The Soil Foodweb

Last updated 15th December 2017.

The Soil Foodweb.

In natural ecosystems, organic material is deposited on the soil's surface as animal waste and the decomposing remains of dead plants and animals.  It feeds the soil's microorganisms and through them provides food for plants.  The plants in turn provide food for animals and other creatures living above and below ground.  It's a cyclic system and it's completely sustainable, however it is more wonderfully complex than it seems.

Like most organisms on the planet, bacteria and fungi are designed to exploit all the resources available to them, and since life began billions of years ago, they have been busily breaking down the earth's bedrock into nutrients to sustain themselves.  These nutrients are essential components in the structure of their bodies, but are of no use to plants in this form.  When their predators (nematodes, arthropods and protozoa) consume them, waste is excreted into the soil in a nutrient rich plant available form.


When this relationship between microbes, plants and animals is working naturally, it is totally self sustaining and abundantly productive.  Scientists call this relationship the Soil Food Web

Maintaining Healthy and Fertile Ecobed Soil.

Ecobeds were designed primarily to cope with drought conditions by minimising water use.  They have a water tank located beneath the soil which releases water as required by plants.   As a consequence they have a self contained ecosystem isolated from the surrounding soil.  Worm and microbial activity in these ecosystems need to be carefully nurtured to ensure the crops grown in them are healthy, productive and full of essential minerals, vitamins and other nutrients.

With the assistance of a mulch of sugar cane straw, the Ecobed's soil is kept constantly moist providing an ideal habitat for worms and soil microbes.  Their vitality and diversity is maintained by frequent applications of generous quantities of high quality homemade compost onto the soil.

Seedlings are planted and seeds sown directly into this active layer of soil and compost and the microbes readily engage with the roots of young plants and begin to build mutualistic relationships with them  (more about this in "Beneficial Microbes" below).
 
What is Soil?

The basic constituents of soil are very small particles of rock broken down over millions of years by volcanic activity, glacial action, erosion (wind and rain) and biological activity.

These particles are the original source of most of the chemical elements found in the bodies of living organisms, and soil is created continuously from it by the activities of fungi, bacteria and other micro-organisms.

Soil scientists classify soil types as sand, silt or clay (in descending order of particle size).  Most soils are a mixture of them all with perhaps one dominating.  Dr Elaine Ingham, a world leading soil microbiologist, teaches us that all soils wherever they are to be found in the world hold abundant supplies of the minerals required by microbes, plants and animals living in or on them.

When soil laboratories conduct tests for farmers, they measure the amount of soluble nutrients in the soil, and often declare soils as deficient in one important mineral or another.  They then advise the farmer to add artificial "amendments" to correct this, but usually the soil is simply deficiency in the appropriate soil biology.
 
Natural soils contain large quantities of organic material which is being broken down by an amazing number of small animals, insects and microorganisms.  Earthworms and microorganisms add structure to the soil so that air and water can get down to the plant's roots where they are needed.  These living soils feed and nurture the plants growing in them and, in turn, the creatures living above the ground.
 
Beneficial Microbes.

Natural soil contains lots of organic matter with billions of bacteria and fungi breaking it down and sustaining themselves with it.  These microorganisms also manufacture enzymes which break down parent rock (sand, silt and clay) into the minerals they use in their growth.

Larger microbes and small animals in the soil, feed on the bacteria and fungi and their waste (like worm castings for example) becomes food for plants.  The above ground animals who eat the plants get the benefit of these micronutrients and in time return them to the ground in their waste and in their bodies when they die.

Bacteria glue themselves to minute rock particles (sand, silt or clay particles) in the soil to form aggregates, and fungi wrap their hyphae (very fine roots) around them to form even larger aggregates..  These porous aggregates behave like large particles capable of giving the soil structure.  This open structure provides space for water and air to easily penetrate the soil to the root zone of the plants.  Surplus water drains readily to the subsoil, but enough is retained on and in the aggregates to keep the soil moist and friable.

Symbiotic relationships exist between plants and microbes, and in return for help harvesting water and micronutrients from the soil, plants feed the microbes with exudates of sugars, proteins and complex carbohydrates which they have manufactured in their leaves using photosynthesis.

Plants need different nutrients at different stages in their growth (i.e. growing new leaves, growing flowers or producing seeds) and they need special nutrients to produce (for example) toxins so they can defend themselves against pests and diseases.  Plants can regulate the composition of the nutrients they receive by attracting or activating the appropriate microbes.  They change the composition of the food they make available to the microbes as their needs change, so only those microbes producing the requisite nutrients required will flourish.

The root zone of a plant is occupied by lots of beneficial microbes in these symbiotic relationships with plants.  Their sheer numbers suppress the activities of plant pathogens, and when a plant's foliage is sprayed with aerated compost tea, beneficial bacteria do the same thing.  Plants feed these microbes with the same exudates as they do in their root zone, and in return are supplied with nitrogen based nutrients extracted from the atmosphere using enzymes called nitrogenase.

An amazing relationship is developed between Mycorrhizal fungi and most plants.  The fungi set up nutrient exchange centres in the plant's roots and produce masses of fine root like hyphae to forage for plant nutrients and water - out of the plant's reach.  In return the plant manufactures energy food for the fungi.  Mycorrhizal fungi form these relationships with more than 85% of plant species, and can increase their nutrient and water gathering capacity by several hundred times. They form symbiotic relationships with bacteria in their mycelium to mine for minerals in the same way as the plants do in their root zone.

