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The Soil Food Web: The Life Beneath Our Feet

soil
A handful of soil. Photo by Bluedog Studio

As we tread upon our soil, plant into it and harvest from it, it’s hard to imagine the myriad of creatures that live within it. Teeming with life, the sheer number and diversity of creatures in our soil is mind-boggling, and these creatures are crucial to the health and vitality of our soil. This interdependent circle of life is known as the soil food web.

The soil food web is the living component of our soil, a living ecosystem which is closely linked to the health of the plants that grow in the soil. If the soil biology is healthy, the plants we grow and eat will be healthy, and if the plants are healthy, then we humans can be healthy. We are more connected to and reliant on our soil than we think.

WHO MAKES UP THE SOIL FOOD WEB?

The soil food web is made up of a range of beneficial organisms including:

Bacteria

A teaspoon of productive soil generally contains between 100 million and 1 billion bacteria. Bacteria convert energy in soil organic matter into food for other microbes. They extract nutrients by breaking down sand, silt and clay, making a glue which holds these mineral fractions together with organic matter to form the base of the ‘soil house’ or soil structure. Without these microbes to build the ‘bricks’, none of the larger pore structures, air, water or root channels could be built.

Fungi

Also microscopic, soil fungi usually grow as long threads. Like bacteria, fungi help retain nutrients in the soil, breaking down the crystalline structure of rocks using an array of enzymes and strong acids. Fungi make the mortar that holds the soil house together, and arrange ‘windows’, ‘doors’ and passageways for air, water, roots and soil organisms to move through.

Protozoa

Single-celled animals that eat bacteria, fungi and other protozoa. Because bacteria have a higher concentration of nutrients than protozoa, when protozoa eat bacteria they release the excess nutrients in a form that plants can use. And like bacteria and fungi they can prevent diseases, pests and parasites from establishing on plants.

Nematodes

Worm-like creatures, some of whom can grow to 1 mm, but in soil are usually microscopic. Like protozoa, nematodes consume bacteria or fungi and release the excess nutrients in a form that plants can use. Some undesirable species of nematodes, called root-feeders, have given the rest of the nematodes a bad rap. Most nematodes are helpful and can consume the bad guys or deter them from finding the root system.

Arthropods

They get their name from their jointed legs, and range in size from microscopic up to mites, millipedes, spiders and dung beetles. Arthropods aerate and mix the soil, regulate the population size of other soil organisms and shred organic material.

Annelids

Needing no introduction, earthworms are the epitome of how the soil food web creatures work. They shred plant residues, mix and aggregate soil, improve water-holding capacity of soil and simulate microbial activity.

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Photo by Ronemmons
worms
Photo by The Sustainable Gardener

Clockwise from top left: Adding biochar to compost; Worms are an essential part of the soil food web.

FUNCTIONS OF THE SOIL FOOD WEB

The organisms in the soil food web make it possible to have clean water, clean air, healthy plants and good soil water retention. These organisms have many important functions.

They break down organic compounds in the soil, like manure, plant residues and each other, and with the help of the predators they make nutrients available to plants (called nutrient cycling). The more healthy and diverse the soil community, the more likely it is to have the range of organisms necessary to break down even the most resistant materials in the most extreme environments.

Certain groups of bacteria fix nitrogen from the atmosphere, and with the help of their predators make it available to plants. This is an opportunity for plants to use the existing nitrogen that is either bound in the organic matter, atmospheric gases or rock itself, therefore negating the need to add synthetic fertiliser.

They also help sequester carbon. When you return biomass to the soil, (i.e. adding compost, turning in green manure crops or cover plants), reduce tillage and keep the soil covered and moist, then you’re building your soil organic matter. This is food for microbes and they will do their job of converting the soil organic matter to humus, a stable form of carbon which can be sequestered in soils for centuries.

These organisms change how water is retained in your soil by increasing soil porosity. This allows plants to grow their roots more freely through the soil and access more nutrients and water. It also assists in reducing run-off, purifying the water and retaining as much of it as possible, especially when there’s not much to go around. They also break down pesticides and prevent them entering waterways and becoming pollutants.

A healthy soil food web can help control pests by improving plant health. This happens when nutrient cycling increases, therefore increasing the sugar content of plants which pests can’t digest (pests like low-sugar plant material).

By improving your soil food web you can also reduce weeds. Weedy species require poor soil life diversity and activity to thrive. So you can remove weeds by not tilling and increasing organic matter in the soil and cycling nutrients (as nature has done for billions of years).

WHERE DO THE ORGANISMS LIVE?

Soil organisms live both in the top 10 cm of soil (where dead plant materials drop and are turned into soil organic matter) and concentrated around the roots of plants. The soil directly around the roots of plants is called the rhizosphere and this is teeming with microbes feeding on sloughed root cells, proteins and sugars released by roots. This is another very important reason to have your soil covered with plants and not bare.

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Soil food web. Photo by Soil Solutions

HOW TO BUILD SOIL FOOD WEB HEALTH

The primary needs for soil organisms are not that different to our own.

