· by Michael Blair, Nashville's Drainage and Excavation
A low spot in the yard is easy to see and tempting to fix with a truckload of dirt. The questions start when you go to order it: fill dirt or topsoil, how much, and how thick it can go over the grass. The answer depends on how deep the low spot is and what is growing there. There is also a question most people only ask after the first heavy rain, which is where the water that used to sit in that low spot goes next. This post covers how we choose the dirt on yard grading work, how thick each layer goes, and what we check before the low spot is filled.
What is the difference between fill dirt and topsoil?
Fill dirt is subsoil with little organic matter, sold to take up space and pack down firmly, while topsoil is the darker upper layer that holds the organic matter plants need. UT Extension’s guide to preparing ground for new turfgrass, written in 2007 for Tennessee lawns, notes that topsoil usually carries more organic matter and more beneficial organisms than the subsoil below it.
That difference decides where each one goes. Fill dirt is for depth: it compacts and holds its shape, which is what you want under a lawn, a walk or a patio. Topsoil is for the top layer, where the roots live. Because its organic matter breaks down over time, a deep pile of topsoil keeps settling for longer than compacted fill does, and a hole filled with topsoil alone tends to sink again.
Around Nashville, the fill that comes off a job is often clay. Clay compacts well and sheds water, which is useful against a foundation and less useful in an open lawn, where a low spot packed with clay over clay can stay wet just as it did before.
Which dirt levels a low spot in the lawn?
It depends on the depth. For a deep low area, a filled-in hole or a trench that settled, we bring the level up with fill dirt in thin layers, packing each one before the next, and finish with a layer of topsoil for the grass. UT’s guide follows the same order for a new lawn: set the rough grade first, loosen hard subsoil before spreading topsoil, then spread the topsoil evenly over it.
The UT guide gives a quantity for that top layer: about 12 1/2 cubic yards of topsoil spreads a 4-inch layer over 1,000 square feet. It also says turf rooted in fertile loam is often healthier and better at handling weather stress than turf growing in poorly drained, clayey subsoil, which is the reason we do not finish a yard in bare fill. Where the existing topsoil is good, UT’s advice is to strip and stockpile it before regrading and spread it back afterward.
For a low area in between, a few inches deep under living grass, the usual answer is to cut and roll back the sod, fill and firm the soil beneath it, then lay the sod back down at the new level and water it in.
How thick can new soil go on top of grass?
Not much at a time. Clemson Cooperative Extension’s fact sheet on topdressing a lawn, written for South Carolina lawns, says to keep each application light, no more than 1/2 inch of material, because a thicker layer can stunt the grass or shade it out. For low spots and small eroded areas, it suggests topsoil or sand where appropriate, put down lightly and often.
That makes topdressing the tool for shallow dips, not deep ones. Spread a thin layer, work it down into the grass with the back of a rake so the blades still show, and let the lawn grow through before adding more. A dip an inch or two deep may take a few rounds over a growing season.
Sand is sometimes sold for this. On heavy clay, a layer of sand over clay does not change how the ground below drains, so for most dips we topdress with a screened topsoil close to what is already in the yard.
Before the dirt arrives: where does the water go?
A low spot is often where water already collects or runs, and filling it sends that water somewhere else. If the yard drains through that dip toward the street or a ditch, raising it can turn the flow back toward the house, a neighbor’s yard or a patio. If the spot stays wet because water comes up from below or runs in from uphill, more dirt will not dry it; our post on a soggy spot that never dries covers what does.
So before we order anything, we look at the whole slope: where the water enters the yard, where it leaves, and where the new grade will send it. Near the house, the rule is different and stricter, and our post on grading around a foundation explains how fill against the walls should slope and what it should be made of.
If you have a low area in Franklin, Nashville or anywhere around them, send us a photo after the next rain. We will look at where the water goes, tell you whether it needs fill, topdressing or a drain, and give you a price for the dirt and the work.
Sources
- UT Extension W160-A, Turfgrass Establishment: Preparing to Plant (Samples and Sorochan, 2007) — Topsoil has more organic matter and beneficial organisms than subsoil; stockpile topsoil, set the rough grade, loosen subsoil, redistribute topsoil; about 12 1/2 cubic yards for 4 inches over 1,000 sq ft; turf in fertile loam healthier than in clayey subsoil
- Clemson Cooperative Extension, Home and Garden Information Center, Topdressing a Home Lawn (South Carolina) — Topdress lightly, no more than 1/2 inch at a time; thicker layers can stunt or shade turf; topsoil or sand for low and eroded spots, light and frequent
FAQ
Fill Dirt or Topsoil to Level a Low Yard — questions
Can you use topsoil as fill dirt?
For a shallow fill, yes, but topsoil is a poor choice for depth. Its organic matter keeps breaking down, so a deep hole filled with topsoil alone tends to settle and sink again. Fill the depth with fill dirt packed in layers, then finish the top few inches with topsoil for the grass.
Can grass grow in fill dirt?
It can, but it struggles. Fill dirt is subsoil with little organic matter, and UT Extension notes that turf rooted in fertile loam is often healthier and more tolerant of weather stress than turf in poorly drained, clayey subsoil. Cap fill with a layer of topsoil before seeding or sodding.
Should I remove the sod before adding dirt?
For a low spot more than about half an inch deep, yes. Cut and roll back the sod, fill and firm the soil beneath, and lay the sod back at the new level. For a shallower dip, leave the grass and topdress lightly, no more than half an inch at a time.
How much topsoil do I need to cover 1,000 square feet?
UT Extension’s turfgrass guide puts it at about 12 1/2 cubic yards of topsoil for a 4-inch layer over 1,000 square feet. A thinner layer takes proportionally less. Order a little extra, since topsoil settles after it is spread and watered, and spread it over a rough grade that is already set.