How to Select Good Top Soil for Lawns and Gardens

Quality topsoil is the basis of quality landscapes. The growth rate, health, and visual appearance of landscape plants are directly related to soil quality, water and nutrient availability, atmospheric temperature and the amount of sunlight received.

You may have just finished construction of your new home. The next step is landscaping. Since the top soil is generally removed during leveling and home construction, you may need to buy topsoil, which is the fertile top layer of soil in your yard that will support the lawn and garden. You may have a garden at your existing home or lawn that has a poor soil. Then you may consider buying a good topsoil to improve the soil or by adding organic matter and other amendments required like nutrients, lime and gypsum to improve the poor soil. A source of confusion for homeowners is knowing the characteristics of the topsoil to choose.

Topsoil is commonly available either bagged or in bulk. Bagged topsoil is usually sold in 40- to 50-lb quantities and has been amended with lime, fertilizer and organic matter and is available in the local garden centers. Bulk topsoil is generally is a native soil taken from the surface and sold in truckloads. You can look under yellow pages or newspaper adds to find bulk topsoil suppliers.

Topsoil is the uppermost part of soil ranging from 3 to 10 inches. Although most surface soils have higher organic matter content than subsoil, not all surface soil is ideal for your garden or lawns. While the plants will grow in a fairly wide range of materials, you should ensure that the topsoil you are buying has suitable physical and chemical properties for your purpose.

Topsoil Quality Guidelines: The American Society of Landscape Architects has specifications for topsoil commonly used in contracts for landscape projects. These specifications commonly require topsoil to have acceptable ranges of organic matter, clay, and pH (a measure of soil acidity). Topsoil is often preferred to be free of weeds, plant disease pathogens, and is required to contain stones below a certain diameter as determined by sieving.

Three main chemical and physical properties influence topsoil quality in Missouri soils:

pH, texture and organic matter. It is important that the topsoil meet specific standards for these properties. If soil falls outside of acceptable ranges for any one of the properties, reject the material or realize the need for spending a considerable amount of money and time for improving the soil conditions for plants to do well.

pH: Soil pH is an indication of the acidity or alkalinity of soil. Soil pH must be measured with an electrode to obtain accurate value. Paper test strips will not accurately measure soil pH. A pH of 7 is neutral, while values below 7.0 are acidic and values above 7 are alkaline or basic. Ideal pH is between 5.5 to 7.5. A wide variety of plants will grow well in this pH range. However, some acid loving plants like azaleas, rhododendrons, blueberries and raspberries prefer pH below 5.5.

Texture: Soil texture refers to proportion of (percent.) sand, silt and clay sized particles in soil. The percentage by weight of sand, silt and clay are used with a textural triangle in assigning soils to a specific textural class. Texture influences the water holding capacity, aeration, drainage, tilth, compaction and nutrient holding capacity of the soil (Cation Exchange Capacity- CEC). Ideal soil texture is loam and silt loam. Soil texture is measured in the lab by Hydrometer method or Pipette method.

Organic Matter: Soil organic matter is essential in the formation of soil structure, reducing compaction, and for retaining plant nutrients. It helps in improving the water holding capacity of the soil, aeration, and tilth. Ideal is to have an organic matter content of 3 % or greater. Soil organic matter can be measured in the by Loss on Ignition, or by Walkley Black method.

Evaluating Topsoil Quality: A soil test is the most reliable way to determine the quality of topsoil. If you are buying lot of topsoil have it tested to check the soil pH, organic matter, nutrient levels and the soil texture. The University of Missouri Soil and Plant Testing laboratory located at 23 Mumford Hall of the UMC campus offers soil testing for determining the soil fertility status (pH, buffer pH, organic matter, P, K, Ca, Mg, and CEC) and textural analysis (particle size analysis ). For more information call the laboratory at 573-882-0623, or e-mail soiltestingservices@missouri.edu or visit the lab’s Web site at http://soilplantlab.missouri.edu/soil.

Manjula V. Nathan, Director
MU Soil Testing and Plant Diagnostic Service Laboratory