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How much topsoil do I need for new lawn?

Posted by peakbagger66 Washington State (My Page) on
Mon, Mar 30, 09 at 14:12

Hey Master Gardeners!
We are planning on seeding a new lawn, about 1500 square feet, in our backyard this spring. We live in a new development, so it is pretty barren in the backyard. There are a lot of rocks - from 1/2 inch to 12". We have removed most of the larger ones that were 3" or more in diameter from the surface. We plan to rototill down to 6" or more and we will remove any larger ones that we come across.

I see recommendations on various sites and books for 4" to 6" of top soil. What conditions would determine if we need 4" vs. 6"? It does make a big difference in the amount and therefore the cost of the initial topsoil. Are there cheaper substitutes for topsoil?

Also, we are planning on (but not married to the idea of) 2 tilling sessions - the first to loosen up the soil and help find big rocks, and then the second to mix about 2" topsoil with the existing surface. At what point can we add weed killer?


Follow-Up Postings:

RE: How much topsoil do I need for new lawn?

Peakbagger66, as far as the time to weed, what is your intention about accomplishing that....dig them out one by one, or spraying a broadleaf weed killer or a glyphosate, such as RoundUp, to kill everything green?

I would imagine in your neck of the woods, spring has come in earnest so let's just say weeds are tackled better when we wait until they are growing their best.
Let the soil warm up well.
You might think since you can go out and work the soil, the time is ripe to attack the weeds. Not so, wait until the weeds are actively growing.

You have 1500 square feet of area and wish to add 2" of topsoil over the area.
First thing, you speak of digging in 4 - 6 inches. That should be a minimum to consider. Digging down 4 - 6 inches would be ideal and if you can add lots of organic matter which will help retain moisture, do that. Peat moss is such an amendment. Hay, straw, newspaper, lots of things can be rototilled into the top 6". But, if you can afford it, look for prices for "composted cattle manure (or sheep manure) which can make a lot of difference when we want things to grow.
Don't be tempted to use fresh or near fresh manure--unless the area so given, can lie fallow and not be expected to grow things for at least a year.

Trees are always a problem. Trees take what moisture they like and leave very little for what's above.
Some trees are worse than others....maples, birches, oaks, and poplars (some) are notorious water stealers.

Hopefully the rototilling wont harm what tree roots might be close to the surface. Most trees have their roots in the top 18"--and some major roots are a lot closer than that. So be careful. There is a formulae that one can use when needing to cut tree roots without endangering the stability of the tree.

There is also a way to figure out how much soil is needed to cover a particular amount of area.
You have 1500 square feet and wish to put down 2" of topsoil.
Multipy the 1500 square feet X the 2 inches....resulting in a figure of 3000. Divide that figure by 1000 and multipy by 3.
The resulting figure is the number of cubic yards of soil you will need to accomplish the layering.
In this case....9 cubic yards would do the trick.
That is a awful lot of soil...would be necessary for delivery by truck and dumped on your driveway if he cant get to the site. Then you wheelbarrow it in.

As far as trying to use something other than topsoil..look around your neighborhood, try to find construction sites.
Often they remove such topsoil and replace very little when a buidling is going up.
Homesites might be a little harder to take such soil from.
But you can always ask --maybe you can get a not-caring-individual-worker who couldn't care less about that pile he has to walk around or climb over.
There are always people who advertise "wanting clean fill"...maybe find out where someone is getting it and you go get some.

Any digging up of soil --say from a nearby forest, might only get you lots of weeds.

The first thing to do is to remove as much as possible those rocks that surely could damage the tines on the tiller. Then gather what organic material you can and till it in down 6" or more.
If you plan on seeding the area, you cant spray for weeds.
If you spray for weeds, you cant overseed for at least 2 - 3 weeks. If you use RoundUp, you cant put seed down until the ground is improved and a week or two has gone by.
But on any weed killer package or bottle, read and understand the directions. Where RoundUp is concerned, more is not better.

RE: How much topsoil do I need for new lawn?

Well I am going to go the other route.