Shelter

Continuous ground cover is critical to a healthy functioning soil food web. Cover your soil, grow more plants, add more biomass. The more diverse the better. If it means letting the weeds grow, as long as your crop is dominating the sunlight, you’ll be far better off than if you have a neatly trimmed garden with exposed bare soil. If a soil is exposed to the sun and wind, moisture is lost to the atmosphere as CO2.

Food

If you leave a section of your garden or farm fallow then you’re creating a famine for your soil food web creatures and they will die or hibernate. Replace fallow ground with green manure crops. Practice or support planned grazing management, such as rotational grazing, that will allow full ground cover and the development of large root systems. The roots of plants are just as important as what’s above the soil as they are feeding your soil food web. If you’re not feeding your soil microbes, nutrient cycling and carbon cycling declines.

Water

Bacteria and other micro-organisms thrive in a moist environment. If your compost pile and vegetable patch dry out, the soil food web starts to lose efficiency. You don’t want to wash your garden away in a flood every time you water either, as this will leach nutrients. It’s about keeping soil moisture at good levels to allow microbes to thrive and continually feed your plants.

Avoid Tillage & Compaction

Untilled soil has continuous pore space built by microbes which tightly holds water, allowing soil life to flourish. Tillage also results in loss of organic matter. This is one of the reasons why a broadfork just loosening up the soil a little is better than turning the soil over. Also avoid walking on garden beds, as compaction reduces oxygen around plant roots and makes it harder for plants to get their roots through the soil to access water and nutrients. Stress on plants affects the community living around the rhizosphere (root zone).

Avoid Pesticides/Herbicides

There is significant literature showing the effects pesticides have on soil organisms. Roundup Weedkiller for example has been shown to result in the death of fungi.

Encourage Diversity

You can add inputs to help activate your soil organisms and promote diversity and a more complex soil food web. A few of the more popular inputs are biochar, fish and seaweed, molasses and inoculants like IM (indigenous micro-organisms). How you use these inputs will depend on the condition of your soil and how they’re combined with other management practices.

ADD TO YOUR SOIL FOOD WEB

Biochar

This is burnt or charred wood remnants that are structurally complex, difficult to break down and high in carbon. It can be added to soil, potting mixes and composts. Biochar provides protected spaces for microbes to live, allowing microbes to build better soil structure. This results in greater water holding capacity and a more rapid rate of nutrient cycling. Biochar adds stable carbon to the soil, and if the correct biology is present, results in ever-increasing amounts of carbon and other nutrient sequestration.

Fish and Seaweed

These products are food for bacteria and fungi. When fish is ground to milligram size, particles and the fish oils are left intact—it’s called fish hydrolysate and encourages fungal growth. When fish is finely ground into microgram particle sizes, it’s called fish emulsion—this will encourage bacteria to grow.

Molasses

A high quality non-sulfured blackstrap molasses can contain as many as 150 different fairly complex sugars. This type of blackstrap molasses benefits fungi for the most part. Care should be taken not to use too much molasses as it can unbalance soil to favour weeds instead of your desired plants.

Inoculants

Add biology to your soils in the form of either homemade or commercial products high in beneficial microbes, such as compost teas, effective microorganisms (EM, a facultative anaerobic bacterial inoculum) and mycorrhizal fungi. Adding these microbes can boost the uptake of nutrients by plants.

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Seaweed provides food for bacteria and fungi. Photo by Ryan Benoit

TESTING YOUR SOIL

It’s difficult to assess the number and diversity of organisms that live beneath our feet. There are a few simple tests you can do at home (see below), or you can send a sample of your soil away to a soil testing laboratory. A lab will give you an indication of the mineral balance and level of soil organic matter, and some labs can tell you which microbial groups are present in your soil and if they’re in adequate numbers to cycle enough nutrients to your plants.

Any type of soil test you do will be beneficial because it means you will be observing, touching, smelling and even tasting (if you’re willing!) your soil, bringing you into closer contact and intimacy with your soil food web. Over time you will get to know your soil and be better able to assess it and increase its health.

HOME SOIL TESTS

Squeeze Test

Take a handful of moist soil and squeeze it, then open your hand to find out if your soil is loamy (holds its shape but crumbles with a light poke), clay (holds its shape and sits stubbornly when poked) or sandy (falls apart as soon as you open your hand).

Earthworm Test

Dig a hole the width and depth of a shovel. Spread the soil out on a tarpaulin and count earthworms—if you have at least ten, that’s good. Less than ten means you may not have enough soil biology present, as worms eat bacteria and fungi that decompose soil organic matter.

pH Test

You can buy a pH kit from any gardening store. Most plants grow best in soils of 6-7pH. Where to get soil tests done

www.scu.edu.au/environmental-analysis-laboratory—eal/

www.soilfoodweb.com.au (biological testing)

Dr Elaine Ingham runs online soil biology workshops on www.environmentcelebration.com

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