DIGGING IN

  • Topdressing is the process of applying a material in a thin, even layer to the surface of natural turfgrass to accomplish a variety of purposes. The term topdressing also refers to the materials applied in the process, typically sand, sandy topsoil, compost or a prepared combination of any of these materials, sometimes incorporating calcined or vitrified clay.
  • When you topdress and what you use for topdressing material depends on what you want to accomplish.
  • Topdressing can be used to develop or maintain a smooth playing surface by eliminating holes or ridges that negatively impact player safety and ball performance.
  • Topdressing can reduce excessive thatch buildup (greater than 0.5 inches) by modifying the thatch layer to encourage the microbial activity that leads to its decomposition (breaking down).
  • When applied following core or spike aeration, the topdressing material can filter into the holes opened by the aeration process, speeding turfgrass recovery.
  • Topdressing can be used following seeding, overseeding or sprigging to increase soil contact and help retain moisture to protect the developing plants from desiccation (drying out) during the establishment process.
  • Topdressing can also be used on open, windswept turfgrass fields to help avoid winter desiccation.
  • Typically, for all of these uses, the topdressing material should closely match the composition of the soil profile.
  • Topdressing also may be used to modify the soil profile of a native soil field to alleviate compaction and improve water and air movement, though this will take multiple applications over a number of years. For this use, the topdressing material should differ from the composition of the soil to which it’s applied.

Finding a good, local supplier that consistently provides uniform materials is essential to a successful topdressing program.

MODIFY THE SOIL PROFILE

  • Soil is made up of rocks, minerals, organic matter and water. The mineral particles are broken into three categories defined by size, with sand the largest, silt the mid-size and clay the smallest.
  • Soil texture describes the proportion of these soil particles in soil types, which refer to the fineness or coarseness of a soil. The soil texture determines characteristics that impact plant growth: water-holding capacity (the amount of water the soil will retain), permeability (how easily and quickly water and air are absorbed into the soil and move through it) and workability (how easily the soil can be tilled or penetrated).
  • Typically, topdressing for modification is combined with core aeration, with the cores collected and removed from the field. A coarse-textured material ( frequently sand or sand-organic matter combinations) is applied to soils with high silt/clay content ( fine-textured). Over time, this changes the soil texture to improve root-zone conditions, which results in more vigorous turfgrass and a better surface for play.
  • In the short term, the addition of coarse-textured topdressing reduces bulk density (the dry weight of both the solids and the pore space within a soil) and opens drainage channels to improve infiltration (the movement of water, air and nutrients into the soil) and percolation (the movement of water, air and nutrients through the soil).
  • When budgets allow, sand, calcined or vitrified clay, or a combination of these may be installed in a field in slits (narrow channels) that extend through the top 6 to 12 inches of the soil profile. These help alleviate compaction, move water from the surface more quickly and direct excess water to subsurface drainage pipes. Regular topdressing with the same material used to fill the slits is necessary to prevent “capping” of the slits by native soil, which would keep them from working.
  • Compost is a topdressing material that is used alone or added to sand or soil. Unless you’re trying to convert a native soil field to a sand or sand-capped field, Dr. Andy McNitt (Penn State University) builds a case for using only straight compost topdressing on native silt/clay soil fields. He describes compost as organic matter that can act like glue, binding soil particles together to form larger particles. These larger particles then stack against each other, creating pore space between the particles.

Topdressing can reduce excessive thatch buildup (greater than 0.5 inches) by modifying the thatch layer to encourage the microbial activity that leads to its decomposition (breaking down).

CAUTIONS

  • The use of varying topdressing materials can result in layering, with different textured soils stacked in layers within the soil profile. The greatest problem occurs with a fine-textured soil forming a layer on top of a coarse-textured soil. This can hinder water’s ability to reach the turfgrass roots, since water moves throughout one layer of soil texture before infiltrating the differing texture of the level below it.
  • Topdressing for soil modification can be detrimental if done improperly. Once the topdressing material is determined, that same material must be used throughout the process to ensure the consistency that facilitates water movement and root growth.
  • Problems also can occur if the process of applying the topdressing materials is done improperly, resulting in uneven depths of material across the field. This can impede water and nutrient accessibility, creating wet and dry areas that impact turfgrass growth and the overall appearance of the field.
  • Topdressing to maintain a smooth, consistent playing field, without also maintaining a consistent slope, could disrupt surface drainage.
  • Applications of too much topdressing material, or applications made too frequently, over an existing thatch layer can bury the thatch layer. Without proper aeration to rectify this, turfgrass roots may grow into and within the thatch layer rather than extending into the soil profile. This makes the turfgrass more susceptible to drought, heat and other stresses.

