Restoring Healthy Soils
Notes prepared by Craig Castree, 2025
Diagnosing soil deficiencies or not?
In the past, we have looked at the leaves of plants to diagnose deficiencies and amended the soil thinking that the soil was deficient in theses minerals. We have had that wrong, most soil is not deficient and any minerals it’s not functioning. Through our misunderstanding of soil, we have managed to change it from functioning healthy to a lesser form and turned it is not dirt.
What is healthy soi?
The layers beneath our feet are as follows: organic layer, topsoil, subsoil, parent material, bedrock. Topsoil is the thinnest of the layers beneath us, it is between 100mm – 230mm deep, 4 – around 8 inches deep, some parts of Australia it is much less than this. We only use 3% of the earths topsoil to grow food. Soil consists of sand, silt, & clay, 4% humus, minerals, gas and water. Dirt on the other hand is Sand, Silt, & Clay. Researchers say that topsoil is being lost at 7-10tonnes per hectare per year. We only have 65 years of it left. We need to understand what healthy soil is. We need to understand how it functions, to regenerate it.
Soil composition
This break down refers to healthy soil, remarkably though 50% of its make up I am sure most are surprised at, (25% water & 25% Air). Air is a very interesting component given every breathe we take in consists of 78% nitrogen. Now if you are a gardener, I am sure you will be like me and have spent money on Nitrogen based fertilisers to feed you plants. Whilst all this is probably starting to seem complex, I assure you it is much simpler that you think. We have managed to over complicate gardening with our incessant need to interfere with nature thinking we have the answers.
Soil compaction
0-200 PSI Air can get into soil microbes prefer 100-150 PSI
200-300 PSI Restricted Air flow
Plants roots now struggle to penetrate; microbes stop operating.
300-800 PSI weeds penetrate to fix compaction and aeration problems, and they can penetrate down to 800 PSI. They can often be indicator plants of what is happening or not happening in your soil. If you are getting long deep-rooted thistles and dandelions, you have compaction. Weeds are the mechanics of the plant kingdom and are dispersed t solve a problem.
Use a garden fork to check your soil for compaction, it should easily penetrate the soil with light hand pressure. If you have to use your foot to apply pressure to penetrate your soil, you have soil compaction. Without air, you now don’t get the 25% water infiltration that healthy soil should have. This has a follow-on effect given that is also stops microbes from functioning.
The weather system has a big part to play in healthy soil driving air into the soil as deep as 200PSI if the soil is not compact.
A high-pressure system is heavy sinking air, driving that valuable commodity oxygen into the soil and of course along with it that all important free nitrogen.
A low-pressure system is light rising air, and it lifts the CO2 and other gases as a result of the carbon cycle up into the atmosphere and along with it microscopic bacteria called aerosols which form the nuclei for water droplets to form around and build clouds. When the water droplets get heavy enough, they fall back to the earth in precipitation (rain).
The carbon cycle at work
It looks complex but it is very simple, and it does best when we stop disrupting it. The plants photosynthesise and make some of the food they need for themselves and the make 40% more than they need to feed the microbes. In return the microbes solubilise minerals and make the available to the plants in a format they can use (microbes feed the plants). Microbes live for moments, minutes, some for days and others for weeks, when they die the become microbial residue. It then becomes Soil organic stable carbon and can remain there for many years provided it is not liberated by tilling. This process creates CO2 and liberates other gases and that make their way into the atmosphere. Plants and trees release far aerosols that are carried up into the atmosphere particularly with a low-pressure system with that light rising air.
Stop turning soil over
Tilling soil oxidises the stored carbon and liberates it back into the atmosphere, this happens due to carbon being exposed to oxygen and 2 oxygen molecules (O2) are attracted to a carbon molecule (C), making it CO2. The otherwise heavy carbon molecule is now light due to the oxygen and makes its way up into the air. It is tilling that is liberating a great deal of this from soil and we need to leave the carbon where it belongs. Topsoil does the heavy lifting when it comes to carbon sequestration, more so that anything else on earth.
Bacteria, fungi, protozoa, nematodes, archaea, viruses, microscopic insects, etc. We still only have named about 1.5% of what exists in healthy soil. This is a new frontier that is still largely unexplored. This coupled with an extensive fungal hyphae network makes up the soil microbiome.
Microbes decompose organic matter and make nutrients available to plants by converting them into usable forms like nitrogen, phosphorus, and potassium.
Soil microbe function
Water Purification
When water passes through soil, it is cleaned via physical, chemical and biological processes. In addition to soil’s physical filtration capacity, soil contains important biota that helps transform and decompose certain chemicals and other contaminants from soil, thus helping filter them out of the water.
Carbon sequestration:
Soil microbes help store carbon in the soil by breaking down organic matter and incorporating it into soil particles, the become themselves stable carbon when the die if not disturbed, contributing to climate regulation.
