Know thy Soil
Notes prepared by Adam Grubb of Very Edible Gardens, 2024
“Humanity, despite their artistic pretensions, their sophistication, and their many accomplishments – owes their existence to a six inch layer of topsoil and the fact that it rains.” -Anon
“Ironically, however, the question [of soil condition] is mostly academic. The cure for heavy clay soil is to add organic matter to break up the clay and improve drainage. The cure for sandy, dry soil is to add organic matter to retain water. The cure for acid soil is to add organic matter, particularly bird manure, to bring the pH up and make more nutrients available. The cure for alkaline soil is to add organic matter to buffer it. The cure for hardpan is to add organic matter to feed earthworms.” – Linda Woodrow, p.48 The Permaculture Home Garden, Viking 1996
Soil & its Aspects
Soil is a complex living system that best understood from at least three aspects: the physical or structural, the chemical, and the biological.
The Structural Aspect of Soil
Let’s start by asking what is soil, exactly? Soil is “the naturally occurring, unconsolidated or loose covering of broken rock particles and decaying organic matter on the surface of the Earth, capable of supporting life.” A good soil is about half air and water, 5% or more organic matter and 45% mineral particles – and is host to huge variety of living organisms. It formed by the interaction of geological and biological processes – the grinding of rocks into particles by erosion, glaciers and volcanic activity, and the efforts of plants, animals, fungi and soil microbes.
Sand, Silt & Clay
The soil categorisation triangle (above right) is used by structural engineers to assess soils before building. You can use a ribbon test to roughly assess the percentages of your soil. The longer the ribbon you can form while letting gravity take its weight, the more clay there is in the sample. Clay feels ‘plasticiny’, silt has a smooth silky feeling, while you can feel and hear the grittiness of sand.
Besides sand, silt and clay, as gardeners we are also interested in organic matter, air, water and the structure of the soil. Clay soils tend to be good had holding water and nutrients. But they can be water-logging i.e., too much water will ‘drown’ the plant roots in wet weather. The heaviness of clays can make it difficult for roots to penetrate and extract the nutrients. Sand based soils are free draining and easy for plant roots and to penetrate but they do not hold nutrients or water effectively.
Having a nice mix of sand, silt and clay gives you some of the best of all words and these soils are known as loams. But we’re usually in a situation where the soil is dominated by sand or clay.
Amazingly, increasing the organic matter of the soil helps address the issues of both sandy and clay soil!e
Dispersive clays
Dispersive clays are clays which crumble apart or create ‘halos’ in water. They are prone to erosion but perhaps more importantly they will collapse in on themselves when wet, destroying the structure of the soil. Placing a piece of clay in water and waiting reveals how dispersive it is. We’ll talk about how to improve dispersive clays below.
Soil layers
Plants and microbes concentrate nutrients and build organic matter in the upper layers of the soil. We call this nutrient and organic material rich layer the topsoil.
Fresh minerals can also be mined by chemical and biological activity from the source rock from below.
The subsoil has less organic matter. It usually has a deficiency or imbalance of minerals. Plants have mined it for what they need and brought those nutrients closer to the surface. Abundant or excessive nutrients are left to accumulate below via leaching. It is important not to mix the layers when working the soil!
Organic matter
Soil Organic matter (OM) in the soil is produced from plant and animal residues decomposed mostly by bacteria and fungi. It gives fertile soils their open, crumbly sponge-like texture and dark chocolatey colour. A good soil should be at least 5% organic matter. Where does this organic matter come from? It comes from above as dead plant and animal material in the litter layer are broken down by microbes and incorporated below. And it comes from beneath, as plants’ roots release sugary exudates for the soil life which is ultimately turned into dark decomposed organic matter.
OM can hold the equivalent of 80-90% of its weight in moisture, and helps open the soil allowing air and plant roots to penetrate the soil. OM helps the soil hold and circulate nutrients, and is a perfect home for essential soil microbes. OM has a fairly neutral pH so helps to moderates soil pH imbalances. OM can immobilise toxic heavy metals so they are no longer available to plants.
Thanks to organic matter and the action of the microbes that build it, a good soil should have the colour and crumbly texture of a chocolate cake!
