The Dirt on Garden Soils
- learn which planting medium to choose for any gardening task -

Posted on March 15, 2019

The Dirt on Garden Soils
Walk into any garden center and what do you see? Plants, of course, but what else? Usually, you are also confronted with stacks upon stacks of different types of garden soil. What you should buy and how much depends, of course, on what gardening tasks you hope to accomplish. Use this quick guide to help you decide which type of planting medium you need:

Top Soil

Hands holding top soil

Definition: The bagged top soil you find in garden centers is most often a manufactured product, usually soil-less, containing organic fillers and amendments like fertilizers. Bulk topsoil might also be manufactured or it might be scraped from the first few inches of a construction site or other ground and contain sand, silt, chopped roots, and bits of other debris. The quality of the product varies greatly depending on from where it came.

Uses: To build up or replenish garden beds, fill larger outdoor containers, or spread over large areas before planting lawns or other large landscape features.

Advice: Try to find a local, environmentally friendly source for purchases of bulk products. Test your soil regularly to make sure topsoil from unknown sources is not impacting the health of your plantings.

Potting Mix

Containers filled with light-weight potting mix

Definition: A soilless mix of substances such as vermiculite, perlite, pine bark, or coir, designed to provide a growing medium that doesn't compact easily or dry out too quickly. Some potting mixes also include amendments such as chemical or organic fertilizers.

Uses: Growing medium for smaller container plants, indoor plants.

Advice: Make sure your potting mix does not include soil, which is not sterile and can cause drainage problems, both serious issues for container plants. A heavy bag is a sure giveaway that there's some soil in your potting mix.

Compost

Hands holding rich, black composted soil

Definition: Compost, sometimes referred to as "black gold" by savvy gardeners, is organic matter and garden waste that's in the process of decomposing and hence is rich with beneficial organisms and earthworms. Compost is available in bags, but there's a great variation in quality, and often the product is no better than top soil and contains sewage sludge or other unnatural amendments.

Uses: Good compost can serve in your garden as plant food, mulch, an amendment to increase your soil's water retention qualities, and a disease shield all at once.

Advice: Read the labels on any bagged compost carefully, and if they are incomplete or unclear, stay away from that product. Also beware of compost that looks to have been sitting in bags for a long time. The anaerobic environment will kill off any beneficial organisms. A good way to ensure high quality compost is to make your own or buy in bulk from a reputable producer.

Garden Soil

Raised garden bed planter filled with garden soil

Definition: Describing garden soil, like describing top soil, can be both straightforward and highly complicated. On the one hand, garden soil is exactly what it sounds like: the mix of topsoil and compost and other amendments that fills your garden beds. But garden soil sold in bulk or bags, again like top soil, can vary greatly in quality and ingredients.

Uses: High quality garden soil is the bulk of what should fill garden beds and large containers.

Advice: Read the labels on any purchased garden soil carefully. As you work your garden beds through the years, adding amendments and fresh compost, it's a good idea to test it to make sure it has the right balance of acidity and nutrients. Tests are available at most garden centers.

Seed Starter

Peat pots filled with seed starter soil and tiny plant sprouts

Definition: Seed starter mix is generally a light, fast-draining combination of perlite and/or vermiculite, milled peat moss, coconut coir fiber, and trace amounts of lime (to balance acidity). Many such mixes also include plant nutrients, preferably high quality compost or worm castings. Though some seed starters do contain soil, that is not ideal as it can introduce diseases and weeds, both unhealthy for baby plants.

Uses: To germinate seeds, usually in small pots or trays, for later transfer to the garden.

Advice: Beware of chemical fertilizers incorporated into seed mixes. Often these are too harsh for young plants, which are not able to access the nutrients at such an early stage of their growth cycle.

Vermiculite and Perlite

Perlite and vermiculite close up

Definition: Both are mined minerals used in potting and seed starting mixes or as an amendment to garden beds.

Uses: To improve drainage and water retention of a growing medium. One or the other should be a major component of any soil-less mixes and can be worked into beds to improve the garden soil's quality.

Advice: The two minerals are very similar, but some say perlite is better in wet areas, because it enhances drainage more, while vermiculite is better in dry climates because it increases the soil's water retention capabilities.

Sand

Sand being mixed with soil

Definition: Sand is another soil amendment that both improves a mix or garden soil's drainage and increases water retention.

Uses: Work sand into the top two inches of container plants or dig into beds. Root vegetables like beets and carrots especially appreciate the looser texture sand brings to their growing area.

Advice: Look for bags of coarse sand, also known as "builder's sand" or "yellow sand" for best results. Avoid beach sand or clay sand (the type often sold for children's sand boxes).

Peat Moss

Sphagnum peat moss close up

Definition: Peat moss is a type of decomposing moss found in bogs. It's sold compressed in blocks and bagged in most garden centers.

Uses: Like compost, peat moss is an organic growing medium that is also light and suitable for posting and seed starting mixes. It has high acidity, so it's an especially good soil amendment around acid loving plants like azaleas and blueberries. However, with growing global concerns about the destruction of peat bogs, you might want to consider mulched leaves or choir as an alternative to peat.

Advice: When using peat moss, test your soil regularly and if it gets too acid (in general, plants like a pH of around 6.5), add a small amount of alkaline wood ash or lime. You can also use a few pinches of peat moss on the top of seed starting cells to combat damping off and other diseases.

Specialty Soil Mixes

Potting a succulent with cactus soil mix

Definition: Most garden centers carry bags of specialty soil mixes (orchid mix and cactus mix are two common examples). These products, often marketed for house plants, are designed to meet the unique nutrient and drainage needs of specific plants.

Uses: Most often used for indoor potted plants.

Advice: As with any growing medium purchase, read the label carefully to understand what's in the mix you're buying.

Navigating the many choices in soils and soil amendments available at the garden center doesn't have to feel impossible. Use this guide as a starting point, read the bag labels, and know what you're plants need to thrive and you should have no problem finding the perfect growing medium for your needs.




Is it time to refresh the potting mix in your houseplants? Get all the information you need in our article, How to Repot a Houseplant.

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