low in organic matter (<3%) with little or no living organisms present.
Features of healthy vs compacted soil (courtesy of The Organic Farmer) Colour of poor vs healthy soil
Healthy soil holds a multitude of living organisms which mostly reside in the top 6 inches (15 cm). Some you can see with the naked eye (earthworms, centipedes, ants, beetles etc.) others can only be revealed under a microscope (nematodes, protists, fungi, bacteria). It is estimated that a teaspoon of healthy soil contains a billion bacteria, tens of metres of fungal threads (hyphae), thousands of protists (amoeba, flagellates, ciliates) and dozens of nematodes.
Earthworms (left); Centipedes (right)
Microscope images of soil organisms – (a) various amoeba; (b) fungal thread; (c) nematode
All life needs energy to survive – in the vast majority of cases this energy comes from carbon-based materials supplied by plants, waste products from other organisms or the bodies of dead organisms. Most organisms consume more than one food source creating a complex web of interactions between them. In the context of soil, we call this the soil food web. Each soil environment, with its differing set of soil organisms, produces a different soil food web but, all webs have one thing in common, they always start with the plant.
Diagram representing the relationship between organisms in the soil food web (courtesy of USDA)
Plants are at the top of the food pyramid (the first trophic level). They use energy from the sun (via photosynthesis) to generate carbon-based molecules (sugars) from carbon dioxide and water. Some of these sugars are used by the plant but much of them are transferred to the roots where they are secreted as more complex carbon-based molecules such as carbohydrates and proteins (exudates).
These exudates have a purpose – they wake up specific soil bacteria and fungi by providing them with energy to grow and multiply within a zone around each root, a couple of millimetres wide, which is called the rhizosphere. The bacteria and fungi absorb minerals from the surrounding soil which are not directly accessible to the plant. These bacteria and fungi are in turn eaten by other microscopic soil organisms (e.g. nematodes and protists). These higher organisms create waste which is rich in in essential nutrients that the plant needs (e.g. nitrogen, phosphorous, mineral elements including potassium, calcium, iron etc.). These are absorbed from the rhizosphere by the plant roots. So, in a sense the rhizosphere provides an environment where nutrients can be “traded” between organisms for mutual benefit.
The key point to note is that the higher organisms (nematodes, protists etc.) are needed in order to release the nutrients “gathered” from the surrounding soil environment by the bacteria and fungi. Without them the system breaks down; the plant needs a fully functioning soil food web to be healthy.
Representation of the plant rhizosphere (courtesy of Raw Lab)
The function of soil food web organisms extends beyond the “trading” of nutrients. They are also important for establishing soil structure – soil that is porous, well aerated, able to soak up and hold water like a sponge. Soil bacteria generate sticky glues and fungi create long thin threads both of which serve to bind soil particles together to form aggregates. Higher organisms such as earth worms, insect larvae and burrowing animals create pathways within the soil of varying sizes serving to mix the soil as well as generate a porous sponge-like structure.
Soil life serves to cycle nutrients through the entire soil food web. When plants and any other soil food web organisms die, they decompose and the nutrients that they contain are recycled, their bodies are consumed by other soil organisms – one organism’s waste is another organism’s food. Without this recycling system essential nutrients would eventually be washed out from the soil.
It is useful to note that if we (as gardeners or growers) apply a chemical fertilizer to the soil only a tiny fraction of it reaches the rhizosphere and is absorbed by the plant. The rest drains through the soil reaching the water table and eventually ending up in rivers and oceans where it has a negative environmental impact. In contrast a functioning soil food web ensures that essential nutrients (many of which are not present in chemical fertilizers) are released within the rhizosphere where and when they are needed by the plant – the addition of external nutrients is not needed!
A healthy soil food web also serves to control disease causing (pathogenic) organisms. Although present in the soil, pathogenic bacteria and fungi are held in check by the vast array of beneficial organisms which outcompete with them for food. Soil health, and consequently plant health, is dependent on the diversity of beneficial organisms which can flourish under a wide variety of conditions. Every part of the soil food web has its place, keeping the other members in balance – it is a self-regulating system. If external forces (such as pesticides and fungicides) start to disrupt this balance then the web no longer functions effectively, nutrient cycling becomes disrupted, and the door is opened to plant diseases.
Whilst it is fairly apparent that some conventional gardening practices (such as the use of pesticides) kill soil food web members at all levels from bacteria up to earth worms and beyond. It is less obvious that using chemical fertilizers is also harmful to the function of the soil food web. Chemically fed plants bypass the microbial assisted method of obtaining nutrients. Microbe numbers fall as the plants stop releasing exudates into the soil; consequently, the higher organisms which feed on the microbes also become depleted. Nutrient cycling within the soil food web starts to fail. Higher organisms such as earth worms, which are irritated by chemical additives, move elsewhere. Waste materials (such as dead plant matter) are no longer broken down, the soil becomes compacted, less aerated and unable to absorb water. Conditions for growing plants become less and less ideal and any interventions such as digging to aerate the soil only serve to make matter worse by breaking up the fragile networks of fungal hyphae - our soil slowly turns to dirt.
However, all is not lost; we have the potential to reverse the decline to dirt and regenerate our soil by making a few simple interventions.
Avoiding further disruption to the soil food web by phasing out the use of toxins such as pesticides, herbicides and NPK fertilisers
Increasing the level of soil organic matter by addition of composts and mulches
Encouraging the return of soil food web members by use of good quality composts (aerobic, balanced, mature), compost extracts and compost teas [beware - many commercial composts are not what you need!]
Maintaining the soil food web by minimal disruption, employing no-dig or minimum till approaches; leaving roots in the ground after harvest; keeping the soil covered all year round with mulches or cover crops
There is much to say on these regenerative approaches; too much to cover here but, fear not, we will cover more on composts and other bio-amendments in a subsequent blog post.
So, why should we care about the health of our soils? Plants grown in dirt are weak and prone to disease; plants grown in healthy soil are lush and rich in nutrients. As, gardeners and growers, we want our plants to be healthy with a minimal need for costly interventions. As consumers we need healthy plants because we are what we eat; we benefit from the nutrients and healthy gut microbiome that we can only get from plants raised in healthy soil.
healthy soil = healthy plants = healthy food = healthy people
If you would like to dig deeper into this topic you can find a wealth of information in the following resources:
Video by Dr Elaine Ingham introducing the Soil Food Web - https://www.youtube.com/watch?v=vIQwy0Xn9AU
Book by Jeff Lowenfels & Wayne Lewis – Teaming with Microbes - https://www.jefflowenfels.com/books-by-jeff-lownfels/