How Much Topsoil Do You Need to Grow Plants?

Plant Root Zones

• Most garden plants and vegetables need a 12- to 18-inch layer of organically rich topsoil to develop strong root systems. Quality topsoil is often referred to as loam. Loam contains the optimal mix of soil particles and organic matter to support vigorous plant growth. Unfortunately, builders often scrape off and remove topsoil from subsoil layers during construction of residential areas. Investing some time and effort to improve topsoil conditions in these situations will reward you with healthier, more productive plants.

Topsoil

• You can find many topsoil products at local garden centers. Landscape companies also offer bulk topsoil for larger scale horticultural and agricultural development. However, topsoil production is poorly regulated and there are no uniform sets of standards to grade it. Only purchase topsoil that has been tested for chemistry, pH, organic matter content and the presence of contaminants. Ask any topsoil vendor for the results of these tests and if they are not available, ask that they be conducted before purchasing any.

Amending Your Soil

• You may also use many other organic amendments to improve the quality of your native landscape, creating your own topsoil. These include finished compost and humus, finished manure, commercial soil conditioners, grass clippings, leaf mold and chopped wood products. Don't simply spread topsoil or other amendments over your existing soil before planting. Work or till in amendments at a 25 to 50 percent ratio with your native soil. For example, if the goal is to create a 12- to 18-inch layer of productive soil, add 3 to 6 inches of amendments over your existing soil, then mix them thoroughly to the desired depth.

Maintaining Good Topsoil

• Organic soil components break down over time. Weathering processes and soil microbes convert organic material into ever smaller particles that eventually reach a stable stage called humus. You can maintain a healthy topsoil layer by regularly adding compost and mulches to your garden or landscape. Annually in spring or biannually in spring and fall, apply a 1- to 2-inch layer of finished compost, then cover this with a 3- to 4-inch layer of organic mulch material such as pine needles, ground hardwood or bark or leaf mold. This technique helps continually enrich your topsoil, conserves soil moisture and mitigates the effects of deep winter freezes.