What types of soil are in your garden? If you’ve ever had your soil tested, read the ingredients on a potting soil bag, or tried to order bulk garden soil, chances are you’ve come across some of these terms for the different types of soil.
Whether you’re checking out the soil in a potential garden or shopping for some potting mix, make sure you know what you’re getting!
Types of soil
What is soil anyway? Here are some of the most common types of soil including definitions and descriptions of how they’re used in the garden.
Plain garden soil
“Soil” is the term for what’s already there in the ground before you start gardening. Soil is a mix of mineral particles (sand, silt, clay), organic matter (remains of plants, animals, and waste), gases (like air), liquids (like water), and a variety of organisms. It is also the medium in which plants grow. The matrix of materials in the soil provides the structure for the roots while also storing water and providing a habitat for beneficial organisms like worms and fungi.
Ideally, garden soil contains a nice mix of sand, silt, clay, organic matter, air, water, and beneficial organisms. Soil that has too much clay or not enough air may be difficult to garden in. That being said, all plants thrive in different environments, so it may just be a matter of choosing plants and methods that are appropriate to soil conditions.
Loam: Soil with minerals in ideal proportions
Loam is soil that contains a balanced mix of sand, silt, and clay particles. No type of mineral soil particle is too out of balance in loam soil. The soil has enough small particles to retain a good amount of water and nutrients while also having enough sand to allow for the drainage of excess water.
An average loamy soil may contain 40% sand, 40% silt, and 20% clay by weight. A loam that strays too far from this general range might be called sandy loam, clay loam, et cetera. Gardeners love loam because of the balance between the particle size. If you’ve got nice loamy soil on your ground, consider yourself lucky!
Sand: The big bits of garden soil
Sand particles are generally the largest particles in the soil unless there is gravel or cobbles or other visible rocks. Sand is smaller than gravel, but the eye can still see the different particles. Soil which is mostly sand can feel gritty to the touch.
Sandy soil warms up quickly in the springtime. It drains well too because of the air space between the particles. Because it drains so well, sandy-type soil may not retain nutrients as they wash through with the water. Plant roots may not have the opportunity to absorb water and nutrients before they drain away. Sandy soil can be improved by adding organic matter such as compost to help the soil retain a bit of water and some nutrients.
Silt: The happy medium-sized soil particles
Silt particles are the middle-sized portion of garden soil. Silt is smaller than sand but larger than clay. Silt feels like flour when it is dry but can feel a bit slippery when wet.
Some silt in the garden is a good thing as it can balance out the free-draining nature of sand with the heaviness of clay. A silty soil generally holds more water than sandy soil but is easier to work in than clay soil. That’s the best of both worlds!
Clay: The small, slippery bits of garden soil
Clay soil particles are very small. The tiny, plate-like bits of clay nest together tightly to create a dense matrix of soil. Clay-type soil drains poorly due to the lack of air voids. Clay is sticky and malleable when wet but becomes brittle and hard when dry.
Some clay in your soil is helpful to plants as it holds water and nutrients. Too much clay, however, can be difficult to work with. It’s easiest to avoid gardening in really heavy clay soil if possible. If you must garden or put in a lawn on heavy clay soil, try adding a top dressing of compost or good quality topsoil to improve aeration and drainage for your plants. Clay can be great for growing as long as there is also air in the soil to support healthy roots and enable water drainage.
Humus: The part of garden soil that used to be alive
Humus is the dark, rich organic matter (a.k.a. decayed plant and animal material) that is no longer recognizable as the plants or animals that it once was. Natually-created humus is essentially in-place compost that has developed in topsoil as part of the nutrient cycle.
Humus helps soil retain moisture and nutrients, making it a very valuable portion of the soil to gardeners! If you’ve got lots of humus in your garden, take it as a good sign. It is helping your plants have a very happy home.
In commercial potting soils, humus can be a bit of a general term for any organic matter. If humus is listed as an ingredient, it’s worth asking what the source material is. It may have been mined from topsoil but is more likely just some sort of compost product.
