The Soil Foodweb

Last updated 18th April 2021.

The Soil Foodweb.

In natural ecosystems, organic material is deposited on the soil's surface as animal excreta 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 etc) consume them, their waste is excreted into the soil in a nutrient rich plant available form.

When these relationships between microbes, plants and animals are working as nature intended, they are totally self sustaining and abundantly productive.  Scientists call this 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 regular applications of high quality homemade compost onto the surface of the soil.

Currently I purchase a small bale of sugar cane straw every month to use as a moisture retaining barrier on top of my soil.  It's recycled, after only being used once in this role, through my hot composting system and provides a substantial proportion of the carbon rich material needed.  As my soil carbon increases over time, I plan to rely less on this mulch and retain moisture using horticultural fabric.  Instead of producing a batch of hot compost every month, I may manage with a batch every 3 months.  This will reduce my reliance on an organic input sourced nearly 3000 kms away in far north Queensland.

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 on them.
 
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 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 will flourish. There is little or no waste of these nutrients to the subsoil or subterranean aquifers as the plants regulate quantity as well as quality of the nutrients being made available to them.

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 above ground.

Plants feed foliar microbes with the same exudates as they do in the ground, and in return the microbes supply atmospheric nitrogen based nutrients extracted using enzymes called nitrogenase.

An amazing relationship is developed between Mycorrhizal fungi and most plants.  The fungi set up nutrient exchange centres in and on 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 the plant's nutrient and water gathering capacity by several hundred times.

Soil Structure.

Compacted and saturated soils provide conditions which favour anaerobic bacteria (the main cause of plant diseases).  Compaction also leads to rainwater runoff and restricts the depth to which plant's roots can penetrate.

The fungal hyphae in healthy organic soil is responsible (with the help of bacteria) for the formation of aggregate structures.  These structures transformed the soil's ability to provides free movement of air, water and nutrients to the plant's roots, and readily drain surplus water to the subsoil.  They are essential if strong, healthy, nutrient rich crops are to be grown.

Tilling either by large industrial farming equipment or with a spade in the average veggie patch breaks down soil structure by chopping up this fungal hyphae.  For this reason I leave my soil as undisturbed as possible.  I don't dig it and I even leave roots behind when harvesting crops.  An interesting bonus is that weeding is no longer a chore in my garden beds or even in my organic lawn.

By planting new seedlings in organic soil where a crop has recently been harvested, my plants benefit from the infrastructure left behind.  Beneficial microbes and fungal hyphae are keen to reestablish their mutualistic relationships with the new plants as they grow. 

A Step Too Far.

Modern farming practices use powerful chemicals as fertilisers, pesticides and herbicides.  They 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 and microorganisms from the soil, 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 as populations grow and consumption increases in emerging economies like China and India.  At the same time supplies of these resources are getting harder to find and will become more expensive in the future.

Modern agriculture is a major user 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 and 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 mainstream farmers have stopped returning organic waste to the soil and have destroyed most of the soil's biology and structure.  Worse, crop residues are burned off adding even more carbon to the atmosphere.

Solutions to These Problems.

Organic farmers and gardeners recycle carbon by returning crop residues and household waste to the soil as humus and other organic materials.  In my own small suburban garden, I recycle about 400 litres of household and garden organic waste every month and use the (approx) 280litres of high quality compost it generates on my garden.  That's 3300 litres of carbon rich compost sequestered into my soil every year.  I believe a move to organic crop production, small and large scale, would mitigate many of the looming climate, health and economic problems of the world.

The biomass present in healthy organic soil is more than the dead remains of plants and animals or compost produced by farmers and gardeners.  It also includes an even larger mass of living micro-organisms fed by this organic waste and fed directly by plant exudates.

Plants are said to divert up to 40% of their output from photosynthesis as exudates to feed micro-organisms through their roots and foliage, and this amounts to a major sequestration of carbon from the atmosphere to the soil not accounted for.

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 means feeding the soil microbes.

Its so important to maintain the soil food web, and so disastrous to use industrial pesticides, herbicides, fertilisers and tillage on our soil.

It is easier than you think to grow some organic veggies in your own backyard.  Your body will benefit from the wholesome physical exercise and the wonderful tasty nutritious food you grow.  You will be reducing your carbon footprint and helping the environment by recycling waste and by sequestering carbon into your soil.  Most of all you will have loads of fun doing it.

Resources

• Dr Elaine Ingham on The Origins of Compost . 
  
• 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.