Is mulch or topsoil better for new lawn in clay soil?

Question

We just built a new home and the back yard has quite a bit of clay in it. We are planning on seeding the yard in the next month (September). I'm trying to decide between topsoil (4-6 inches) or tilling in organic matter. The advantage of the organic matter is that I don't have to take soil away from the edges of the sidewalk to make room for the topsoil. Topsoil is also pretty expensive. I've heard that the mulch can burn the seeds though and I don't want to do something that will force me to do sod (see expensive).

Any advice would be much appreciated!

Answer 1

Not sure where you're from but in the USA you can send a soil sample to your local university extension office and for a nominal fee they will give you a report on the soil that you can use to improve it for your intended purpose. Two key things to look for is the existing organic matter content and cation exchange capacity. This is always a good thing to do before doing major work for a lawn or garden.

When you say "mulch" I assume you're talking compost and this is what it's called in your area?

Compost is pretty much the closest thing to a silver bullet when it comes to improving your soil. It can fix all sorts of problems. The only time compost will burn the seed or the seedlings is if it has not been composted properly. This is more of a problem with composted manures which are "hotter" or more nutrient rich. A good blend of compost from different sources such as manures, leaves and other plant materials that has been composted completely shouldn't be a problem.

Tilling your soil causes problems but if you need to seriously amend your soil it's the quickest option. A couple of issues are tilling will wind up leaving your lawn uneven and you'll need to fill in areas in the future to level it. Tilling will also bring up a ton of weed seeds that were buried too deep to germinate before so before you plant your grass seed water the soil and give the weeds a couple of weeks to germinate then deal with them before putting down your grass seed.

Clay soil isn't that bad for your lawn as long as you have a decent amount of organic matter already in the soil. You may not want to go to the expensive and effort of tilling in a lot of organic matter (2-6") into your soil all at once. Another option if your soil has at least 5% organic matter and decent cation exchange capacity you can just top dress with about a 1/4" layer of compost. This would require 1 cubic yard of compost per 1,000 sq ft. Do this every year for the next 3 years or so and then start doing it less frequently if you are happy with what you're seeing. Time, worms and other organisms will help work it further into the soil.

You should also do a percolation test to see how well the water drains. See: http://www.gardenguides.com/126137-test-soil-percolation.html http://www.walterreeves.com/landscaping/soil-percolation-rate/

Answer 2

Is mulch or topsoil better for new lawn in clay soil?

Assuming the new topsoil is decent topsoil, the topsoil is "better". Even if you had outstanding topsoil now, additional topsoil would still be better because it would be a thicker layer of good topsoil. You simply can't have too much topsoil.

I'm trying to imagine what you have now and I'm picturing a muddy mess with tracks from heavy equipment and probably much of the topsoil is gone. Since it's described as "clay", I'm also picturing a high-rainfall area (40-50 inches annually). If my guesses are true, then the dirt is acidic, lacking in calcium, low in organics and will take a long time to bring up to par. Lime takes at minimum 6 months and most likely 1-1.5 years to add calcium in useable form to the soil.

I'm having to make a lot of assumptions to make a recommendation, but a soil test is a prudent place to begin, as OrganicLawnDIY said. In addition to CEC (or TEC which should be at absolute bare minimum 10-15), pay particular attention to calcium % and hydrogen % in the base saturation table. If calcium is low and hydrogen is high, you may as well get new topsoil. Even with the new topsoil, I'd still apply lime to the soil you have now, then cover with the new soil. Lime is fairly cheap. If you look on craiglist, topsoil isn't too expensive... about $30 a pickup truck load in my area.

I also agree that tilling is going to be a lot of work with little gain and will result in humps and bump and lots of weeds. It will break up the soil for a while, but it will go right back to its compacted nature since the problem of compaction is due to lack of calcium, which tilling won't fix and neither will mulch. Mulch (or compost) will add friability to the soil until the mulch decays, then you're right back where you started in terms of compaction.