Soil Structure.

Consolidated and saturated soils create conditions which favour anaerobic bacteria that can cause plant diseases.  Compaction also leads to rainwater runoff which limits the depth to which plant's roots can penetrate.

An open structure is created by bacteria in organic soil.  They glue themselves to soil particles to form aggregates and fungi use their hyphae to clump these aggregates together to form even larger soil bodies.  In the same way as in coarse sandy soil, these aggregates enables free passage of air and water to the plant's roots, and readily drain excess water to the subsoil.
   
Soil aggregates provide refuge for bacteria from their predators.  They retain water, and insoluble mineral nutrients in the bodies of the live bacteria. The water is available whenever the plants need it, but to obtain the minerals they need, they supply exudates from their roots which attract the required types of bacteria they need out of their refuges and start a feeding frenzy as their predators go into action.  The plants get the nutrients they want, where they want it, in their root zone.

Worms, whose main food is bacteria, and are part of this process of liberating mineral rich nutrients to the plants in their worm casts, also leave tunnels behind them as they burrow.  This activity provides conduits for the rapid development of plant roots and fungal hyphae and more space for water and air to move freely through the soil.
 
An important feature of an Ecobed is the way fresh air is drawn into the soil as the 450 litres of water in the tank are used up.  When the tank is refilled with water, the air in the tank is forced up through the soil and discharged into the atmosphere.  This helps keep the soil aerobic so that beneficial microbes dominate the root zone and the plant's roots have easy access to nitrogen, oxygen and other gaseous elements.

Good soil structure is essential for efficient water wicking in an Ecobed, and I recommend a policy of minimum disturbance and digging of the soil. 

A Step Too Far.

Modern farming practices using powerful chemicals as fertilisers, pesticides and herbicides which damage the soil food web, and over time, deplete the living organisms in the soil.  This and the use of heavy equipment like ploughs and mechanical harvesting equipment destroys the soils structure and leads to compaction in its lower levels.
  
With few microbes left to supply vital micro-nutrients and maintain soil structure, and with an impermeable compaction layer in place below the cultivated topsoil, water and soluble nutrients drain quickly through the lifeless topsoil and are diverted by this barrier to rivers and streams.  The plant's roots are restricted to growth above the barrier and the plants become totally dependent on irrigation and synthetic fertilisers for survival.  They don't get the full spectrum of minerals they need to maintain excellent health, and as a result, our food is deficient in micronutrients.

By removing organic materials from the soil and the microbes living on it, we not only deny plants the micronutrients they need, the soil's structure collapses too.  We are losing productive farming land worldwide at an alarming rate by denaturing the soil.  We need to react to this problem with great urgency especially in view of the rapidly growing world population.

Economic and Environmental Impacts.

Global petrochemical demand is growing strongly along with population growth and the growthe of consumption in emerging economies like China and India.  Supplies, at the same time are getting harder to find and will become more expensive.

Modern agriculture is a major consumer of unsustainable fossil fuel derivatives including petroleum, synthetic fertilisers, herbicides and pesticides.  Eventually increasing costs will force farmers to change to a healthier and more sustainable agrarian culture.

As we all should know by now, greenhouse gas concentrations in the atmosphere are directly affected by the high use of petroleum, natural gas, coal and their derivatives.
Industrial agriculture is a big contributor, not only because of the fuels, fertilisers, pesticides and other chemicals it uses, but because billions of tonnes of carbon normally locked up in the soil as biomass has been and continues to be lost to the atmosphere as greenhouse gasses.

The release of these gasses is part of the natural decaying process of organic materials by the soil's micro fauna, but since the mid 1940's this carbon is staying in the atmosphere because most "modern" farmers have stopped returning organic waste to the soil and have destroyed most of the soil's biology and structure.

Solutions to These Problems.

Organic farmers and gardeners recycle carbon by returning it to the soil as humus and other organic materials.  In my own small suburban garden, I recycle about 250 litres of household and garden organic waste every month and use it all on my garden.  That's 3000 litres of carbon rich compost sequestered into my soil every yearI believe a move to organics would mitigate many of the looming climate, health and economic problems of the world.

It’s a well known saying among organic gardeners that you should feed the soil not the plants.  I don’t know whether they realise this mean feeding the soil microbes.  Its so important to maintain the soil food web, and why using industrial pesticides, herbicides, fertilisers and tillage on your soil is such a bad idea.

Grow some organic veggies in your own backyard, its win win, you can't lose.  Look after your body by getting good physical exercise in the garden and eating wonderful tasty nutritious food.  Reduce your carbon footprint and help the environment by recycling waste and by sequestering carbon into the soil.  Most of all have loads of fun doing it.

Resources
  • An inexpensive e-book on the subject of the living soil entitled "Teaming with Microbes" by Wayne Lewis and Jeff Lowenfels explores this subject in much more detail.  It gets quite technical at times, but the authors are ever mindful of the need to keep a very complex subject as readable as possible.
  • A BBC film featuring Rebecca Hosking entitled Future Permaculture in Britain is about transitioning from a high energy driven economy to a more sustainable one.
  • Thanks to the "Soil Foodweb Institute" for the use of their diagram.