- Find out your soil type
- Urban Farming: How to Determine Your Soil Type
- Different soil types
- How to determine your soil type
- How Pottery Works
- Dry Clay Harvest Method
- Wet Clay Harvest Method
- Processing Clay for Pottery
- How to identify clay?
- Determine Soil Type and pH
- Soil Texture
- K-12 Soil Science Teacher Resources
Find out your soil type
Knowing your soil type is very important, as it determines which plants will thrive and to choose the plants best suited to your garden.
There are six main types of soil: chalky, clay, loamy, peaty, sandy and silty. To test your soil, you need to take a look at it and feel it. Add water and try rolling it between your hands. Observe how your soil looks and feels, and whether it’s sticky, gritty, friable, or slimy. Watch our video guide to testing your soil texture.
Depending on the size of your plot, test the soil from different areas, as it can vary enormously. If possible, create main planting areas where the soil is good, saving poorer conditions for hard landscaping, where soil quality is less important.
Most soils benefit from some improvement. You can improve soil by incorporating organic matter, such as manure or home-grown compost.
In addition to knowing your soil type, you’ll also need to check its pH. Find out how to check your soil pH.
Here’s how to identify the main soil types.
Knowing your soil type enables to you to choose the plants best suited to your garden.
Stony, chalky soil in open palms
Chalky soil is alkaline, stony and free draining, as it often overlays a chalk or limestone bedrock. Minerals such as iron and manganese will quickly leach out of the soil, but this can be remedied to an extent by regularly adding fertiliser. Chalky or lime-rich soil may be light or heavy. Find out more about plants for alkaline soils.
A ball of wet clay soil
Clay soil warms up slowly in spring and goes hard and cracks when dry. It also drains poorly. Although it’s hard to dig, it’s very high in nutrients. It feels lumpy, slimy and sticky when wet. It rolls into a ball easily and stays in shape. Find out more about flowering plants for clay soil.
Loamy soil held in cupped hands
Loam is the perfect soil type, as it’s easy to work, is not too free draining or prone to waterlogging, and is packed with nutrients. It also warms up quickly in spring. Loam is made up of a mixture of clay, sand and silt, which each have differently sized soil particles. It rolls into a ball easily, but won’t keep its shape as well as clay soil.
Trickling peaty soil between fingertips
Soils containing lots of peat are acidic and high in organic matter, but low in nutrients. Peaty soil holds plenty of moisture and can get waterlogged, but it’s ideal for growing acid lovers such as rhododendrons and azaleas. Peaty soil is dark in colour and feels spongy if squeezed. It’s rarely found in gardens.
Rubbing sandy soil between thumb and fingers
Sandy soil is free draining, easy to work and warms up quickly in spring. However it dries out rapidly and leaches nutrients when it rains, so it needs plenty of added organic matter to retain moisture and feed the plants. It’s gritty to touch. A rolled ball of sandy soil will crumble away easily. Discover plants for light and stony soil.
Silty soil is made from quite fine particles, so is free draining but also retains moisture. It’s also higher in nutrients than sandy soil. It can get compacted easily. Silty soil is smooth to the touch. It rolls into a ball easily, but won’t keep its shape as well as clay soil.
Urban Farming: How to Determine Your Soil Type
By The National Gardening Association, Paul Simon, Charlie Nardozzi
When talking soil, a little knowledge goes a long way. And most important for urban farmers is recognizing your soil type and its health. Some soils are naturally fertile and need little altering, but others need an overhaul. Knowing where you stand with your soil helps you determine what fertilizers and amendments you need to add before you get started.
Urban soils have special considerations you need to know about. Certain contaminants like lead and other hazardous substances are potential pollutants in some city soils. Testing can tell you whether you have any cause for concern. Luckily, if your soil has elevated toxic levels, many tips and techniques can help fix the contaminated soil and build it back to optimum health.
Different soil types
Just like plants and people, soils have different characteristics that make them unique. Knowing the kind of soil you have helps you determine its strengths and weaknesses. While soil is composed of many elements, the place to begin is with your soil type. You just have to observe the composition of the soil’s particles.
The following three types of particles can make up your soil:
Clay: Clay is essential to your soil. Clay soil is naturally high in nutrients and holds moisture well, keeping your plants hydrated.
However, clay soil often gets a bad rap because of some of its characteristics. Wet clay soils stick to your shoes. And because the individual soil particles are so small, clay has smaller air spaces. As a result, it drains water slowly and is slow to warm up in spring. When dry, clay soil cracks and makes your garden look like the Mojave Desert.
