Topsoil is the upper layer of soil in a backyard or garden. It may look like dirt but it's not exactly the same. It has the highest concentration of microorganisms and organic matter, including mineral particles. Good topsoil is loose and well-draining and plants need it to thrive.

However, some yards might be missing topsoil entirely and you'll have to bring some in. So what exactly is it and how much do you need? Read on for all the dirt on topsoil.

## How Much Topsoil Do I Use?

If your yard lacks the topsoil you need, you may decide to bring in topsoil. One cost effective way is to buy it in bulk from a landscape design company. The amount you need depends on how you will use the land. To add topsoil to an existing lawn, you'll only need a few inches. For a lawn, you only need about six inches of topsoil. But for a vegetable garden, count on 12 inches; use 18 inches for raised beds.

When you buy a truckload of topsoil, it is often sold by the cubic yard. One cubic yard of topsoil is equal to 27 cubic feet. How far this will go toward meeting your needs depends on the depth of application. For example, if you spread a yard of topsoil six inches thick, it will cover twice as much ground as it will if you spread it in a 12 inch layer. It is easiest to use one of the many online topsoil calculators. But you can also figure it out yourself if you prefer.

## Formula to Figure Cubic Yards of Topsoil

To figure out how many cubic yards of topsoil you need without using the calculator, apply this formula: square feet times soil depth = cubic feet. Dividing the cubic feet by 27 gives you cubic yards. Here are your three easy steps:

1) Figure out the square footage of the area where you want topsoil by multiplying the length time the width. For example, if you intend to use the topsoil in a vegetable garden that measures 9 feet by 30 feet, you need the topsoil to cover a space of 270 square feet.

2) Multiply the square footage by the depth of the topsoil to come up with cubic feet. Generally garden topsoil should be about 12 inches (1 foot) deep. That means you multiply the square foot calculation by 1. Here, 270 square feet times 1 foot is 270 cubic feet.

3) Divide the total cubic feet by 27, since there are 27 cubic feet in a cubic yard. Here, 270 cubic feet divided by 27 gives you 10 cubic yards. You will need 10 cubic yards of topsoil to put a 12-inch layer on a 9 by 30 foot garden bed.

## Why Do I Need Topsoil?

Good topsoil is essential to plant growth. While undisturbed soils are likely to contain topsoil, it is often removed from a yard during construction. It can also be absent in mountainous or rocky areas. Even in areas where you find topsoil, you may not have the amount you need to allow a top quality garden to grow.

The more organic material in the soil, the better nutrients the topsoil provides for your plants. However, a very high percentage of organic matter decreases the strength of the soil structure and allows it to condense and settle. So a balance of organic and inorganic matter works best.

## Buying Topsoil

If you don't need much topsoil, you may not need to buy a truckload. You'll find smaller amounts in your garden store in bags. This soil may not be true topsoil. Some is termed organic topsoil and may contain matter like shredded wood, moss, or peat. The other popular type is blended topsoil. This is usually soil excavated from a construction site blended with organic compost.

Since the organic mix contains little mineral material, blended topsoil is often the better choice. Look at the label to find out what amendments the topsoil contains. If there is an open back, take out a handful and feel it. The best topsoil is loamy and crumbles in your hand. A 40-pound bag of topsoil usually contains a bit less than one cubic foot of topsoil.

From Alaska to California, from France's Basque Country to Mexico's Pacific Coast, Teo Spengler has dug the soil, planted seeds and helped trees, flowers and veggies thrive. A professional writer and consummate gardener, Spengler has written about home and garden for Gardening Know How, San Francisco Chronicle, Gardening Guide and Go Banking Rates. She earned a BA from U.C. Santa Cruz, a law degree from U.C. Berkeley's Boalt Hall, and an MA and MFA from San Francisco State. She currently divides her life between San Francisco and southwestern France.