Increasing OM is the number one thing to get started on in most soils. The easiest way of rapidly increasing organic matter is to buy some compost. But you can make your own compost. And you can use plants to create organic matter in situ.
Australian soils
Underneath our feet, deep down beneath the soil we hit rocks. And they type of rock you’ll hit depends on where you are in the country. The geology type changes and Melbourne has four main very distinct types of geology radically different from each other. The mineral component of soils is really important, and it’s this rock beneath it that often gives soils their nutritional character.
Soils also ‘age’. Over millennia rain leaches nutrients from the mineral components and they become less fertile. New soils are created in three main ways:
- volcanoes, which bring fresh minerals to the surface of the planet in the form of lava
- glaciers, which crush existing rocks into dust rapidly creating new soil material
- and geological uplift, e., mountain creation where geological plates collide which causes rocks to erode into fresh soil material.
Since Australia doesn’t have any of the above really our soils are considered the oldest in the world. Many of our soils have just a thin layer of topsoil with a sudden transition to a low nutrient leached subsoil below. However there are several rich patches of volcanic rocks all along the east coast, and the western plains of Victoria which form the third largest volcanic plains in the world.
The grey is Silurian era (around 430 million year old) sediments consisting mainly of siltstones. The pink and orange are much more recent volcanic rocks (from lava flows), consisting of basalt, which formed in the last 65 million years up to only a few thousand years ago. The yellow areas are Tertiary (between 2 and 65 million years ago) era sands. The greens are recently formed ‘alluvial’ soils (soils created by rivers and creeks). Outside the reach of this map there are some granites such as in the Dandenongs. Some notes on each:
- Silurian siltstones. Dominate the Eastern hills. They produce pale heavy silty clays with poor mineral balances and thin topsoils (duplex). They look pale yellow or grey with chips of rock in them. Generally deficient in calcium, manganese and boron. This area of Melbourne is generally wetter and was originally Better suited for fruit trees than veggies in general but in both cases liming and lots of organic matter is recommended. Basalt rock dust is a good amendment or other broad spectrum fertilisers.
- Western plains basalt. The basalt produces heavy clays known for cracking and drying out in summer and waterlogging in They range in colour from black to brown to rust red. They are rich in most nutrients but usually have poor structure and are very calcium deficient. These areas are generally dryer and traditionally grasslands or grassy woodlands, they are suited to grazing primarily, but with organic matter and lime can make excellent garden soils.
- Some of the sand belt in Melbourne’s south east was former swamp and has good drainage, organic content and a layer of clay beneath. Some is essentially like beach sand with little water or nutrient holding capacity! They are some of Melbourne’s best and worst soils. There still is much market gardening on some of the good stuff. If you’re on the latter you need lots of organic matter, mulch and partial shade can help.
- Alluvial Often on flat areas near creeks and streams. They tend to be rich in organic matter, minerals and with good loamy structure. Alluvial soils fed from the volcanic areas are best, such as the former Werribee River delta where much of Victoria’s veggies are grown.
The Chemical Aspect of Soil
Mineral balances
Ultimately, the minerals in our bodies come from the soil. High mineral levels and a good soil mineral balance is important for healthy plants, and healthy animals, and healthy humans. There are two approaches:
- Law of minimum; the most deficient nutrient will set the level of productivities
- The ratio or balance of certain minerals may be as important as absolute levels. The capacity of soils to contain certain minerals is measured as ‘Cation Exchange Capacity’. (A cation is a positively-charged mineral).
| Calcium | Magnesium | Potassium | Sodium |
|---|---|---|---|
| 68% | 12% | 2-5% | <1% |
Table 1. William Albrecht argued that soils with certain proportions of exchangeable cations will show excellent structure and optimal nutrient availability, maximising productivity and making management easier. It can’t hurt to aim for these levels, however addressing the minimum available nutrient is probably a more important step, since scientific research has shown that plants can thrive in a wider range of ratios. (The above ratios are modern recommendations from Holmgren’s Permaculture Principles and Pathways)
You can have your soil tested for these at a lab such as APAL, see Resources. You can diagnose some deficiencies from signs on the leaves such as different colours and forms of mottling. See table below.