Topsoil: The top layer of the soil
Topsoil is the upper layer of the ground made up of a mix of mineral soil particles (sand, silt, clay) and organic matter such as humus and partially decayed plant matter. Topsoil develops naturally over time. The top few inches of topsoil generally contain lots of organic matter in comparison to the soil below it. Topsoil thickness can vary from a thin skiff of less than an inch to a rich, deep topsoil of 6″-12″ (if you’re very lucky!).
Gardeners love topsoil because it often has a nice mix of sand, silt, clay, and organic matter. It also contains enough air to keep plant roots happy. The mix of particles in topsoil can allow it to retain enough moisture to support plants while also draining excess water to ensure there is still some air in the soil. Naturally occurring topsoil is a rare soil type, making it gardening gold!
As with humus, topsoil is a bit of a general term at garden centers. Clarify what you’re getting before buying topsoil. It may be the top foot or so of soil from a development site, but it may also just be a mix of quarried minerals and some sort of compost. If it actually is topsoil that has been collected from some sort of development site, check to see where it’s from and if it’s been screened of large chunks like gravel.
Compost: Yard waste becomes garden gold!
Compost is a dark, rich, soil-like product created as we attempt to replicate and speed up the process of plant decay. Good compost is made up of nutrient-rich decayed plant matter similar to the humus which develops naturally on the ground.
Compost really is garden gold. It’s the perfect way to take all those leaves and trimmings from around the yard and recycle them into nutrient-rich plant food for your garden. A top dressing of compost can keep your garden happy and even revive a tired lawn. If you’re not already making your own compost, check out how to compost your fall leaves into nutrient-rich plant food.
Composted mixed yard waste: Municipal magic soil improver
Municipalities or arborists generally make mixed yard waste compost and consists of composted leaves, sticks, grass clippings, and other yard waste. Large branches and logs are shredded and combined with smaller bits. The whole mix is then hot composted in a large facility. It can be purchased in bulk and is often a good option if you can’t make as much compost as you need.
Mixed-yard waste compost can be great because there are generally many different types of plants in the mix. Unfortunately, it can be hard to know whether chemicals were used in those plants and whether those chemicals are still present in the compost following the composting process. Still, composted mixed yard waste can be a great option for adding some nutrient-rich organic matter to your garden, especially if you’re not making as much homemade compost as you’d like.
Composted forest products in potting soil
“Composted Forest Products” is a broad term used to describe a common ingredient in potting soil which is just that…composted bits of trees and other brush. The finished product is similar to mixed yard waste compost, although it may contain fewer different types of plants.
Composted forest products are often made from waste byproduct material from sawmills and the pulp and paper industry. Different areas have different forest management practices leading to varying levels of chemical residue in the finished product. If you’re buying potting soil that contains composted forest products, check to ensure that it is OMRI-listed for organic use.
Soil-free mix: High-quality potting soil for controlled container gardening
Soil-Free Mix is a bagged potting mix product used in container gardening. The soil-free mix is free of soil – meaning the mix is free of the sand, silt, and clay which make up the mineral component of garden soil. That said, most gardeners prefer to the soil-free mix as a type of potting soil.
Soil-free mix is perfect for container gardening because it is sterile, drains well, and holds nutrients. It’s free of any potential diseases and doesn’t contain any weed seeds. It’s great for small, controlled gardens such as indoor herb gardens, patio vegetable gardens, and seed-starting applications. As with all pre-purchased soils, check to make sure the potting soil you select is OMRI-listed for organic use.
Mulch: Magnificent ground cover to place on top of bare soil
Mulch refers to any type of material used to cover existing soil for the purposes of keeping moisture in the soil and/or reducing weeds. Mulches made from organic materials such as arborist wood chips, homemade compost, or bark mulch can contribute to the organic portion of soil in the garden.
