Owners of newly constructed homes in North Carolina commonly confront two issues when landscaping: soil infertility due to poor nutrient availability, and physical restrictions created by compaction and improper drainage.

During most new home construction, the topsoil and subsoil are removed from the construction area. If the floor plan includes a basement, additional subsoil material is removed from the site. Removed materials are often placed adjacent to the house and used around the foundation during the final stages of construction. It is necessary to compact the soil to increase soil strength for construction, but compacted soil can inhibit plant growth. Removing topsoil and using compacted subsoil materials to construct lawns and planting beds can present problems for homeowners.

Much of the subsoil in the piedmont and mountain regions has high clay content—commonly called “heavy” soil. If the native soil has been removed and replaced around your home site during construction, this subsoil may now be at the surface. Soils with high clay content tend to have slow water movement and may remain wet for long periods of time. Construction activities also compact the soil, which further reduces the rate of water movement and restricts plant root growth (see Figure 1).

Plants require water storage in soil for use between rainfall and irrigation. It is important that excess water in the soil pore space (the voids around and between soil particles) drains to allow adequate aeration for plant roots. Without air movement and exchange, plant roots cannot survive, and the plants will, in effect, drown.

Soils in the coastal plain region tend to have high sand content and are commonly referred to as “light” soils. Sandy soil allows for better water movement under natural conditions, but compaction from construction and traffic can still reduce infiltration rates drastically.

How can you improve soil physical properties?

Increasing the amount and size of pore spaces is an important first step toward improving your soil. A healthy soil typically has more than 40% pore space, ranging from large pores, which allow drainage, to small pores, which help store water. This combination provides adequate water infiltration and aeration, and it also promotes root growth. Compaction reduces the total amount of soil pore space and the proportion of large pores. Soils composed of small soil particles, such as heavy clay soils, naturally tend to have more small pores, even without compaction, making them difficult to work with, difficult to wet, and hard to aerate. Although the sandy soils of the coastal plain may not be as restricted from small pore spaces, their overall amount of pore space can still be low when they are compacted.

Several methods can increase pore space in your soil. In clay soil, adding sand will increase the size range of the soil particles and pore spaces. However, it takes a large quantity of sand to produce a measurable effect, and this method is generally cost prohibitive. Unless you can add sand at four to five times the volume of the clay, smaller amounts may be worse than no sand at all, producing a concrete-like material. Likewise, adding topsoil from offsite might provide some benefits, but this option tends to be expensive for large areas. Topsoil application is best suited for smaller areas such as raised plant beds. It is recommended to apply 6 to 8 inches of topsoil to raised beds because many plants require 6 to 8 inches of soil for healthy root growth. Homeowners should understand that there are no legal standards for topsoil. What is sold as topsoil may be no better than what you already have—and may even introduce new problems.

A more cost-effective solution for larger areas is to add organic materials, such as composted leaves, grass, or wood bark. These materials will, over time, improve the structure and porosity of the soil and may enhance both drainage and nutrient retention. It is important to use materials that have first been composted, as fresh organic materials will settle as they decompose. In the short term, adding organic materials may also increase the need for nitrogen, which is consumed by microbial activity during decomposition. However, in the long term, organic material added over several growing seasons will lead to richer soil. Adding compost to sandy soils typically requires more frequent and sustained repetition, because the organic materials tend to decompose more quickly. Information about preparing compost can be found in NC State Extension Publication AG-791, Backyard Composting of Yard, Garden, and Food Discards.

Peat is sometimes added to soil for the same purposes as compost. However, peat is not recommended for soils with high clay content. It tends to behave somewhat like clay in that peat is slow to absorb water, but it also tends to retain water and remain wet. Composted materials provide a better alternative for improving physical properties.

Core aeration (a machine used to remove plugs of soil from the lawn) is commonly used to aerate soil in managed turfgrass settings. However, cores aeration in a compacted soil does not provide a direct benefit for increasing water movement through the soil.