Mineral balances can affect soil structure. The level of sodium and the ratio of calcium to magnesium can be particularly important on many clay soils. Not enough calcium can lead to dispersive qualities. High ratios of sodium creates ‘sodic’ clays, which are generally sticky masses with little structure. Too much Magnesium can have similar effects. A Ca:Mg ratio of around 6:1 means that your clay can form good crumb structure (peds). Gypsum and/or lime can help in both displacing sodium and lifting the Ca:Mg ratio, if your soil test or local knowledge suggests that would be a good thing. Dolomite, which is high in magnesium, can make some clay soils more dispersive, and the structure of them worse.
Soil pH
pH is a measure of soil acidity, where 0 is an extremely strong acid, 14 is a very strong base, and 7 is neutral (like water). Different nutrients are more or less available at different pHs. Some plants like acid soils (e.g., blueberries like it in the range of 5). Veggies like soils around neutral, or slightly acidic (between 5.5-7.5, with 6.5 about ideal). Most fruit trees generally like this pH range too, if perhaps erring on slightly more acidic.
In the diagram to the right, the width of the bar represents how available the nutrient is at various pHs.
Testing pH
For under $20 you can buy a soil testing kit at hardware stores and nurseries which contains a colour changing liquid, and chart for reading the pH of your soil. You can get around 100 uses out of a single kit. There are also electronic testers available. These kits can give you a general idea of acidity or alkalinity, although they can misread on highly organic soils.
Adjusting pH
Organic matter is a pH ‘buffer’ – whether your soil is alkaline or acidic, it tends help bring it towards neutral. If your soil is extreme though you may need to consider other options.
If you have overly acidic soils (low pH), garden lime is the most common solution. Garden lime is readily available at nurseries with directions on application rates.
If you have overly alkaline soils (high pH), one solution is elemental sulphur. Elemental sulphur is sometimes less available, and may be sold as a stock feed nutrient, rather than as a garden additive. (Here’s one place it is available by post)
To drop the pH by roughly one point, spread 100 grams of elemental sulphur per each square metre. Soil bacteria then need to process it, which means the pH change happens more quickly in warm, wetter conditions. It can take up to 6 months before the full effects are felt, but you should see some changes within a week.
| Symptoms appear first in the OLDEST leaves; indicates nutrient is mobile | |
| Nitrogen (N) | General yellowing; stunting; leaf drop; |
| Phosphorus (P) | Loss of sheen/darkening; red/purple/bluish tones (may affect leaf underside more) with some yellowing of lower leaves; stunting; erect habit; |
| Potassium (K) | Scorched margins; spots surrounded by pale zones; small fruit |
| Magnesium (Mg) | Interveinal yellowing; brilliant colours or bronzing, in extreme cases, burning around edge in V shape, often with green triangle tongue from petiole along midrib. |
| Molybdenum (Mo) | Mottling over whole leaf; little pigmentation; cupping of leaves; distorted stems (Uncommon) |
| Cobalt (Co) | Legumes only, as for nitrogen (Uncommon) |
| Excess salt | Marginal scorching, generally no spotting |
| Symptoms appear either in OLDEST or YOUNGEST leaves | |
| Manganese (Mn) | Interveinal yellowing. Similar to Iron but less detailed yellowing, broader band of green along veins, no leaf shrinkage |
| Symptoms appear first in YOUNGEST leaves; indicates nutrient is immobile | |
| Calcium (Ca) | Tiphooking; blackening & death; newly emerged leaves develop abnormally |
| Sulphur (S) | Yellowing; small size; rolled down |
| Iron (Fe) | Interveinal yellowing; veins green; youngest leaves almost white if severe |
| Copper (Cu) | Dark blue-green, curling; twisting; death of tips |
| Zinc (Zn) | Smallness; bunching; yellow-white mottling |
| Boron (B) | Yellowing margins; crumpling; blackening; distortion |
Table 2 – Nutrient deficiency symptoms of plants. Adapted from the work of Kevin Handreck (What’s Wrong With My Soil, 1977, CSIRO) and Carol Curtis.
A note on soil toxins
Unfortunately urban soils often contain toxins. Potential toxins include heavy metals such as lead and cadmium, residual pesticides (such as DDT and Dieldrin), petrochemical residues such as polyaromatic hydrocarbons (PAHs) and others. In general, it’s worth being aware but not alarmed.
