What Every Gardener Should Know About Topsoil

What Exactly Is Topsoil?

Topsoil is the top portion of the soil consisting of minerals, organic matter and microorganisms. It can range from a few inches deep in some areas to a few feet deep in the Corn Belt. Topsoil has accumulated over millennia, but erosion is a serious problem. Erosion can deplete topsoil quickly, which is why it is important to cover bare soil with mulch or a ground cover plant.

What's in Topsoil?

Topsoil is made up of sand, clay and silt. The proportions vary. An ideal topsoil, called loam, is soft and crumbly and has roughly equal parts sand, clay and silt (a fine, dust-like sediment of rock and mineral particles). However, many gardeners struggle with less-than-ideal topsoil that tilts towards heavy clay (slow draining and less oxygen available to plant roots) or sand (fast draining but less able to hold moisture and nutrients). Surprisingly, compost can improve clay and sandy soils.

Topsoil vs. Dirt

Topsoil is for growing things. Dirt is for filling, which is why it's often called fill dirt. Some people erroneously use "dirt" interchangeably with "soil."

Topsoil vs. Garden Soil

Available in bags at your favorite nursery or big box store, garden soil is a topsoil that's been screened and amended with compost. It's too heavy for use in pots, but it's well-suited for garden beds and lawns ready to be reseeded.

Topsoil vs. Potting Soil

Topsoil is used for planting in the garden, potting soil (also called potting mix) is used in containers. It is a lightweight, soil-less mixture of peat moss and perlite. Some mixes also contain wetting agents to help the peat moss absorb moisture, as well as slow-release fertilizer and moisture-holding crystals.

Potting Soil vs. Raised-Bed Soil

With the popularity of raised-bed gardening, suppliers have introduced raised-bed soil. It is lighter than topsoil but heavier than potting soil/potting mix and may contain compost and wood fines (small pieces of wood that are already composted).

How Can I Improve Topsoil?

Amend topsoil regularly by digging compost into the top six or eight inches. Compost supplies nutrients and beneficial microorganisms to create a "living soil" that is better for plant growth and health. Sound like too much work? Then start mulching with shredded leaves. They will be food for earthworms, who will aerate the soil with their tunneling and add to its fertility with their castings (waste).