Spinner topdressers cover a wider swath and speed the process of topdressing, which is important when many fields need to be covered or when time is limited.

SELECTING THE MATERIAL

  • Finding a good, local supplier that consistently provides uniform materials is essential to a successful topdressing program.
  • If soil profile modification isn’t the goal, topdressing material should match the material in the underlying soil.
  • On sand root zones, topdressing materials are typically pure, clean sand. A combination of sand and calcined clay may be used to help retain moisture.
  • Sands vary and there are specific, differing, criteria for sands used on native soil or sand rootzones. Key points to consider are particle size and particle distribution, uniformity, shape and chemical makeup.
  • Gravel and sands are classified by particle size. Pamela Sherratt (The Ohio State University) reports that typically, medium-coarse, uniform sands, which adhere to the United States Golf Association (USGA) recommendations, are used on native-soil fields, with slightly smaller particles introduced on pure sand rootzones to ensure surface stability. She notes the coarser and more uniform the sand, the less stable, or firm, it is.
  • Dr. Peter Landschoot’s (Penn State University) fact sheet on selecting quality compost for topdressing provides key details to consider.
  • Because compost quality varies depending on the source, the initial materials used and how it’s produced, you need some basis for determining its suitability for use on turfgrass. Landschoot recommends selecting compost that has been field tested at a university and/or has been used successfully by other turf managers and is consistent from batch to batch.
  • McNitt reports quality compost should contain at least 30 percent organic matter and have a carbon-to-nitrogen ratio less than 30-to-1. Particle size should range between 0.25 to 0.375 inches in diameter, with the ideal pH 6.0 to 7.0. Salt content and nutrient value also should be tested and monitored.
  • Some sports turf managers apply recycled crumb rubber to high-traffic areas to protect the turfgrass growth point (the crown) and prevent loss of ground cover. Sherratt notes that it’s less abrasive than sand, but cautions that when adding any type of foreign material to the field, you must consider its effect on soil and turf performance. Particle size must be monitored to prevent layering. The recommended depth of crumb rubber is 0.25 inches or less.

Typically, topdressing for modification is combined with core aeration, with the cores collected and removed from the field. A coarse-textured material (frequently sand or sand-organic matter combinations) is applied to soils with high silt/clay content (fine-textured).

WHEN TO TOPDRESS

  • Dr. John ‘Trey’ Rogers (Michigan State University) explains the best time to topdress athletic fields is during periods of active growth — in other words, during time where there’s little, or no, play.
  • To be effective, Rogers reports topdressing must migrate to the soil/plant interface, with the speed of migration critical. Irrigation helps migration, but the best aid is actively growing turfgrass. Thus, he recommends the best timing for topdressing is during the spring and early summer for both cool- and warm-season turfgrasses. Faster migration reduces the risk of an unstable surface, which could create footing problems and limit the effects of abrasive materials, such as sand, damaging the turfgrass leaf tissue.
  • If soil cultivation and topdressing of cool-season grasses must be done during the heat of summer, it’s best to allow at least 10 days for recovery before field use.

APPLYING MATERIALS

  • Dry topdressing materials can be applied with a rotary spreader if the operator is skilled enough to ensure it’s applied uniformly. A dedicated topdressing machine will provide uniform application and can be programmed to apply the desired amount of material.
  • Because compost is light and bulky, a spreader with a large hopper cuts down on refill time. Modified manure spreaders with conveyor belts and brushes mounted on the back also work well.
  • Landschoot recommends applying the compost first, followed by several passes with a hollow-tine aerator with a heavy drag mat attached, to break up the cores, mix the compost with the soil and drag some of the mix into the aeration holes.
  • Sherratt suggests considering a recycle dresser, which pulls cores, pulverizes the soil and then reapplies it as topdressing in one pass.

MATCHING RATES AND FREQUENCY

  • Sherratt reports the amount of topdressing applied must factor in the degree of damage to the field surface, the mowing height of the grass, the physical texture of the underlying soil and the budget, with a typical application rate of 0.125 to 0.375 inches.
  • Topdressing Athletic Fields,” found at Iowa State University’s Extension Store, includes a modified table based on one originally created by former ISU professor Dr. Dave Minner. It outlines the amount of material required for topdressing only, as well as for topdressing and filling aeration holes of differing sizes, depths and spacing.

Author’s note: This article is a compilation of information provided through a wide range of resources developed by: The Lawn Institute; Dr. John ‘Trey’ Rogers, Michigan State University; Pamela Sherratt, The Ohio State University; Dr. Andy McNitt and Dr. Peter Landschoot, Penn State University; and Ryan S. Adams, Iowa State University.