Lead is probably the most common soil contaminant. It is particularly a concern for children as it can affect mental development if ingested. It occurs naturally in soils, but as it was used as a petrol additive, in paints and elsewhere, it can be in toxic excess in the city. Assume you have excessive levels in the soil wherever your land was settled pre-1970 (when lead paint was phased out), or if you are on a major road. One piece of good news is that lead rarely makes it into fruit in any appreciable quantities (including tomatoes, eggplants, pumpkins etc.), however some leafy plants such as lettuce will take some up from the soil. If soil is suspect, root vegetables should be peeled. Most exposure comes from breathing in or eating dust, so wash your produce. The main strategies for limiting plant uptake is lots of organic matter, and liming the soil if it is acidic, so that it becomes neutral. Similar strategies apply to cadmium and other heavy metals.
We use Amal Analytical for broad spectrum soil toxin testing, but it can be expensive. There’s also a very cheap service called GardenSafe which tests for heavy metals.
See our top 11 tips on dealing with lead contamination here.
The Soil Food Web
The soil is brimming with microbes, fungi, nematodes, mites, springtails and worms. A complex ecosystem in microcosm. In nature, plants absolutely depend on this for nutrients and sometimes water access, for breaking down organic matter, for disease protection and for building soil structure. Good soil has good ‘crumb structure’ and the biology aids this:
- Bacteria make micro-aggregates with glues
- Fungi make macro-aggregates by tying things together with their hyphal strands
A single teaspoon of good soil contains anywhere up to 1 billion organisms of 10,000 species. The combined weight of these organisms is as much mass as two cows per acre. Complex microbial ecosystems existed in the soil long before plants and animals evolved. By numbers or biomass they still rule the earth.
To care for soil we need to appreciate that it is very much alive. Like us, it needs:
- Food – in the form of compost and mulch
- Water – when soil dries out, much of your life and organic matter is lost
- Air – soil should be damp, not The ‘good guy’ soil life requires air. Anaerobic (no oxygen) bacteria produce stinky and toxic acids and attack plant roots.
- Shelter – mulch holds in moisture, and protects the surface of the soil from UV light and temperature extremes
- No poisons – pesticides, herbicides and artificial fertilisers all tend to be antagonistic to soil life
Great, but what to do?
Luckily, there are some simple strategies that apply in almost any situation:
- Compost! It introduces organic matter, recycles nutrients, and is chock full of good soil life. It must be aerobic (well oxygenated) and with a good carbon to nitrogen balance of ingredients for good soil life. (See compost link below for more info.)
- Mulch! It mimics the natural process of a forest or plains, protecting the soil and feeding Use straw mulch for veggies, woody mulches for trees and perennials.
- Plants! Plant roots build soil and feed soil life with their exudates and dead roots. Plant foliage protects soil from the elements and mulches the soil. Aim to have 100% plant coverage of soil as much as Some plants are better soil builders than others, but anything is always better than nothing. Many acacias are great soil builders.
- Water! Keep it moist to maintain In sandy soils it’s all the more important to keep the soil moist. They can become water repellent if allowed to dry out. Regular shallow waterings for sandy soils are recommended.
- Sandy soils note: Kitty litter. Seriously! For those high productive areas, for instance where you are growing veggies and water retention is a problem, it’s worth considering adding a little bentonite The easiest way is to add a kitty litter. Not all kitty litters are made of bentonite, and some have extra additives. We’re told the Coles brand clumping unscented is a good one.
- Clay soils note: First do a dispersion test! If it’s dispersive you’ll need to add But in what form? Next do a pH test. If the soil is alkaline or neutral you’ll want to add gypsum. If it’s excessively acid you may want to garden lime. Physical aeration with a garden fork (be sure to do it when the soil is slightly moist but not sticky and wet) can help kick start opening up a heavy clay-based soil, and it’s a good time to add your garden lime or gypsum.
- Fertilise! you not only need to keep up the organic matter but you need to bring in minerals and fertilisers. Basalt rock dust is crushed lava! If you’re not on volcanic soil already we recommend adding it to your Blood & bone and chicken poo based fertilisers bring in some of the richer fertilisers needed, but read about your plant’s requirements.
Further resources
Nutrient balance soil testing labs: Apal Analytical and SWEP Analytical Laboratories