Sand: Sand is the opposite of clay in many ways. Because of the large particle size, sand has lots of air spaces, so it drains water quickly and warms up fast. These characteristics make it ready to plant in spring sooner than clay. However, it’s also the first type of soil to dry out in summer and doesn’t hold nutrients as well as clay.
Silt: Silt is like the right bed in Goldilocks and the Three Bears. It has medium-sized particles, so it holds some water, but not too much. It holds some nutrients, but not as many as clay. It warms up fast in spring, but not as quickly as sand. A soil dominated by silt is a gardener’s friend.
Most soils are a combination of these three particles, but the particle type that dominates dictates many of the properties of your soil.
The ideal soil is 40 percent sand, 40 percent silt, and 20 percent clay. You’ll hear this mixture referred to as loam. It takes the best from each soil particle type. It has good water drainage and allows air to infiltrate the soil like sand, but it also holds moisture well and is fertile like silt and clay.
Loam is the ideal, but if your soil falls a little short, don’t worry. Through the addition of organic matter, you can create a loamier soil that has all the attributes you desire.
How to determine your soil type
You can try a few tests (from simple to more complex) to get a general idea of your soil type. Choose one or two tests to help you get an idea of your soil type.
The squeeze test
To do this test, be sure your soil is damp, but not soaking wet. Grab a small handful of the soil in your hand. Rub some of the soil between your fingers. If it feels gritty, it’s mostly sand. If it feels slick and slimy, it’s mostly clay.
The ribbon test
Take a handful of damp soil and make a ribbon by rolling the soil between your hands.
If you can form a ribbon and hold it vertically without it breaking, you have mostly clay soil. If you can make a ribbon, but it breaks off when you try to hold it up, you probably have somewhere between 25 and 50 percent clay in your soil. If you can’t make a ribbon at all, chances are your soil is more than half sand.
The jar test
The jar test is for the scientists in the crowd. It’s a bit more precise than the other tests. To do this test, take soil from a number of places in your garden and mix the samples together in a bucket. Scoop up a cup of your soil and follow these steps:
Let the soil dry out on a flat surface until it becomes crumbly.
Remove any roots, stones, or debris and crush it into a powder with a mortar.
Place a 1-inch-thick layer in the bottom of a quart-sized clear glass jar.
Fill the jar two-thirds full with water and add a pinch of salt (or 1 teaspoon of liquid dish detergent) to help the soil particles separate. Shake vigorously.
Let the solution settle into different layers.
The sand will settle quickly (within a few minutes) to form the bottom layer. A few hours later, the silt will settle. You should be able to see a visual difference between the large sand particles and the smaller silt particles. The clay may take days to settle out.
Measure the total amount of soil, and then measure each layer.
To determine the percentage of each soil type, you need to do a little math. If, for example, the total amount of soil is 1 inch deep and you had a 1/2-inch-thick layer of sand, your soil is 50 percent sand. If the next layer (silt) is 1/4 inch deep, you have 25 percent silt. The remaining 25 percent, then, is clay.
Many people ask me, how I can find clay in nature and how do I know it’s clay.
What is clay? You can google it, you can find a geological description on Wikipedia, but I will give you a different explanation. Clay are rocks, crushed and weathered to smallest particles – smaller than 2 μm. Between the particles is water, which makes the material plastic and workable. I think of clay as liquified rocks. So depending on the origin rock, there are many different clays. During the process of weathering, clays can stay on site, and stay very clean, or, they can travel from the original place, mixing with all sorts of other clays, metal oxides and organic matter. I think of clay as natural material available to almost anyone who shows some effort in finding it. Clay is Earth’s flesh.
For a geologist clay needs to have a clay mineral. But from a potter’s perspective, clay can be any kind of plastic material that you can work with and after fire, stay in the same shape. I met Australian potter Steve Harrison few years ago. He tries to be as self sufficient as possible with ceramic materials. Since he didn’t have any clay in his direct surroundings, he made it from rocks, which included milling, soaking in water and ageing with vinegar for about a year, if I remember correctly. He made a kind of porcelain.
- How do you know it’s clay?
There are several indicators. I like to go clay hunting after rain. After rain terrain is wet, colours get more contrasted and you can try the wet material if it is sticky and plastic. There is a simple test: grab a handful of material, roll a coil and bend it like U letter. If the coil is plastic enough, it will not crack. That means it is most likely clay. But again, it needs to be moist enough. Do not confuse too dry with not plastic. They both crack.