Soil structure improvement:
Microbial activity helps create a healthy soil structure by binding soil particles together, improving water infiltration and aeration.
Pathogen suppression:
Certain microbes can inhibit the growth of plant pathogens by competing for nutrients or producing antimicrobial compounds.
Plant growth promotion:
Beneficial microbes can directly enhance plant growth by producing plant hormones, facilitating nutrient uptake, and protecting roots from pathogens.
Ecosystem resilience
Microbes can help plants withstand abiotic stresses like drought, salinity, and extreme temperatures by regulating water uptake and producing protective compounds.
Tilling soil and leaving it bare has consequences
Degenerative: On a day when the ambient temperature is 35C, the surface soil temperature is 65C, this rapidly evaporates the water in soil and air near the surface and is replaced with cool air and then heated up again evaporated and so the cycle goes, this is a heat bowl effect. It causes terrible soil compaction. There are no aerosols being expelled by the microbes and there are no plants growing in this soil.
Regenerative: In contrast, soil with a cover crop or plants growing in It covering soil, the temperature at soil level is 28C, a very big difference with little to no evaporation occurring. Now aerosols are being expelled into the atmosphere to form the nuclei for precipitation.
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We turn the shallow living microbes on their heads, and they die!
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the microbes that live deeper also get turned on their heads and they die!
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We oxidise the stored carbon in the soil, and it is released back into the atmosphere!
750 km rabbit proof fence divides 13 million acres cleared for agriculture, gets 40% less precipitation than the eastern side with vegetation.
What we have misunderstood in soil is the relationship with plants roots and microbes. Plants don’t just make simple starch sugars (food) for themselves, they make more than the need to food microbes.
Plants exude these sugars through their roots to feed microbes called exudates. Plants entice the type of solubilising microbe they need the mineral from by offering the exudate that attracts that microbe. So in effect, the plant knows what it needs and it gets the exact amount it needs when it is needed through this process, but this only happens in functioning healthy soil.
Root zone = Rhizosphere
The root zone of has been redefined as the rhizosphere (rhizo = root, sphere = area) this is where in healthy functioning soil the microbes inhabit waiting for their daily feed of exudates from the plant of tree above. Some of the microbes are internalised by the plant, cloned, and invested in and around each seed so the young seed when dispersed and it germinates it has all it needs to start out life.
Exudates build aggregates, you see living plant roots feed microbes that produce glue like substance that stick the sand, silt, and clay, particles together, and humus, this forms aggregation along the roots of plants. This becomes a home for microbes and this symbiotic relationship between the two is strengthened.
This is a crucial step to understand in the relationship, because to interfere with it, stops this communication and nutrient exchange from taking place. When we feed plants fertilisers organic, or inorganic, the plants stop feeding microbes. The other problem is that plants can only take up small amounts of solubilised nutrient and the rest gest locked up in soil unavailable to them. This also stops the plants from getting the protection offered from the soil microbiome, as their food source has stopped being so. That’s what a microbiome does, it protects its food source from harmful bacteria and pathogens, these weakens the plants response to this sort of attack. It also lowers the nutrients density of the plant, the opposite of what one would assume happens by feeding plants.
The fungal hyphae network
This microscopic network is not visible to the naked eye, so it is no surprise we don’t either know it exists or see it, and they are some of the largest organisms on the planet, some stretching kilometres. The only time we get a hint that they are there is when we see mushrooms or toadstools, puff balls, and a whole host of other reproductive stages, etc. pop up in the garden, or in fields, forests and the bush.
The orange strand in the picture is a plant root, that is a nitrogen host plant, like a legume, the rest of the network are fine filament threads (hyphae) of a nitrogen fixing fungi, and it is fixing nitrogen nodules to the plant.
The microbiome of the soil coexists with the soil food web of insects, a complex community of organisms living in the soil, where different species interact by consuming each other, with bacteria, fungi, insects, earthworms, and other microorganisms breaking down organic matter and providing nutrients to plants, essentially creating a food chain within the soil ecosystem.
Feeding plants or Soil
We have misunderstood how plants feed, thinking that they drink up nutrients from soil, this is incorrect, in fact they cannot solubilise minerals, microbial solubilisation makes the otherwise unavailable minerals available to the plants in a format they can take up. As discussed, the plants respond by feeding microbes. When we feed plants solubilised fertilisers, not only can they only take up a small amount of it, but they also stop feeding the microbes. The unused fertiliser becomes locked up and unavailable to the plants. This is particularly bad when the fertiliser is inorganic, as this often ends up in our waterways causing algal blooms and acidifying oceans.
