Turfgrass Establishment Series - Seedbed Preparation
Soil Modification, Grading, and Preparing the Surface for Seed
By Pam Sherratt
Turfgrasses grow best in good quality, free-draining soil with adequate nutrients for healthy growth. The first step to understanding the quality of the soil is to carry out a soil test. If the soil drains well, with adequate nutrients and a pH between 6 and 7, the only soil amendment needed during the seeding process may be a starter fertilizer applied at the time of seeding. If soil test results revealed that lime needs to be added, follow the recommendations outlined in the factsheet Lime & the Home Lawn.
Many lawns in Ohio (especially new lawns) are growing on soils that contain appreciable amount of clay. These soils are considered lower quality because they lack organic matter and are prone to compaction, resulting in poor drainage. These soils, particularly if they are not graded correctly, require a more aggressive approach to soil amendment.
There are two approaches to soil improvement – (1) Topdressing with a soil amendment, or (2) mixing/rototilling a soil amendment into the top 4-6” of topsoil.
1. Topdressing with a soil amendment
This option is good for lawns that have a good grade but need organic matter adding to the soil profile. Topdressing is the process of spreading a thin layer of material (like compost, topsoil or a mix of both) over the surface of the lawn. This is usually done by hand. Larger lawns will require specialized equipment.
- Spread good quality compost/topsoil across the lawn surface to a depth of ¼ to ½ inch.
- Rake, drag, or brush the material into the lawn.
- Water the material in.
Topdressing could be done in conjunction with core aerification, to work the material down into the underlying soil. Once seed has been applied, another fine layer of material can be applied over the top, to keep the seed in place and to retain moisture.
2. Mixing a soil amendment into the top 4-6" of topsoil
This option is good for lawns that need to be completely renovated because the soil quality is poor and the grade needs fixing. In this scenario, the existing lawn is removed by a non-selective herbicide so that a new lawn can be established.
- Remove dead/old vegetation so that only bare soil remains. This can be accomplished with a hard rake, or by lightly rototilling.
- Spread 1-2 inches of good quality compost across the soil surface and rototill to a depth of 4-6 inches. Rototillers can be rented, or hire a local lawn care company to complete this step. Perform this task when the soil is fairly dry.
- During the tilling process, remove rocks, wood pieces, and thick roots.
- Typically, 2-3 passes with the rototiller are enough to mix the materials together.
- After tilling, use a rake to grade soil, sloping away from the property. A typical slope is 2-5%.
- The finished seedbed surface should comprise of a soil that has small aggregates, is even (no bumps or depressions) with a loose, friable appearance.
Preparing small seedbeds
Preparing a seedbed in small patches (dog pee spots, high traffic areas under swing sets, etc.) can be easily accomplished with a garden weasel. Remove dead grass/weeds and hand-till the soil with a weasel until it creates a crumbly tilth. Mix seed with topsoil & starter fertilizer to create a divot mix, and patch those small areas by hand.
Types of soil amendments
Choose a good quality compost or topsoil material that will ultimately improve soil drainage and soil fertility levels. Composts are derived from a variety of materials like sewage sludge (called biosolids), manure, yard waste, spent mushroom compost, coffee grounds, and food waste. Composting facilities are regulated by the EPA and suppliers will help you choose the best source. Ideally, a lawn compost would have a bulking agent like woodchip, have no weed seeds present, and not be too odoriferous. Biosolids and manures will have an odor for a couple of weeks after application. Work with your local soil supplier to pick a good quality topsoil or compost material for your yard.
A starter fertilizer contains phosphorus (P), as well as other macronutrients. Even if the soil is amended with compost or topsoil (or a combination of both), a starter fertilizer applied at the same time as seeding will provide the germinated seedlings the best possible scenario for nutrient uptake because of its proximity to the seed. A typical application rate for starter fertilizer is 1 lb. nitrogen per 1,000 sq.ft
There are synthetic and natural organic fertilizers that can be used during establishment. Keep in mind, the type of fertilizer makes no difference to the grass, as grasses absorb nitrogen (N) only as nitrate (NO3-) or ammoniacal nitrogen (NH4+). Synthetic sources of nitrogen are readily available to the grasses and come in slow-release forms. Natural organic sources of nitrogen need to be broken down by soil microbes and converted to those chemical forms before grasses can use them. As such, natural organic sources of nitrogen are considered slow-release and safe to use (have a low burn potential). However, some organic sources, like biosolids and manures, can contain high levels of salt, which can cause turf burn in hot weather. Natural organic sources of nitrogen can also contain substantial levels of phosphorus, so closely monitor soil phosphorus levels if you are using composts as your primary source of nutrition.
The Golden Rule of seedbed preparation is to create a soil surface that maximizes seed-to- soil contact. The seedbed must have a friable, aggregated soil surface so that seed can be lightly raked or dragged into the soil surface. The ideal seedbed is created by either topdressing, rototilling, or on a small scale, using a tool like a weasel.