How Much Topsoil Should Be Put Down?

A healthy garden or lawn begins with a good base of topsoil. How much topsoil you put down depends on the quality of your existing soil, the nature of the plants -- grass or garden vegetables and flowers -- and the size of your yard or garden plot. It takes Mother Nature 500 years to produce a single inch of topsoil, according to the University of New Hampshire Cooperative Extension. Fortunately, today's gardeners can replenish their plots with a trip to the local nursery.

The optimal level of topsoil in a traditional garden plot is 6 to 8 inches. When preparing garden soil for planting, the gardener should cultivate the topsoil to this depth --- or add topsoil to bring the soil up to this level --- and then incorporate a 2- to 4-inch layer of organic matter into the soil. Most gardens will benefit from the addition of another 2 inches of organic matter each year.

Raised-bed gardening offers many benefits to the backyard gardener. Raised beds provide gardeners working with poor soil in water-logged areas a healthy environment for plant and root growth. They also extend the growing season by speeding soil warming. This is especially beneficial in cool climates. According to the University of Missouri Extension, most plants need a minimum of 6 to 12 inches for optimal root growth, although deeper is better. Raised beds should be filled with good quality topsoil; however, most topsoil benefits from the addition of organic matter such as compost, peat moss or decomposed manures.

The ideal topsoil depth for growing a new lawn from seed is 4 to 8 inches. How much topsoil you add to your yard when planting grass seed depends on the existing topsoil, if any, on your property. Sod lawns often don't require the addition of any topsoil. If your existing soil is in relatively good shape, you can improve it by working 1 to 2 inches of organic matter into the top 4 to 6 inches of soil. If your soil is very poor and you decide to bring in topsoil, plan on adding at least 4 to 6 inches as the grass roots will grow to a depth of about 5 to 6 inches.

Once you determine how many inches of topsoil you want, multiply this number by the length and width of the plot to determine the volume of topsoil you need. For example, if you need to fill a flower box that is 10 feet long, 3 feet wide and 1 foot deep, your volume is 30 cubic feet. Most nurseries and home improvement stores sell topsoil by the cubic yard. One cubic yard equals 27 cubic feet. To calculate the number of cubic yards in our example, we would divide 30 by 27 to get 1.11 cubic yards. If you can't remember the math, most vendors will help you figure it out if you provide them with the basic measurements of your plot.