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 composted sawdust.
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. Potting soil lets water drain easily, so it 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. Some potting soils include vermiculite or perlite; flakes of fluffy featherweight rock that’s been puffed up so it holds lots of air. 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. “Pulverized” topsoil 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. It’s not good for gardens or containers, because it is so gooey and dense when it gets wet.
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. Depending on what you’re growing, we can blend different ingredients in the right proportions.
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. A great way to find out the ideal soil for any plant is to look closely at the container mix that plant is grown in by the nursery. Nurseries strive to find the ideal soil mix for each crop, so the closer your planting soil is to the professional grower mix, the more likely the plant will be happy in it.
Steve Boehme is a landscape designer/installer specializing in landscape “makeovers”. “Let’s Grow” is published weekly; column archives are online at www.goodseedfarm.com. For more information call GoodSeed Farm Landscapes at (937) 587-7021.