What we tried to replace
Post World War 2, there was a great deal of chemicals left over from making bombs etc, A very clever scientist worked out at set ratios this was fantastic plant fertiliser, this started the Green Revolution and a modern chemical and mechanical farming method was adopted all over the planet. It is still in place today, and farmer are now applying 10X times the amount they started out applying to get the same result. Their input costs are through the roof and their profitability through the floor. Gardeners followed the farmers lead and adopted the chemical way of feeding and fertilising plants. We are now starting to understand the consequences of having gone down this path. We incorrectly thought that we could replace 74 natural occurring minerals, nutrients, and trace elements with just three, N = Nitrogen, P =Phosphorus, K = Potassium. Whilst the fertiliser does do what the packaging says, it makes plants grow, the consequences are such that it depletes healthy soil, decreases the microbial population, weakens plants nutrient density leaving them susceptible to insect pest and disease attack, along with a whole host of others/ The most important one is that we are no longer getting the nutrition form food we should be when it is grown like this.
Polyculture (growing different species with one another) is a regenerative approach towards soil health, traditional methods have convinced us to grow plants in rows of the same type. This is called (monoculture) and is degenerative approach to soil depleting the life within it.
Four or more plant families and the microbial population explodes in numbers.
Leaving the roots of spent plants in the ground, leave the microbiome intact and it adds wonderful organic matter to the soil. The beauty is though the microbes around the old root system (rhizosphere) will migrate to the new seed, seedling or plant, when you water it in with a biostimulant.
We need to stop buying chemical fertilisers and soil amendments and use your kitchen scraps, and garden waste and turn it into the GOLD that it is as compost. A forest floor is in a constant state of decomposition, and that us what we need to aim for. Whilst we cannot feed our garden mulch daily and replicate what happens on a forest floor, we can go close by helping it along a little. Making compost is easy either using a bin, or a heap, use 1-part brown material (low nitrogen) to 3-parts green material (high nitrogen) and build the compost in layers of these. I don’t put anything (including manures) on my garden unless they have gone through the compost first.
I use a cheap sieve to separate the finished compost from the bottom of the heap form the unfinished compost. The larger material needs further breaking down and is return to the compost. I make approximately 2 x 10 litre buckets of finished compost to use over the garden per month.
I also mix in the worm castings form the worm farm into the compost, this dries out the moisture in the castings making them easier to use. I then broadcast the mis over the plants and beds alike and only do a small area (about 2 square metres) and immediately water it in to not kill the microbes, this allows the mix to settle down into the mulch layer and some will make it to the soil. The soil food web does the rest, and this feeds the soil instead of feeding plants individually like many of us including me have done since we started gardening. This gets your soil back to a functioning state and it mimics more closely the constant state of decomposition that a forest floor has.
Humus plays a big role in healthy soil
Humus is that chocolate coloured sweet smelling substance that most gardeners strive for, It has a huge water holding capacity, and is a home for microbes. It forms 5% of healthy soil but unfortunately, we have managed to lose two thirds of it, if is globally down to 1.5% and here is Australia its more likely to be less than that.
Worms make it 4 times faster than any other decomposition process we know, so using compost as described earlier will feed them. It is said that a healthy worm population should number 25 per shovel full of healthy soil If you can manage to raise the humus level in your soil by just 1% of it, each square metre of soil will hold 17 more litres of water than it could beforehand down 200mm deep. This means that if you can regenerate your soil back to where it should be at 5% humus, that same square metre could hold as much as 85 litres.
Feeding soil and the soil food web
Using this method of feeding soil through the mulch layer feeds the ecosystem beneath our feet that allows this organic material to be decomposed even father into food for a whole host of other living organisms that have their role and function to carry out, all the while contributing to the health of the soil. This will also increase the worm population as within the compost and the worm castings there are worm eggs so by the very nature of you feeding the soil with this material often will help populate your soil with many of them. They are an incredibly valuable asset to have more of, it is the deep digging worms that are the biggest value as they are the ones that aerate the soil on their way to feed from our broadcast compost.
Worm farms
If you haven’t got one, consider getting one, most council have rebates on their cost, and actively encourage their use. Keep them nearest the kitchen outside so that they are fed often. You will get the liquid at the bottom that you can use on you garden and you can make a worm tea from soaking the castings in a bag like a tea bag and use this as a biostimulant. This stimulates soil microbes and particularly essential when planting new plants, seeds, or seedlings. Then you can also harvest the castings to mix with your compost. If you can’t have a worm farm or make your own compost you can buy it, however when you make it you know what goes in it.
All of this will make a huge difference to your soil the overall health of your garden, the health of your plants and your own health if you grow your own food, you will get exactly what you are meant to from food grown this way making you well. This is a stark contrast to food grown on our behalf, hospitals are full of people that shouldn’t be in there and much of the health of the community stems from poor diets and the lack of nutrition and poor gut health. It will allow you to grow nutrient dense produce that can make you well.
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