Tilling compacted soils is highly recommended since tilling helps loosen the soil and reduces the immediate effects of compaction, even without amendments. If topsoil or other soil amendments are going to be used, then they should be tilled into the existing soil, rather than placed on top of existing soil as this facilitates further water movement and encourages deeper root growth. Deeper tillage may increase the depth plant roots can grow down into the soil; however, deeper tilling is not necessarily associated with further increases in water drainage (6- and 12-inch till depths have similar water infiltration; see Figure 1).

Amendments for larger areas, such as lawns, should be added to no more than half the depth of the tiller tines (usually no more than 3 to 4 inches) in a single application. Without the addition of organic amendments, the benefits of tilling may not last due to repeated traffic (mowing, walking, or other activities) and the natural settling of the soil. Organic amendments additionally provide nutrients for plant growth.

Figure 1. Infiltration rate for compacted, tilled, and compost-amended soils.

From Mohammadshirazi et al. 2016; Alshraah 2020

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Figure 1. Infiltration rate for compacted, tilled, and compost-amended soils.

From Mohammadshirazi et al. 2016; Alshraah 2020

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If the soil around your yard, landscape, or gardens has never been amended for plant growth, they may lack essential nutrients, especially nitrogen, phosphorus, and potassium. These nutrients must be available in proper amounts to meet plants’ requirements. Native soil may also have an improper soil pH—a measure of acidity or alkalinity—for good plant growth and nutrient availability. Generally, native soil and fill materials are low in available nutrients and soil pH. All plants require a balanced nutrient supply, but soil pH requirements can be quite different between plant species.

How can you improve soil fertility?

After soil amendments are applied, the homeowner needs to adjust the nutrient status of this newly improved soil. Proper soil testing will reveal nutrient amounts and types. The North Carolina Department of Agriculture and Consumer Services provides soil analysis at no charge. Proper soil sampling is essential for an accurate soil fertility assessment. For complete instructions on how to take a sample, see Extension publications AG-439-30, Careful Soil Sampling—The Key to Reliable Soil Test Information, and AG-614, A Gardener's Guide to Soil Testing, or contact the Cooperative Extension center in your county. Take separate soil samples in each location you have renovated as well as other planting areas around your home. Mail samples to the North Carolina Department of Agriculture and Consumer Services, Agronomic Division (4300 Reedy Creek Road, Raleigh, NC 27607-6465) or drop them off at the same location. Your report will be available online after the analysis is completed.

Your soil report will recommend the types and amounts of fertilizer you need. The report will also indicate whether agricultural lime should be added to adjust the soil pH to an adequate value. Application of lime to achieve proper pH can affect whether soil fertilizer elements are available for plant use. Lime and phosphorus tend to stay in the surface of the soil if not tilled in, thus light tilling will help incorporate these two soil amendments and make them more effective.

Your local N.C. Cooperative Extension center is a valuable source of information on lawn and garden care.

A Gardener’s Guide to Soil Testing (AG-614)

Bed Preparation and Fertilization Recommendations for Bedding Plants in the Landscape

Backyard Composting of Yard, Garden, and Food Discards (AG-791)

How to Make Wildlife-Friendly Landscapes

North Carolina Department of Agriculture and Consumer Services, Agronomic Division

Alshraah, S.H. 2020. Soil Tillage for Stormwater Infiltration: Effects of Amendments and Vegetation Type over Time. [Doctoral dissertation, North Carolina State University.] NC State Theses and Dissertations.

Mohammadshirazi, F., V.K. Brown, J.L. Heitman, and R.A. McLaughlin. 2016. Effects of Tillage and Compost Amendment on Infiltration in Compacted Soils. Journal of Soil and Water Resources 71(6), 443-449.

This publication is an update of a prior publication by Greg Hoyt, Josh Heitman, Deanna Osmond, and Al Cooke.