Soil Presentation by Miranda Liebmann – a geologist and macadamia farmer who is changing the her farm to organic farm practices

Soil is made up of a mixture of rock-derived minerals, plant-derived organic matter, dissolved nutrients, gases like air, earth worms and other bugs like ants, slaters and mites, that we can see, and a lot of microbes that we cannot see with the naked eye. Research has shown that a teaspoon of healthy soil contains millions to a billion bacteria, several metres of fungi, several thousand protozoa and a few dozen nematodes.  That shows how small they are but it is still a very crowded teaspoon!

What are all those microbes doing? A plant photosynthesises and creates carbohydrates and sugars that are extruded through the plant roots. The bacteria and fungi feed on those carbohydrates and in return provide nutrients to the plants in plant available forms.  The bacteria and fungi are concentrated in the root zone, known as the rhizosphere.  The microbes provide nutrients in faecal excretions and also release them when they die. The protozoa and nematodes feed on bacteria and fungi and contribute to the nutrient release that the plants can take up.  Different plants have different microbe associations. Different microbes have different roles.  Take legumes, for example, nitrogen fixing bacteria create nodules on the plant roots. The nodules contain bacteria that fix nitrogen. Gardening Australia did an excellent story on this recently,

Bacteria and fungi produce sticky substances, like glomalin, that is like super glue and binds soil particles together to form rhizosheaths. This helps the soil to retain moisture, retain carbon, retain nutrients and protect the roots.

Other roles of microbes include decomposing organic material, producing soil carbon and protecting plants from disease.  Different microbes have different roles. To encourage different microbes, different plants are required. Australian microbiologist, Dr Christine Jones, who has many excellent presentations on YouTube, promotes growing plants from 6 different family groups together to ensure diverse microbe populations eg brassicas, grasses, legumes and daisy families. Maybe that’s partly why companion planting works?

What affects the soil life? The use of pesticides, herbicides and fungicides, unsurprisingly, knocks back the microbe populations. Over fertilising can affect the microbes by changing the soil pH. Microbes enjoy a pH around 7. Tillage can break up fungal strands, decimate worms and allow the soil to dry out.  Think of an optic fibre network like the NBN or a power grid all linked together. Fungi do the same thing. They link together and link plants together. Research has shown fungi transfer nutrients, moisture and chemical signals through the hyphae. Compaction will adversely affect microbes as well as bare soil and monocultures. Remember microbes have plant affiliations. A monoculture significantly reduces the variety of microbes. On the other hand, microbe numbers and diversity are promoted by adding compost, vermicast and mixed cover crops to your soil.

How can you assess your soil in your garden? You can take soil samples and send them to a laboratory for nutrient analysis. Some labs now analyse soils for the percentage of microbes. More simply, you can do a Soil Health Card Test which was developed some years ago by a local Landcare group for farmers. But, it is equally applicable to gardeners to assess the soils in their gardens. The recommended time of year to do the test is during autumn, a few days after rain so the soil is damp and a bit soft. Select an area in your garden with excellent soil and another area with the poorest soil. Repeat the test every 6 months and repeat it about the same time each year. The Soil Health Card Instructions can be found at There’s also lots of other excellent information on the SoilCare website. The optional calico test is an excellent one to assess the activity of microbes in your soil. I prefer the NSW DPI variation on page 92, where 30cm x 3cm strips of calico or pure cotton are cut. Label each one at an end that will sit up above the soil. Use a spade to cut a 15cm, vertical slot in the ground. Place 3 or more calico strips into the slot by folding the strip of cloth in half over the end of the spade (equal lengths) and push the spade into the vertical slot so that about 2cm of cloth remains out of the soil so you can find it. Firm the soil against the spade with your foot and draw the spade out. If the cloth sticks, shine the spade with some sand paper or steel wool first. After 3 weeks remove the first strip to assess how decomposed it is. Then wait a week or 2 before removing the next one and so on. Over time you will see trends, especially if you are improving the poor soil.

Useful, practical, easy to read references for gardeners, to help improve your soil life:

  1. Soil Food 1372 ways to add fertility to your soil by Jackie French (available in the local library);
  2. Also check out Dr Christine Jones on YouTube “secrets of the Soil Sociobiome” for the most up to date information A bit slow to start but worth watching.


Posted in Special Interests.