Colour and texture. Clay comes in many colours, but it is usually different than the soil of the vegetation cover above it. Observe how the soil and the humus looks like, see the difference from that layer and what is under. Usually vegetational cover is 10-30cm deep. After that it can be rocks, sand or clay.
Colour. Clay can be light grey, dark grey, brown, orange, olive, cream, ochre, red and many other colours.
If the clay is exposed – without that vegetational cover, it is either in dry or moist form.
Dry form has special properties: the upper surface cracks with very clear and distinctive cracks. If you crush this dry clay in hand it breaks to particles that have sharp edges and flat surfaces, it is possible it is clay. If you try and break a piece of material that looks like rock and you manage to break it easily, it is likely to be clay. You can break clay, but not all rocks. Scratching is another test. You can scratch dry chunk of clay with your finger nail and you will leave a scratch line.
There is good possibility that the ground you stand on, has clay under your feet.
Stay with me for part II.
by Marvin Bartel, Emeritus Professor of Art, Goshen College, Indiana, USA
July 7, 2016 update © Marvin Bartel. – CONTACT the author
These high school ceramics students are clay prospecting. They not only made pottery from the clay, they built a kiln and wood fired the pots in the kiln. Eric Good Kaufmann, their teacher, is an accomplished potter and teaches art at Bethany Christian High School, Goshen, Indiana, USA. Mr. Kaufmann is an alumnus, class of ’97, Goshen College.
PROCESS to REWORK good clay that becomes too dry to use. Processing self-dug clay follows below and self-dug clay is shown in the photos to the left on this page.
- To reprocess hard clay it must first be totally dry. There is no need to break up dry clay.
Be sure it has no plaster chips in it – plaster causes pop-outs when bisque fired. Leather hard clay or moist clay does not slake well because it is not porous like dry clay is. Instruct students to handle the dry clay without making dust. Airborne dust is not healthy to breath. Other contaminants such as paper, sponges, and so on, may cause mold to form in the clay, in addition to being a nuisance in the clay.
see health hazards page
- Place the totally dry lumps in clear water in something like garbage cans. Use enough water so clay is totally under water. Just let it set in clear water.
- Never stir it. Stirring clogs up the porosity and prevents good slaking (soaking to mush).
- In a few days or less, even huge chunks of dry clay will slake to mush. Go to step 6 below and dry it enough to use as in steps 7, 8, and 9.
SELF-DUG CLAY VARIATION
If you dig clay yourself, it often has impurities that need to be removed. Most kids love to help with this and there are few better learning experiences. If you are a teacher, invite students to bring in samples for testing. If it works well, ask them to bring more.
- Let the clay become totally dry.
- Slake it as described in 3 above.
- When it is all soft and mushy, stir it until it is a slip. I use a mixer on an electric drill or a blunger. Add water if needed to liquefy it.
- Pour the slip through ordinary window screen available at any building supply store.
- The screening removes stones, roots, and other trash that causes trouble. The chief culprit is limestone. Limestone, like plaster, pieces cause pots to break after firing.
- When the clay has settled and turned to mush, remove extra water from top. Dip water off or siphon it off.
- Spread the mush a few inches thick on clean dry porous surfaces. I use, dry plaster, clean concrete, canvas, denim, etc. Smooth the top to avoid getting small dry pieces on the surface.
- If you want it to dry faster, use a fan and/or set it all on a wire rack to allow air under it.
- When it is nearly dry enough, I make coils as thick as my arm and set them around like big arches (a foot tall) and they are ready to wedge and use in 24 hours or less. This clay can be stored forever in an airtight plastic.
In ancient China, potters stored moist clay in caves for the next generation to improve the plasticity of the clay. If it is to be stored long-term, double wrap it. Double wrapping in plastic bags from the supermarket works. Students can bring in hundreds of these.
notes on digging clay
WHERE IS CLAY? – Check stream banks, construction sites, roadway cuts, and any place that gets slippery after a rain and sticky as it starts to dry. When dry, it is nearly rock-hard. Many of us can find clay under the topsoil in our back yards.
