With the expansion of residential landscapes in the US, the number of homeowners interested in learning how to start and maintain a healthy lawn has increased. Bare soil is usually easier to work with when improving the soil for lawn preparation. Improving the soil in an established lawn can be a little more difficult, but it is possible.
South Carolina soils range from heavy clays in the Upstate to sandy soils from the Midlands to the Lowcountry. Additionally, many South Carolina soils have been poorly managed and damaged through construction activities or agricultural overuse. Regardless of the soil type, most of the soils in SC are highly weathered, eroded, and contain little organic matter. Common soil quality issues include:
- Very low soil pH, except along the coast
- Soil compaction and thus poor soil aeration
- Poor soil structure
- Reduced water holding capacity in sandy soils
- Excessive water saturation (holding capacity) in clay soils
- Low organic matter content
- Low cation exchange capacity (CEC) or ability to hold onto nutrients
- Low nutrient availability
Correcting these soil quality issues when establishing a new lawn is easy when there is no established turfgrass to protect. Homeowners can use a tiller to relieve soil compaction, add soil amendments on top of the tilled soil, and then incorporate the soil amendments to improve soil structure and increase soil organic matter, if necessary. But what is the homeowner with an established lawn to do to improve the soil? Topdressing is a solution.
Each of the soil quality issues listed above can be improved significantly by the routine use of annual topdressings with organic matter, such as compost. Topdressing is the lawn maintenance practice of placing a thin layer of material on top of an established lawn. Changes in soil composition will occur as the material works its way beneath the existing turfgrass and into the soil profile. The topdressing material slowly improves soil structure over time.
Topdressing materials may include topsoil, composted organic matter, or sand, depending on why the topdressing is needed. Historically, sports turf managers have used topdressing to level and smooth playing surfaces. However, landscapers and homeowners now employ topdressing as a management tool to promote healthy turfgrass.
After seeing sand-based topdressing materials used on golf courses and sports fields, homeowner demand has led to the popularity of using sand as a topdressing on home lawns. However, most high-end golf course greens and athletic fields are not grown on native soils as are home lawns. Instead, these turf areas are grown on modified ‘prescription’ systems with an all-sand or sand-on-top-of-gravel base. These systems were developed to quickly drain standing water and resist soil compaction on the playing surface. A sand topdressing material is necessary to keep a clean, uniform sand base layer for optimal performance in this instance.
Topdressing with topsoil or sand is used to fill in gaps and low or bare spots within the home lawn. Topdressing with compost or other organic material promotes a quicker turfgrass recovery from aeration and dethatching, helps control thatch through enhanced microbial activity, and indirectly may reduce diseases. Topdressing with compost can best alter the soil structure in the turfgrass root zone for improved availability of nutrients and water. Other benefits of organic topdressings include:
- Increased organic matter in soil resulting in better soil structure
- Buffering of soil pH
- Improved soil aeration and drainage
- Potential reduction in irrigation demand
- Reduced fertilizer requirement because of the improved ability to hold onto nutrients for turfgrass use
- Improved soil health, resulting in enhanced turfgrass health
Soil consisting of fifty percent solid material (sand, silt, and clay) and fifty percent pore space supports healthy plant growth. Good soil structure, or the aggregation (sticking together) of soil particles and the arrangement of soil aggregates, provides adequate pore space. An ideal soil structure is made up of crumbs that adhere to one another to form larger clumps to provide good soil tilth. This arrangement allows for small pockets of space called micropores to form along with an equal amount of larger spaces called macropores within the root zone. Micropores and macropores have an inverse relationship to one another. As one type of pore space increases, the other decreases in amount.
Micropores are small spaces (<0.08mm) within each soil aggregate. Here, water is tightly bound and only available to plant roots through suction created by evapotranspiration (the process in which water moves out a plant). Macropores are larger spaces (>0.08mm) found between the soil aggregates. Here, water drains by way of gravity, thus providing an oxygen-rich environment for plant roots and soil microorganisms. Micropores hold water making it available to the turfgrass roots, while macropores provide the air spaces needed by the roots for deeper root penetration within the soil. Both types of pore spaces are equally important to growing a healthy stand of turfgrass.
If the native soil is heavy clay, it will be comprised of mostly micropores and can hold water around the roots of the turfgrass. This greater concentration of water holding pore space limits the amount of oxygen available to the root system, can cause root suffocation, and can increase disease incidence in the lawn. Too much water retained in the pore space surrounding the turfgrass roots can potentially cause root rot. Topdressing with organic matter improves pore space balance in heavier clay soils resulting in better soil aeration and internal drainage.
Conversely, native sandy soils have greater macropore space, thus allowing water to drain more quickly. Additionally, sandy soil aggregates have little micropore space available to bind water to plant roots. Ultimately this contributes to sandy soils becoming very droughty during periods of dry weather because of the disproportionate amount of air space and greater soil drainage. Too much air in the soil pore space and the turfgrass roots can become too dry. The addition of organic matter through topdressings improves the pore space balance in sandier soils resulting in better water-holding ability.
How to Topdress the Lawn
Sand should ONLY be used to topdress lawns when the native soil is sandy. If the native soil is NOT sandy, use topsoil as the topdressing material instead. True topsoil has organic matter. Topdressing applications of topsoil should closely mirror the content of the native soil. Compost materials, rich in organic matter, may be used to topdress all soil types.
If working with bare soil, evenly distribute the topdressing material, surface till, and smooth to level the grade. If working with established turfgrass, lightly and evenly rake the topdressing material to allow the material to sift down between the leaf blades. Precaution should be taken to prevent damage to existing healthy turfgrass.
Topdressing applications should be light with no more than ½- inch of material applied at a time. A thicker application can cause problems by stunting the turfgrass’s growth or providing excessive shade to the turfgrass.
Topdressing is often paired with lawn aeration. Lawn aeration involves using equipment to pull small cores or plugs of soil out of the ground to relieve compaction. See HGIC 1200, Aerating Lawns for additional information on aeration.
Allow the soil cores from the aeration to breakdown and become reincorporated into the soil for another good way to topdress home lawns. If the addition of organic matter is necessary, this can be done immediately following core aeration. A soil analysis can provide information regarding the need for any additional organic matter based on the soil CEC (cation exchange capacity). If soil compaction is not a concern, lawn aeration is not necessary prior to topdressing. As with lawn aeration, topdressing should be performed during the lawn’s active growing season.
In summary, topdressing is applied to improve or change the quality of the soil, as well as to level the lawn when low spots are present. For soil quality improvement, apply compost or other organic matter; to level the lawn or fill in gaps and eroded spots, apply topsoil or sand where appropriate. Topdressing applications should be light and frequent, allowing for incremental changes to the soil profile over time. For the average home lawn, compost is the best choice to improve both the soil’s overall structure and quality. It will help to achieve the balance of pore space needed for a healthy lawn. Compost and other sources of organic matter will improve the fertility of the soil and can reduce the fertilizer requirement. For additional information on topdressing and timing, see HGIC 1226, Turfgrass Cultivation.
If compost or topsoil is used to improve the quality of the native soil in the lawn, then plan to get a soil analysis first. It is important to have a soil analysis not only for the existing soil but also for the compost or topsoil that will be applied. Please plan ahead; it takes approximately two weeks to receive soil analysis results. The pH of compost and topsoil can vary significantly, and some nutrients may be present at elevated levels. For more information on soil analysis, see HGIC 1652, Soil Testing. For assistance with soil analysis results, contact the Clemson Home & Garden Information Center.
As a word of caution, topsoil and river bottom sand may contain weed seeds that can introduce a new weed problem into the lawn.