Know your soil: sand, clay, loam and how to tell
Two simple tests tell you whether you have sand, clay or loam. What that means for watering and feeding, plus a guide to Adelaide's soils and a drainage check.
5 min read 路 Patchmates
Every garden starts with the soil you already have. Once you know what it is, a lot of puzzles make sense: why water runs straight off one bed, why another stays soggy for days, why your carrots fork. The good news is you can work it out in ten minutes with your hands and a jar.
The three main soil types
Soil is a mix of mineral particles of different sizes, plus organic matter, air and water. The size of the particles changes how the soil behaves.
| Type | Feels like | Good points | Challenges |
|---|---|---|---|
| Sand | Gritty, falls apart | Drains fast, warms quickly in spring, easy to dig | Dries out fast, nutrients wash through |
| Clay | Sticky when wet, hard when dry | Holds water and nutrients well | Drains slowly, cracks in summer, hard to dig |
| Loam | Soft, crumbly, holds shape loosely | A balance of both | Not much; this is what most gardeners want |
Silt sits between sand and clay. It feels smooth and silky, a bit like flour. Most real garden soils are a blend, so you're looking for which one dominates.
The squeeze test
This is the quickest way to get an answer.
- Take a small handful of soil from about a spade's depth, not the surface.
- Wet it a little and work it in your palm until it's like putty.
- Squeeze it into a ball, then try to push it out between your thumb and finger into a ribbon.
What you'll see:
- Won't hold a ball, feels gritty: sandy soil.
- Makes a ball that crumbles when poked: loam.
- Makes a short ribbon that breaks off: clay loam.
- Makes a long, shiny ribbon that holds together: clay. The longer the ribbon, the more clay.
The jar test
The jar test takes a day or two but shows you the mix more clearly.
- Fill a straight-sided glass jar about one third with soil. Pick out roots and stones.
- Add water until the jar is nearly full, plus a teaspoon of dishwashing liquid to help the particles separate.
- Put the lid on and shake hard for a few minutes.
- Set it on a shelf and leave it.
Sand drops to the bottom within a minute or two. Silt settles over the next few hours. Clay can take a day or more, and the water may stay cloudy for a while. Bits of organic matter float on top. Once it settles, look at the layers. A thick bottom layer means sandy soil. A thick top band of fine material means clay. Roughly even layers point to loam.
Adelaide's soils
Adelaide has a real patchwork of soils, sometimes changing from one street to the next. As a general guide:
- The plains: many suburbs sit on heavy clay. A lot of it is reactive, which means it swells when wet and shrinks and cracks when dry. That's why you see cracked ground in late summer.
- Near the coast: sandier soils, often low in organic matter. They drain well but dry out fast and need regular feeding.
- Alkaline soils and calcrete: many Adelaide suburbs have alkaline soil, and some have a hard layer of calcrete (limestone rubble) not far below the surface. If your spade hits a pale, rocky layer, that's probably it.
- The Adelaide Hills: soils there tend to be more acidic, which suits plants like blueberries, rhododendrons and camellias.
Your own block may also have fill brought in when the house was built, so test a few spots.
The drainage test
Knowing your soil type is half the story. The other half is how well water moves through it.
- Dig a hole about 30 cm deep and 30 cm wide.
- Fill it with water and let it drain completely. This wets the surrounding soil.
- Fill it again and time how long it takes to empty.
As a rough guide:
- Empties within an hour or so: free-draining, typical of sandy soil.
- Empties over a few hours: reasonable drainage for most plants.
- Still holding water the next day: poor drainage. Raised beds or mounded rows will save you a lot of grief.
If the water sits on top of a hard layer, you may have calcrete or a compacted clay pan. Breaking through it or building up is usually easier than fighting it.
What to do with what you've got
- Sandy soil: add compost and well-rotted manure every season. Mulch well. Water little and often, and consider a soil wetter if water beads on the surface.
- Clay soil: add compost and gypsum to help it crumble, don't dig it when it's wet, and never walk on beds. Raised beds are a great shortcut.
- Loam: keep it that way with compost and mulch.
Whatever you start with, organic matter is the fix. It helps sand hold water and helps clay break up and drain. Root crops like carrots and potatoes enjoy loose, crumbly soil, while tough plants like olives cope with much harder ground.
In Patchmates
Add a photo or note to each bed with your soil type and drainage test result, so you remember what you're working with.