PLASTICITY – Some clay is too sandy and some is too sticky. When I prospect I look for clay that can be rolled between my hands into a pencil thick coil of soft clay and wrapped around my finger without cracking. If the coil cracks, it may be too sandy or its clay particles may be too large. Sticky clay tends to be cling to my hands too much. It will often have severe drying shrinkage and tend to crack during drying. Potters often blend several clays to get the right properties. See photos on left.
Commercial clays can be added to balance the mix. Commercial ball clay adds plasticity (so it is less apt to crack when bending it). On the other hand, crude ground fireclay, china clay (kaolin) fine sand, and/or grog reduce plasticity (make it less sticky and shrink less). Do a web search of “ceramic chemicals and clay” for sources of commercially available clay types near you. See photos on left.
IMPURITIES – Most common clay contains impurities, often in the form of iron oxide, sand, roots, and other debris. Troublesome impurities can be removed by making a thin slip. The sand settles to the bottom first. Allow the sand to settle a short time. Then decant the clay water (the good slip from the top down to the sand) and discard the sand in the bottom. Allow the clay (slip) to settle and process it as described in the 9 steps above.
Iron impurities are very common and not easily removed. Iron gives it the reddish brown color when fired and causes the clay to melt more easily. It may not work for stoneware, but most common clays are fine for earthenware. Most of it will fire to cone 05 without problems.
GOOD USES OF IMPURE CLAY – Potters who make high fire stoneware sometimes add small amounts of impure local clay to their clay body to add character and blemishes. I regularly add some common brick clay to add character to my pottery. Color and iron spots look more natural and give a warmer feeling. Stoneware potters also use local clay as a source of glaze material. These “slip glazes” have been used for hundreds of years for lining jugs and traditional crockery.
AESTHETICS OF SELF-DUG CLAY: Most native self-dug clay fires to look like common clay flowerpots. Some potters burnish it (rub the nearly dry pieces with a polished stone or back of a spoon). Some Native American potters make beautiful polished black pottery from self-dug clay. Black is achieved by smothering the fire at the end with ashes so that no air reaches the hot pottery and the carbon from remaining fuel blackens the pottery. Typically, tribal pottery is not glazed and is fired without kilns. Sometimes the potters use colored and white clay (slip) to decorate. Search (Google) Terra Sigillata for more information on how to get a highly polished surface without glaze.
HINT: Clay that is thick or not dry enough often explodes as moisture turns to steam when it heated rapidly. If this happens, make it thinner, dry it better, heat it slower at first, and/or add something like sand to the clay to open the clay body more and let the steam out.
OUTDOOR FIRING WITHOUT A KILN
Responsible Adult Supervision is Required
Never leave an outdoor fire unattended
Never fire if there is any chance of a wildfire
Have emergency fire quenching equipment on hand
Leave the site cleaner than you found it
Obey all laws and codes
WHY IS CLAY FIRED?
Clay becomes pottery at temperatures at about 1,000 degrees F (the beginning of glowing red heat – about 540 C). Traditionally, tribal earthenware is fired to about 1,400 degrees F (760 C). Heat removes the molecular water in the clay. The heat converts clay molecules to molecules that do not dissolve or slake in water. In modern societies pottery and brick is fired in kilns to temperatures ranging from 1,800 F to 2,400 F. Most of the common clays like clay shown here on the left found in our back yards start to deform and melt if they are fired higher than about 1,900 F. Modern toilets are fired from clay that has fewer contaminants. It is fired to 2,300 to 2,400 F., making it very strong and impervious.
FIRING WITHOUT A KILN
Kilns were invented to contain heat to reach higher temperature with less fuel. In tribal settings it is traditional to use an outdoor bonfire type of firing that is fueled with enough wood kindling under the pottery to exceed red glowing heat during the burn. The tempreatures of the pottery reach 1,000 F and hotter.
WHERE and how to do it SAFELY. Consider fire safety and local fire codes. Many cities and communities are very strict about open fires. In any case, do not do this where there is any chance that the fire will spread from your firing. Have an ample supply of water close at hand. Have a shovel and dirt that can quickly be used to put out an accidental fire. Do not leave it unattended. Teach careful and strict safety habits to children and students. Temperatures are much hotter than a cooking fire.
THE ‘UNKILN’ CONSRUCTION STEPS
1. The ‘unkiln’ firing begins with a pile of dry kindling wood. Some potters put this in a shallow pit or within a ring of steel, brick, or stone. In any case clean the area to prevent fire spread.