Why on earth are you thinking about rototilling for? It does more harm than good especially if you are going to bring in more soil or amendments. Rototilling destroys the soil structure, organisms, brings a bumper crop of weeds, and leaves you a bumpy unlevel surface that gets worse as the soil settles.

You will be far ahead of the game bringing in a tractor and box blade. Doing that you can bring in all those dump trucks of soil and amendments you want and an operator will have it all done in a hours leaving you with a perfect level smooth seedbeds ready to plant.

1 cubic yard will raise the grade of 1000/ft2 inch. A dump truck holds anywhere from 3 to 7 cubic yards depending on the size of the bucket and truck. So that gives you some idea of how many truck loads of dirt it will take.

RE: How much topsoil do I need for new lawn?

I mostly agree with texasweed in hesitating to till. I realize you can find hundreds of articles on that topic, but you will never find a reliable landscaper that tills before installing a garden. They always use a tractor and box blade to develop perfect drainage and seed bed. If you are in a new development, your soil was probably graded already. If you add more soil you are almost guaranteed to ruin the drainage. If you end up with water draining into the house, you are in for a long time of headaches. I'm am assuming you don't need more soil.

No matter what soil the contractor left you with, it is adequate to start a lawn. Pure sand is what I use as a replacement for my lost topsoil. Sand will never be "topsoil" but it works great for grass. Removing the rocks is a good idea, but I would not go way overboard on that either. My bedrock protrudes from the surface in places and is as much as 18 inches deep at the deepest. You can't dig holes in my garden without a pick or jack hammer. But I digress. Whatever soil you are starting with is fine. It will become topsoil before you know it.

As for seeding, the most I would do, assuming your soil is properly graded from the contractor, loosen the very top with a leaf rake, apply a mix of Kentucky bluegrass seed and turf-type tall fescue, roll it down with a water filled roller, and water. KBG takes a little longer to sprout than fescue so keep watering daily until the KBG comes in. Set your mower to the highest setting and mow when the grass is tall enough to be mowed. Back off on watering frequency as you can but increase the time you water. Eventually you should be watering every other week unless your part of WA is in the desert. Then during the hot part of summer you might need to water weekly. Fertilize after you have mowed the grass for the second time.

You don't add weed killer unless and until you have weeds to kill. Don't worry about not having weeds. Spring seeded lawns are very susceptible to crabgrass sprouts. Keep your grass growing up tall and that will help keep the crabgrass out.

RE: How much topsoil do I need for new lawn?

Thanks everyone, for their suggestions. Now I'm *really* confused with the two different approaches :-).

Our lawn was roughly graded by the developer. After removing the large surface rocks last fall, it has compacted into an uneven surface. That is one of the reasons I was planning on tilling. I am not sure if we can fit a tractor in there - there are 3 sections of the yard and the sections range from 7' to 10' in width with tight corners, not to mention the pain of taking down the fence, although that is probably a minor task (I think). I see a raking was suggested but, the surface is pretty compact and does require a fair amount of energy to break up. Is there a different tool to break up the surface? If we do rototill, would simply adding earthworms help the microflora re-establish themselves?

Post-tilling, we were planning on compacting it down with a waterfilled barrel.

We went to a lawn and garden shop last night and after looking at our soil sample they recommended just adding an inch of high value soil/compost blend. Maybe we don't need top soil at all. I will have the soil tests completed tonight.

For weeds - I sprayed last fall and picked the majority of the remnants during the winter. I'm absolutely positive there are vestigial crabgrass root systems waiting to see the light of day. I have a lawn safe weed killer from Spectracide with crabgrass control which I was hoping to use as a preemergent. Should I not spray at all? ALthough the weedkiller is advertised as lawn safe, I do understand that grass seedlings may have different requirements and sensitivities.

Thanks again, everyone, for your advice!

RE: How much topsoil do I need for new lawn?

We went to a lawn and garden shop last night and after looking at our soil sample they recommended just adding an inch of high value soil/compost blend. Maybe we don't need top soil at all. I will have the soil tests completed tonight.

That is basically what dchall and I were telling you. The best machine to do that is a tractor and box blade along with a dump truck to deliver the material.

RE: How much topsoil do I need for new lawn?

