Improving Clay Soil: What Our Soil Test Chemistry Results Show
We compared the soil chemistry of three areas on our Victorian clay. Organic matter is 8 times higher and sodium far lower where we’ve planted.
Improving Clay Soil: What Our Soil Test Results Show
Heavy clay is one of the hardest soils to grow in. It sets like brick in summer, turns to sludge in winter, and seals over so water runs off instead of soaking in. Our property in the Macedon Ranges, Victoria, sits on grey clay. We wanted to know whether our approach to improving clay soil was making a measurable difference, so we sent samples from three areas for a full soil chemistry test.
The short answer is yes. Compared with our untouched baseline, the flower patch has almost 8 times the organic matter, and the sodium that makes clay so difficult has largely disappeared. The test also showed some things we need to fix, and we’ll cover those too.
One note before we start: these are single samples from three areas of one property, tested once. We are comparing different areas at the same point in time, not the same patch before and after. The results show clear differences, but they can’t prove exactly what caused each one.
The three areas we tested
The Baseline - Interrow Samples
We used an area with minimal regenerative interventions to serve as a reference point for comparing the existing system and future outcomes. We chose the spaces between the tree rows. Aside from some decompaction efforts with a deep ripper a number of years ago, these areas have not been actively managed as part of Project Genesis.
Tree Line Samples
Samples are taken along the tree lines of Project Genesis.
Flower Patch
This area is a two-year-old, no-dig flower growing area. Approximately 5 to 10 cm of compost is added each year, using either compost made on-site or imported mushroom compost. Crops are planted directly into the beds.
The area is drip irrigated and managed intensively as a market garden. Compost is added to the surface rather than incorporated into the soil, and the beds are otherwise left undisturbed. Apart from the initial tilling required to establish the area, it has not been cultivated.
Soil organic matter: the biggest difference
Soil organic matter is the part of the soil that comes from living things: rotted leaves, roots, compost and the microbes that break them down. In clay, it matters a great deal. It glues fine clay particles into crumbs, which opens up space for air and water, and it stores nutrients for plants.
Our baseline has just 0.74% organic matter, which is very low. The tree lines have 2.49%, more than three times as much. The flower patch has 5.74%, nearly eight times the baseline, and above the lab’s guideline of 5.5%.
This is the clearest result in the whole report, and it matches our soil biology tests, where the same areas showed far more fungi and other soil life.
Sodic soil: why our baseline clay is so hard to work
A sodic soil is one with too much sodium attached to its clay. In Australia, soil is usually called sodic when sodium makes up more than 6% of the nutrients held by the clay, and strongly sodic above 15%.
Sodium causes clay particles to push apart when wet. The soil turns to slurry, then dries into a hard crust that water and roots struggle to get through. Many people in Australia blame “clay” for problems that are really caused by sodium.
Our baseline is strongly sodic, at 22%. The tree lines sit right on the edge at 6.5%. The flower patch is at 3%, well clear of the sodic range.
The calcium and magnesium balance
Clay and organic matter hold nutrients on their surfaces, a bit like a store cupboard. The size of that store is called the cation exchange capacity. What matters just as much is which nutrients fill it.
Calcium helps clay form crumbs. Magnesium and sodium have the opposite effect, making clay tighter and stickier. In our baseline, magnesium fills 68% of the store and sodium another 22%, leaving just 6% for calcium. This is the pattern you would expect in heavy, sealed clay.
The tree lines are better, with 23% calcium, but magnesium still dominates. In the flower patch, calcium is the main nutrient at 58%, and the ratio of calcium to magnesium is 3 to 1. The lab’s guideline is higher still (about 78% calcium), so there’s further to go, but the change is large.
Note that the lab’s guidelines are based on the Albrecht approach, which sets fairly demanding “ideal” figures. Many soils grow well without meeting all of them, so we treat them as a guide rather than a pass mark.
Phosphorus and potassium: too little in two areas, too much in one
Not every result is good news. Phosphorus is very low in the baseline and the tree lines. These areas also have low nitrogen and, in the tree lines, low sulfur.
The flower patch has the opposite problem. Phosphorus is almost twice the guideline, potassium is about five times the guideline, and sulfur is very high. This most likely comes from the compost we have added over time. Too much potassium can make it harder for plants to take up calcium and magnesium, and the salt level in the flower patch is also slightly raised.
High phosphorus may also explain something from our soil biology test. The flower patch had no mycorrhizal fungi on its roots. These fungi help plants find phosphorus, and when phosphorus is plentiful, plants tend to stop supporting them.
Soil nutrients we’re short of everywhere
Plants need tiny amounts of trace elements such as zinc, manganese, copper and boron. All three areas are low in most of them. Only zinc in the flower patch reaches the guideline.
The report also measures the total store of these elements in the soil, including the parts plants can’t reach yet. Copper, zinc and manganese are low in that store too, which suggests these shortages come from the underlying clay rather than from how each area has been managed.
Our soil test results, side by side
Measure | Baseline | Tree lines | Flower patch | Lab guideline |
Organic matter (%) | 0.74 | 2.49 | 5.74 | above 5.5 |
Total carbon (%) | 0.42 | 1.42 | 3.28 | above 3.1 |
pH (water) | 6.37 | 5.95 | 7.04 | 6.5 |
Sodium share, ESP (%) | 22 | 6.5 | 3 | 1.5 |
Calcium share (%) | 6.1 | 23 | 58 | 77.6 |
Magnesium share (%) | 68 | 57 | 20 | 11.9 |
Calcium to magnesium ratio | under 0.1 | 0.4 | 3 | 6.5 |
Phosphorus, Colwell (mg/kg) | 5 | 11 | 150 | 80 |
Potassium, exchangeable (mg/kg) | 99 | 169 | 1,160 | 235 |
Nitrate nitrogen (mg/kg) | 0.6 | 0.1 | 9.9 | 15 |
Sulfur (mg/kg) | 30 | 8.1 | 140 | 10 |
Salt level, EC (dS/m) | 0.126 | 0.046 | 0.333 | 0.2 |
Carbon to nitrogen ratio | 12.8 | 15.6 | 12 | 10 to 12 |
What we’re doing next
The results give us a clear job list for each area:
Baseline: The sodium needs dealing with before anything else. Gypsum is commonly recommended for sodic clay, because its calcium pushes sodium off the clay so it can wash away. We will get advice on the right rate before applying it.
Tree lines: These need feeding, especially phosphorus, nitrogen and sulfur. We will discuss with the lab how to rectify this, however my understanding is that organic matter will help with this.
Flower patch: Here we plan to ease off on inputs and let the plants use up some of the extra phosphorus and potassium.
Everywhere: We will look at ways to add trace elements, starting with copper, manganese and boron.
We plan to retest each year and will share the results here, alongside our soil biology results.
Common questions
Is clay soil bad for growing? No. Clay holds water and nutrients better than sandy soil. Its problems are usually poor structure and, in many Australian soils, too much sodium. Fix those and clay can be very productive.
How do I know if my soil is sodic? A soil chemistry test will show the exchangeable sodium percentage (ESP). Above 6% is sodic. Signs in the garden include a hard surface crust, water pooling after rain, and cloudy water in puddles.
Will compost alone fix sodic clay? Organic matter helps a great deal, but it may not be enough on its own for strongly sodic soil. For example, our flower patch also shows that heavy compost use can push some nutrients too high. It's best to talk to a qualified expert to get guidance in this space.
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Read about the soil life in these same three areas in our soil microbes blog post. To see the flower patch itself, visit the Florist and Farmer post.
