Compacted soil under grass lawn


Hello, we have had difficulty growing grass for many years. We believe the issue stems from the earth being compacted. We have patches where the grass refuses to grow. The soil is baked solid. We do not have grubs nor weeds. We have aerated the lawn and de-thatched the grass. What is suggested to improve the soil beneath the lawn? Could we use bone meal, alfalfa pellets or something else? Also, when is the best time of year to feed the soil. Thank you.


Dear gardener, thank you very much for reaching out to the Toronto Master Gardeners on how to remediate your compacted soil so that you can improve your lawn.

Some of the reasons that come to mind about why your soil is so compacted  are the following. The soil itself could be solid clay. This is an issue in Toronto; it is no surprise that many of our houses are built from red brick that comes from the clay quarries throughout the GTA.  The other possibility is that the soil is under a large tree or trees – either deciduous or evergreen – and the roots of the trees have compacted the soil. A third is that the soil has been compacted from overuse, either from constantly being walked or played upon, or repeatedly mowed in the same direction. The other thought I had, is that this part of your property may have a lot of old construction materials that could have been dumped there a long time ago. This has been an issue in some parts of the GTA.

If any of these are the reasons for your compacted soil, then adding alfalfa pellets or bonemeal will not really help. But you have mentioned that you have aerated the soil and you have removed the thatch, and those are very good starts to fixing your lawn. (Hollow-tine core aeration is highly recommended — this article also recommends aeration in early fall or late spring).

Fixing soil is a long-term project. The most important thing to do is to add organic matter slowly, over a number of years. The least expensive way to do this is to collect all the fallen leaves in your neighborhood and pile them up high, cut them up if you can, and let them slowly decompose over the winter in an out of the way spot. The decomposition will take a bit of time depending upon what leaves you are using, but by the middle or end of next-year’s summer, you should have enough organic matter to seed.

A quicker way which will immediately improve your lawn, is to purchase composted soil (topsoil will be too heavy). Clemson cooperative gives the guideline of adding 1/4-1/2″ of compost as a topdressing for lawn.

The addition of organic matter will not immediately change the composition of the original soil, but it will enable you to have a nice lawn this coming spring. Over the years, if you keep aerating and adding organic matter, nature will take over, and the natural biome (beneficial microbes), which will now have be re-introduced, will make this a fertile area. Please take a look at this Master Gardeners Garden Guide on Soil Fertility. From the guide, is a great list of the advantages of adding organic matter to your garden in general, and your lawn in particular:

  • Adding a fresh supply of organic materials feeds the soil microbes and improves soil structure; use what has been grown in your own garden and apply to the soil surface
  • Use moderation when applying compost or fertilizer to the garden. It is better to apply organic material consistently over several years than a large amount all at once.  Excess nutrients in the soil will leach into the groundwater and eventually into the water supply.
  • Mulch with organic materials feeds microbes at the soil surface, buffers the soil from wind and water, and helps hold moisture in the soil.
  • Water deeply and less frequently to stimulate healthy root growth deeper in the soil.
  • To reduce soil compaction, avoid excessive digging, and don’t walk on your garden beds.
  • Try to keep your garden free of pesticides as they can harm beneficial soil organisms.

Here is a link to a basic overview on how to look after a lawn from the Toronto Master Gardeners.

Finally, it is worth emphasizing that Toronto Master Gardeners strongly recommend replacing lawn with groundcover. Maybe this subject is for another day, but please give this idea your serious consideration. You will find more valuable information on how to grow ground covers at this link. 

I wish you luck and enjoyment with this gardening project.