2. A stack of pottery is carefully piled on top of the kindling wood. Stack it so you think it will survive as the wood burns and your pots tumble into the ashes. Optionally, you can try supporting the pottery pile with some carefully placed supporting stones, bricks, or some old pieces of fired pottery; but leave plenty space for kindling to fuel the fire.
3. OPTIONAL: Some potters place broken pot pieces over the pottery pile. You can also cheat with some scraps of tin roofing, flattened tin cans, etc. Leave a generous exhaust opening at the top and several combustion air opening at the base around the perimeter.
4. Cover it with a thick layer of natural material such as tall green swamp grass or animal dung to hold the heat in. Some moisture in the dung and grass keeps it from burning off too soon. This insulating layer holds the heat in long enough to fire the clay, but it does also burn toward the end of the firing.
5. OPTIONAL: In some cases, this insulation layer is smeared with a coating that forms a thin shell. This shell can be made of a clay/sand/straw or grass mixture.
6. A generous exhaust hole is provided at the top of the mound and several vent openings are provided around the bottom so the wood gets air and burns with enough gusto that the clay gets red-hot. The size depends on how large your firing is. The openings around the bottom provide a place to ignite the wood and allow adequate combustion air to enter. The top opening needs to be large enough to allow rapid air flow to enter at the bottom and small enough so the heat is contained.
1. Light the kindling with some wads of paper at the vents. OPTIONAL (if worried about breakage): As soon as you are sure the wood is burning, you can cover the top vent partially with some tin or pottery shards to restrict the burn and heat the contents slower at the beginning. Open this up soon enough to allow most of fuel to burn rapidly and very hot. Most of the fuel is needed to reach a high enough temperature to fire the clay.
2. OPTIONAL: When the fuel is all burned, cover it immediately with a layer of dry dirt (if you have wood ashes these also work). This chokes off the air so the pots come out smokier and darker. Some potters can get totally black pottery this way.
3. When it has cooled to about 500 F or cooler, feel free to use sticks to carefully probe and role out your hot treasures.
Mistakes happen, but enjoy the process. Think about it and try again. Many mistakes turn into new ideas and possibilities.
Experiment and learn. Steam pressure is what breaks most pots. If pots are not made to a uniform thickness, they sometimes crack because the drying shrinkage varies. If pots break it may mean they are too thick or the clay needs some opener. Sand or grog in clay is an opener. It allows the moisture to steam out (to escape easier) at the early stages of heating. Sand must not include any limestone. After firing, clay pieces will pop off as the pieces of lime contaminants expand by absorbing atmospheric moisture.
Modern computer controlled electric kilns use a prolonged heating stage at 200 degrees F. This is just below the point at which moisture turns to steam. Clay that is fired fast must be TOTALLY dry before it hits the steam forming temperature. This prevents the clay explosions that often happen when clay is heated to too rapidly.
When firing without a kiln, it may help to pre-dry you clay pieces in a kitchen oven set to 190 degrees F. With a kitchen oven, the pots are dried by “baking” below the boiling temperature of water for several hours. I set the oven to 190 F. This is NOT firing the pots, but it dries them so they can be fired in an outdoor bonfire or pit firing with less breakage caused by steam explosions.
CAUTION: A kitchen oven cannot be set hot enough to fire pots. Firing pots in any indoor stove is never recommend. It may cause a house fire. The temperatures needed to fire clay are too hot (1,000 F degrees and hotter). This temperature would make any stove red hot and it would exceed the safety designed into any stove. This is much hotter than a self-cleaning oven reaches when it burns the residues in a dirty oven. Clay does not change to pottery unless it is fired to 1,000 F (red hot) or hotter.
What if the fired pots dissolve in water?
This means that the fire was not hot enough. Tribally fired pottery is often fired to about 1,400 F. Clay converts to pottery at about 1,000 F. The water that evaporates as clay dries is simply physical water. However, at about 1,000 F, the chemical water is removed. This produces a molecular change–making the clay into a stone-like substance that no nolonger softens in water.
What if you don’t like the color and texture?
Pit fired and bonfire pots have natural variations. These are not defects. Experiment. Pay attention to everything. Try burnishing. Try coatings. Never use toxic stuff on the inside of pottery that might be used for food or drink by anybody now or in the distant future. If you like boring and reliable uniformity, use an electric kiln.
What if water seeps through the pot?
