Steve Boehme: Topsoil or potting soil

Steve Boehme: Topsoil or potting soil

Be Steve Boehme


Photo courtesy of Steve Boehme. Using the right kind of soil is a key to successful gardening, particularly in raised beds.

What is the difference between topsoil and potting soil, and which one should you use? That depends on what you’re using it for. First, let’s understand what we mean by the words topsoil and potting soil.

Topsoil is dirt, and potting soil isn’t. True potting soil is actually “soil-less.” Topsoil is for planting in the ground. Potting soil is for planting in containers. Topsoil is sand or clay (ground-up rocks), mixed with organic materials like compost. Potting soil is a mixture of peat moss and other organic materials like pine fines or composted sawdust. Some potting soils also contain specialized ingredients like vermiculite or perlite, starter fertilizer and wetting agents.

Topsoil is heavy. Potting soil is mostly air so it’s light. Topsoil holds lots of water, so it will stay moist for a long time, sometimes too moist. Potting soil lets water drain easily, so it allows roots to breathe and dries out quickly. Topsoil is dense and packs down easily. Potting soil is fluffy and hard to pack down.

The word topsoil can mean many different things, because no two topsoils are exactly the same. Topsoil means the very top layer of the earth’s crust, rich in nutrients because plants have lived and died in it, sometimes for thousands of years. The topsoil you find in woods contains lots of rotten vegetation. Topsoil in farm fields has been turned over, mixed, and very often exhausted by repeated crops. Topsoils often contain clay or composted manure. They also contain weed seeds, soil bacteria and funguses.

Potting soils are precisely mixed using strict formulas and recipes. Most potting soils are based on peat moss, with other ingredients added to make them ideal for certain uses. For example, seed starter mixes are very fine and fluffy so that fragile, fine roots can spread easily. Perennial mixes have larger pieces and more bark. Woody plants need a coarse mix with lots of pine bark. Some potting soils include vermiculite or perlite; flakes of fluffy featherweight rock that have been puffed up so they hold lots of air. We call it “rock popcorn.” Good potting soils are sterile, meaning they have absolutely no weed seeds or diseases in them.

Topsoil is ideal for filling in low spots in lawns or along walks and patios. Adding a few inches of topsoil gives lawn grass a better chance than subsoil or clay. When planting trees and shrubs, replacing the existing soil with topsoil can help plants grow better. We sell “pulverized” topsoil, which is perfect for fine-grading because it doesn’t have lumps or clay in it. As long as it’s dry, pulverized topsoil is a breeze to spread and rake.

For raised beds, topsoil is much cheaper since it’s sold in bulk, but it should be mixed with compost, peat moss or vermiculite to make it fluffy and improve drainage. Otherwise, it will pack down, swell and break your raised beds.

Potting soils are for planters, hanging baskets, window boxes, and other containers where drainage is important and weight would be a problem. Potting soils allow excess water to quickly drain out the bottom of the container by gravity, pulling in air to replace the water. Since plants breathe through their roots, they’ll thrive in potting soil as long as they are watered regularly. Some potting soils have “moisture crystals,” bits of polymer that help keep them from drying out so quickly.

Using the right kind of soil for the project you’re doing is one key to successful gardening.

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