I like to mulch my outdoor garden soil with an inch or two of rich homemade compost. Fresh compost in the spring and fall seems to keep my plants happy while keeping the weeds down. Check out how to use organic mulch in your garden.
Composted manure as a soil additive
Composted manure from herbivores is a common soil amendment in organic gardening. It includes manure as well as some of the animal’s bedding materials (straw, et cetera). Look for a farm that feeds their animals a wide variety of organic food. You also must check to ensure the manure has been hot-composted to destroy any seeds that might be in the manure thoroughly.
Aged composted manure is best, as some types of composted manure can “burn” your plants due to their chemical makeup while they are fresh. If you do plan to use manure, do some research on the different types of animals and the features of their manure. All composted manure is not created equal.
Biosolids = sewage
“Biosolids” is a nice word for sewage. Although well-composted sewage could possibly be a good fertilizer, our sewage systems can contain all sorts of nasty chemicals and pharmaceuticals that get put down the drain. Human bio-solids are not approved for use in organic agriculture and many gardeners feel icky about using biosolids in their gardens. Not recommended!
Worm castings: A reliable type of garden soil improver
Worm castings, or Vermicompost, are basically composted worm poop. Worms, wonderful garden allies, feed on your kitchen scraps and newspaper in a special bin to create a nutrient-rich product for your garden. Worm castings are garden gold and are a wonderful addition to garden soil. Indoor vermicompost systems are available for those living in small spaces or without yards large enough for traditional compost heaps.
Worm castings are a great option for feeding your seedlings before they go outside. Save the homemade compost for outdoor applications, like a nice top dressing on the garden or around your baby trees. You can make your own worm castings with a home worm bin or purchase them locally or online (they don’t smell!).
Peat moss: Ancient bulk organic material
Peat moss is a mined organic material composed of partially decayed plant matter. Although this type of “soil” is comprised only of organic material, it is not truly a renewable material as it takes thousands of years for wetlands to develop into peat reservoirs.
If you do choose to use peat moss in your DIY potting soil, do your research to ensure it is coming from a reputable provider in your area. Look for a brand that is practicing ecological conservation to offset the effects of mining out a valuable carbon sink. If you live closer to the tropics than to peat bogs, consider using coconut coir rather than peat moss as a more sustainable local alternative.
Peat moss is an acidic material. Most plants prefer soil with a neutral pH, although there are some that prefer acidic soil (such as blueberries). You may need to offset the acidic pH with limestone if peat moss is used for alkaline-loving plants. Most pre-mixed potting soils and growing mediums that contain peat will also contain some limestone as a pH adjuster. You may even be able to find bulk peat moss which has been pH-adjusted using limestone.
Perlite: Volcano rock popcorn for garden soil
Perlite is a mined mineral that is added to soil mixes to help the soil hold air. Plant roots require access to air in the soil, making perlite a valuable soil amendment. The mineral is “popped” using heat like popcorn to create a light, air-filled medium. The little white bits in different types of soil that can be purchased premixed are often perlite (they can look a little like styrofoam).
Perlite is safe for organic gardening as it is chemically inert. If the soil in container gardens or raised beds is at all heavy, perlite can be a very valuable amendment. It can be purchased at a local garden center or online.
Coconut coir: A renewable potting soil alternative to peat moss
Coconut coir is a growing medium made of shredded coconut skins used to add bulk to the potting soil mix. It has the benefit of not being a mined product and instead provides a use for something that may have been a waste product.
If you’re looking for a more sustainable alternative to peat moss or would like a more pH-neutral product, look into buying coconut coir instead. Coco coir is now used in many different packaged types of garden soil – sometimes in addition to peat moss and sometimes instead of it entirely.
The downside of coconut coir is that it needs to be transported to northern areas where coconuts don’t grow. If you live far away from the production of coconut coir, you’ll have to weigh the sustainability of transporting the coir to your area versus other alternatives. Check out what’s available in your area and decide what’s best for you. Some people choose to mix peat and coir to get the benefits of both and balance out the drawbacks.