This is not a defect. Pit fired pots without glaze on them will all be porous and some water will soak through, but the structure of the clay will be okay if it was fired hot enough. If I want to use a porous pot for a vase, I warm it in an oven and then seal the inside with melted wax by pouring melted paraffin wax in and out of the pot. Porous pottery is used for self-cooling water jars that keep the contents cold by evaporation on the exterior. Porous pottery is also used to filter water. Colloidal silver is added to water filters to help eliminate bacteria. If you do this, buy it from a reputable business (some websites have sold unsafe fake materials as colloidal silver). Water filters are typically fired in kilns in order to reach the correct temperature to function properly. has more on porous pottery filtration for drinking water.
Visit my ideas for teachers and parents
Art & Learning to Think & Feel
– Table of Contents – links to essays & lessons –
Classrooms to Teach Thinking and Emotional Intelligence
Teaching Thinking & Feeling
Sculpture and Collage Thinking
Questions from Readers
How Pottery Works
Anyone who has ever been to summer camp has probably worked with clay. This abundant and naturally occurring resource is the basis of pottery. Pottery clay is mined from the Earth and ground into a powder. This powder is combined with other water and other ingredients to form what’s called the clay body — what you probably picture in your mind when you think of a potter at work. The type of clay, how it’s prepared and the amount of water used are all variables in how the final product turns out. There are lots of clays on the market in a wide spectrum of colors depending on your needs and preferences.
Pottery clay needs to be moist, durable and exhibit a good amount of plasticity. This means that it’s easy to mold and retains its shape. You can buy a clay body that’s ready right out of the box, but many potters prefer to make their own mix specific to a particular piece of pottery. Ingredients like nylon or sand can be added to the clay to aid plasticity, or to lower or raise the clay body’s firing temperature, which also affects the color. And because you don’t want a 20 pound ashtray, you can add materials like sawdust or coffee grounds to help reduce the weight of the finished piece.
Earthenware clay is most similar to what our ancestors used and is the kind of clay you might find in your backyard or that summer camp craft class. Earthenware pottery pieces are porous, so they need a sealing glaze to make them watertight. These clays require the lowest temperatures for firing, and finished products typically turn out in rich reds, browns and oranges. Terracotta planter pots are a good example of earthenware pottery.
Stoneware clay is more heavy-duty and requires higher firing temperatures. These high temperatures yield a pretty cool result — the clay vitrifies, or turns into a glass-like substance. An added benefit of vitrification is that it makes the pottery waterproof. This makes glazing stoneware unnecessary, but still an option for decorative purposes. Stoneware clay is typically used for pottery with practical uses like plates, bowls and vases.
Kaolin clay, also called white clay, is used to make porcelain. It goes by many other names as well, including China clay and white cosmetic clay. It has lower plasticity than earthenware and stoneware clays, making it tricky to work with. If you’re a beginner, you may want to work your way up to working with white clay. Aside from its uses in the arts and crafts world, Kaolin clay is used in soaps, scrubs and facial masks because it’s so mild. These beauty products use Kaolin clay to help reduce swelling and draw impurities from the skin. Next, we’ll look at some techniques used in pottery making.
Dry Clay Harvest Method
How did primitive potters harvest clay? People have been making pottery for millennia, and convenient hobby shops have only been a thing for a few decades at best.
Before that, people harvested their own clay right from the soil.
Clay is present just about everywhere, and even soils described as “sandy loam” can contain as much as 20% clay. To the naked eye, it may look like sandy soil, but with a 20% clay content, every 10 pounds of soil is hiding 2 pounds of clay ready for harvest.
With a little effort, you can harvest your own clay for craft projects or even something as advanced as a backyard bread oven.
Though most soil has some clay content, the yield will obviously be higher if you find high clay soils. Look for areas where the water tends to sit after a heavy rain.
Our soils are very shallow, and there’s usually somewhere between 8 and 18 inches of topsoil before a layer of dense clay. We’ve been working to slow the water runoff from our land, and the clay harvest happens more or less by accident while in the process of digging small slowing ponds.
If you’re curious how much clay your soil contains, try doing a test jar. Fill the jar about halfway with soil, add water and stir to completely break up soil particles. After a few minutes, any sand and silt will settle to the bottom. Anything that’s still left suspended in the water is the clay content.
This jar started at half full, and it’s now 1/4 full with silt, sand, and rock. I’d estimate that my soil sample is roughly 50% clay.
Testing the soil’s clay content in a jar.
There are two traditional methods for harvesting clay: dry and wet.