Well I will say a rototiller is a more personal tool to use where it can dig up and till in amendments that will help retain moisture.
The member that suggests her soil is rock hard...might look at rototilling in some organic matter.
The sand that was dumped in was probably beach sand--playbox sand, and that is directly responsible for what happened to her yard.
This kind of sand does not aid in just does the opposite, especially where such soil is clay.

A lawn roller should not 'compact' the soil--it should compress it sufficiently so that it can drain properly.
A lawn roller should have no more than 1/3 the water volume and be sufficiently light to not tire the operator.

If the soil is presently compacted, then aeration is another option. Compaction will result in early death of grass roots and lousy drainage.
Compaction can result in new home developments from truck traffic and constant walking on by construction crews.
When severe, aeration is the only option to consider.

However the ground is dug, do think to get organic matter into it. Organic matter should be dug in at least 6"--the more the better.
When considering the addition of sand, make sure it is coarse sand, builder's sand and make sure it is well mixed.
Buy it at a quarry or soil dealer that guarantees it is what aids in drainage.

A pre-emergent, to prevent germination of weeds, will also prevent germination of grass seeds.
Pre-emergent is applied either in early spring, or later in the fall.
If pre-emergent is applied now, overseeding cant be done until the fall.

RE: How much topsoil do I need for new lawn?

If your yard is broken up into small areas, then nevermind the tractor. And whatever you do, don't get tempted to bring in a Bobcat or a Skidsteer type of implement. They are not designed right for leveling soil (short wheelbase and top heavy). You can do it by hand. Let's assume first that you don't need soil to bring the grade up to the level of any concrete surrounding the yard. If you have a soil level that is at or above the surrounding concrete, then you are fine. Then all you need the additional soil for is to level the current low spots. With that in mind I would direct your attention to this Gardenweb post by proudx where he details how he leveled his actively growing lawn. By buying sand in bags, he would be able to return any he did not need.

RE: How much topsoil do I need for new lawn?

Thanks everyone.

Jeannie7: the topsoil is not rock hard, just somewhat compressed in some places from the kids and adults tromping around on it. It takes quite a bit of force to get the rake in to loosen it up. There is not too much sand in there. There are however, A LOT of rocks 1/2 inch to a foot. The big ones are easy to remove. The small ones...are more problematic due to numbers. THAT is the subject of my next post :-).

dchall: When you say "by hand" do you mean with a shovel? I just did that the other day on one section just to see what the effort level is like. It actually wasn't too bad, but, I do like the idea of using a tiller to mix amendments in more evenly than if I did it by hand. However, I don't like the idea of bringing up weed seeds and disrupting the microflora below as texas-weed mentioned, but I think the microsystem can re-establish itself (I hope) and I can deal with the weeds when they come. And...doesn't digging with the shovel disrupt the soil structure anyways??

The shovel session has brought up another issue...that of rocks! I mentioned that our site has a lot of rocks, but there are more than I initially thought after digging. I'm going to take a few pictures and make a new post...

RE: How much topsoil do I need for new lawn?

Here's how I prepare soil for sod or seed:

1. Evaluation of the soil: I don't care whether the soil is clay or sand or anything in between. There are excellent lawns in every kind of soil across the country and around the world. The only soil that doesn't seem to work is the Death Valley type soil with boron salts. No matter what condition the soil appears to be in, it can be improved by ONLY using a combination of compost and organic fertilizer ON TOP of the soil. I see no need for incorporating anything into the soil in prep for grass. The only thing better than compost and organic fertilizer is to mulch it 3 inches deep in October/November and letting it sit through the winter like that.

2. Preparation for grass involves grading it if it is not draining properly. If it is already draining properly, then NEVER add any soil to it. Have you ever driven past a yard with the soil sitting 1-3 inches high above the sidewalk? They added soil. Some lawns will grow out of their containers and you'll see soil, but for the most part, lawns show exposed soil because they added soil.

3. Adding amendments (whatever they are) can be done perfectly uniformly by applying them to the surface with a drop or broadcast spreader. Compost can be flung with a shovel and blown around with a leaf blower or broomed around pretty evenly with a push broom. No shovels are needed.

I'll look for the pix of the rocks.

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