The dry method involves completely drying out the soil, sifting it repeatedly and pounding the clay globs until it’s completely uniform and flour-like. This sifting is followed by a few rounds of winnowing the clay onto a collection surface.
The dried clay powder is then rehydrated and kneaded into workable potting clay.
This method is a great option in water-limited areas, but it requires a lot of time and energy. If you have baking hot sun and very little rain, it’s practical to completely dry earth.
None the less, the process of sifting, pounding and winnowing will take hours for just a few pounds of clay.
Wet Clay Harvest Method
The wet harvest method involves adding both water and soil to a bucket. The soil is then stirred into the water, and allowed to sit for a brief period to allow the rock, sand, and silt to settle out. The clay stays suspended in the water for longer.
The clay and water mixture is then filtered through a fine cloth or sheet. What’s left is a ball of smooth clay.
The wet harvest method is much more efficient and allows gravity to do most of the work. If you have access to plenty of water this is the way to go.
Up here in Vermont, we’re never short on water, and it’d be hard to find enough hot rain-free days in a row to completely dry the soil out for the dry extraction method anyway.
The wet extraction method is also a lot more fun, and allows kids to join in on clay processing. My 18-month-old was a huge help loading soil into the bucket and stir it up, and I couldn’t have asked for a more enthusiastic helper.
Processing Clay for Pottery
To use the wet extraction method, start by filling a bucket about 1/3 of the way with soil. Add water and use your hands to break up the soil particles as finely as you can get them.
Allow the soil to hydrate for a few minutes, or preferably a few hours. Then use your hands to break up the soil pieces again.
Give the whole bucket a good stir. A shovel works well for this, or a boat oar, or just an arm.
Our soil has so much clay suspended in the water that an arm in the bucket comes out completely covered in the clay slurry.
Clay and water slurry sticking to my arm as I process the clay using the wet extraction method.
Once the soil is fully suspended in the water, give the bucket a few minutes to settle. The rocks fell out of suspension almost immediately, followed by the sand.
The silt will take 2-5 minutes to settle down, leaving only the smallest clay particles suspended in the water.
While you’re waiting, get a sheet ready inside a bucket or colander. Anything with a fine weave will do, I’m using an old bedsheet.
The sheet has a relatively loose weave, so I’ve folded it into quarters.
The water moved through the sheet quite slowly, and I think next time I’ll just double it instead so I don’t have to wait 24 hours for the water to completely drain.
I’ve read in rural India, women who filter water through 7 layers of sari cloth actually filter out cholera. With 4 layers of bed sheet, I was able to filter a clay slurry to completely clear water.
I wouldn’t imagine that I removed bacteria, but it’s good to know that I can take scummy water and make it clear enough to boil for fresh drinking water.
My daughter has been drinking from our pond all summer using a life straw, mostly for novelty value. She gets a kick out of being able to drink right from the pond, but filtering the water to remove dirt first seems like a much more sustainable long-term solution for water.
With 4 layers of bed sheet, it took about 24 hours for the water to completely drain. As I said, 4 layers is definitely overkill.
Next time I’ll cut this sheet in half and make two clay extractors, each with two layers of a bedsheet.
To speed up the process, I gathered the corners of the sheet and hung it from a tree.
In the end, the top inch of clay was still pretty wet and slimy.
After 24 hours, I pulled all the clay out of the sheet, kneaded it together and let it dry in the sun for a few hours. At that point, the texture was just right.
Since it’s not commercial clay, it’ll take a bit of effort to learn how to fire it correctly. The right firing temperature may be hard to achieve, but our next step is to make a primitive kiln in the backyard.
I’m really inspired by the maker of the primitive technology videos on youtube, and he has a beautiful homemade rocket stove kiln. Temperatures in that won’t be exact either way, so it’ll be a learning curve regardless of the type of clay used.
I’ve read that inconsistencies in homemade clay can make it liable to crack during firing and that some potters actually mix in stones to their pottery vessels to help stabilize them. Soon enough I’ll find out when we go to make primitive pottery.
I can pull an unlimited amount of clay out of our soil without much trouble, so even if it takes a while to get the pottery process down it doesn’t much matter. It’s all part of the journey.
We worked our first lumps of clay into very primitive bowls, and thus far I’ve found that it’s much easier to make clay than it is to make attractive pottery.
It’ll take a lot more practice to make something worth firing, but in the meantime, we’ll try firing these practice bowls to refine our technique.
Overall, the process was almost effortless. Though I haven’t tried the dry method, there’s no way it could be this easy.
By description, it’s pretty grueling and it seems like you’d be inhaling a lot of clay dust in the process.
I’ll stick with the wet method and mud splashing with the kids.
How to identify clay?
Don’t know if this will help but my kids and I have been refining clay from a marginal source so I can tell you what we did.
After digging it up we dried for a couple of days on a old tarp and routinely broke up the bigger peices and stirred the pile to help it dry out. We then smashed the dried clay/dirt/sand and sifted it through a screen. (This later turned out to be unnecessary.)
We then mixed the sifted material into a comparatively large amount of water (think 5 gallon bucket of water to roughly 1-2 gallons of sifted material) and let it hydrate for a couple of days, stirring it whenever we walked by.
After the mix sat we stirred it vigorously and quickly poured off the upper 2/3 of the water to a second container and let that settle for a couple of days as well. The reason being the fine particles of the clay remain suspended in the water and the larger particles, such as sand, settle out faster.
Once the clay settles out and you have a clear layer of water you can siphon the water off and pour the remaining soupy mix through a old cloth and let it hang and drain until it’s a good constancy and put it in sealed containers to prevent it from drying out further until it was needed for projects.
Like I mentioned earlier the smashing and drying portion seems unnecessary to me now. If you just dump the raw material into water and stir it until all the clumps are gone the plant material will float to the surface and the rocks and sand will still settle out when mixed and you can pour the watery mix through a fine screen.
Determine Soil Type and pH
How coarse or fine soil feels depends on the size of the mineral particles. Sand, silt and clay — the major mineral particles — are responsible for the size and number of soil pore spaces. Pore space determines the amount of air and oxygen in soil, the drainage rate and soil’s capacity to hold nutrients. Sand grains are the largest particles and create large pores. Sandy soils drain quickly and do not hold water and nutrients well. While sand can be seen by the naked eye, silt particles are microscopic and feel velvety and smooth. Silt creates smaller pores in the soil and results in better water retention. Clay particles are the tiniest of all. When moist, they cling together and feel sticky. Clay soils have a tremendous capacity to hold water and nutrients, and soils rich in clay tend to suffer from poor air circulation and slow drainage.
Soils are rarely pure sand, silt or clay but rather a mixture of all three. They’re often grouped into one of 12 textural classes based on the relative proportions of these particles. Sands and loamy sands, for example, are more than 70 percent sand and share the characteristics of sand. Clays, sandy clays and silty clays are more than 40 percent clay and exhibit the characteristics of clay. Loams, the ideal soils celebrated in so much gardening literature, share the attributes of both — good aeration, drainage, and moisture and nutrient retention. Most vegetables do best in loamy soil. It is possible to grow a beautiful ornamental garden in any soil type, as long as the plants are adapted to the particular soil conditions.
K-12 Soil Science Teacher Resources
The particles that make up soil are categorized into three groups by size – sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Most soils are a combination of the three. The relative percentages of sand, silt, and clay are what give soil its texture. A clay loam texture soil, for example, has nearly equal parts of sand, slit, and clay. These textural seperates result from the weathering process.
This is an image comparing the sizes of sand, silt, and clay together. Sand is the largest. Clay is the smallest.
There are 12 soil textural classes represented on the soil texture triangle. This triangle is used so that terms like “clay” or “loam” always have the same meaning. Each texture corresponds to specific percentages of sand, silt, or clay. Knowing the texture helps us manage the soil.
Soil structure is the arrangement of soil particles into small clumps, called peds or aggregates. Soil particles (sand, silt, clay and even organic matter) bind together to form peds. Depending on the composition and on the conditions in which the peds formed (getting wet and drying out, or freezing and thawing, foot traffic, farming, etc.), the ped has a specific shape. They could be granular (like gardening soil), blocky, columnar, platy, massive (like modeling clay) or single-grained (like beach sand). Structure correlates to the pore space in the soil which influences root growth and air and water movement.
Read more and download our Soil Texture information sheet.
The color of soil is measured by its hue (actual color), value (how light and dark it is), and chroma (intensity).
Soil color is influenced primarily by soil mineralogy – telling us what is in a specific soil. Soils high in iron are deep orange-brown to yellowish-brown. Those soils that are high in organic matter are dark brown or black. Color can also tell us how a soil “behaves” – a soil that drains well is brightly colored and one that is often wet and soggy will have a mottled pattern of grays, reds, and yellows.