We hired a contractor this spring to redo our yard, which contained a significant amount of backfill from an old renovation done years ago ( a mix of clay and garbage, rusted nails, broken tile etc). Removal of the contaminated backfill was a major objective of the project, but this was not done before other elements of the yard were constructed and the sod laid.
The shape of the garden bed was cut into the sod, but none of the clay full of garbage was excavated, the subcontractor just compacted it with his skid-steer. The contractor is telling me that 6 inches of top soil on top will be able to grow anything. My dream is to have a thriving native perennial shade garden in this space. The compacted clay and garbage layer is about 8 inches deep. After that it transitions into a very fine light colored sand. I am wondering what treatment you would recommend for the soil? I am thinking that I will at least need to dig out the garbage.
Further to this, about an inch of soil was raked over the compacted clay before the sod was laid. No tilling was done. So far the sod is growing well. Should I be concerned?
Thank you so much,
Thanks for contacting the Toronto Master Gardeners. Your dream of a thriving native perennial shade garden sounds so wonderful ! It will take some work and probably more time than you might expect, but I’m sure you can get there. Your main challenge is not the clay and sand in your garden space. Ideal soil (the mineral content only, not the water or air) consists of a balanced mix of sand (approx. 38%), silt (38%) and clay (19%) + about 5% organic matter. Adding organic matter improves the soil structure of both clay and sand. Soil with good structure has about 50% of its volume in pore space or empty space between soil particles. Water and air can get into these pore spaces, and roots can grow into these spaces. Compaction destroys these pore spaces and pushes the air out of the soil. Air is essential for healthy roots. So the real problem you are faced with is rebuilding good soil structure.
There are two ways to approach restoring your soil health. The first is the no-till approach. This is the best strategy to preserve soil structure (whatever might still exist in your soil after the severe compaction), prevent soil erosion, minimize weeds, and improve drainage and air flow. No-till means no digging or turning the soil. You need to remove the garbage as best you can from your soil because it will interfere with long term plant growth. The best no-till way to do this is by pushing a pitchfork into the soil and moving it back and forth but not digging up the soil. This will help break up compacted soil but not disturb the soil layers. Do this about every six inches or so throughout your garden bed, loosening the soil and removing whatever garbage you can. Then put about a 4” or even 6” deep layer of organic matter (compost or shredded leaves) on the surface of the soil and gently rake it into the top of the soil. Then let nature take its course. Organic matter will work its way down into the soil, improving drainage and aeration in clay soil and improving the water-holding capacity of sandy soil,. It will also help with the release of nutrients which plants need to grow. Organic matter supplies food for beneficial soil organisms which help with plant nutrient uptake and to move the organic matter deeper into the soil. If you spread the organic matter now, your garden bed will have time over the winter to begin the soil improvement process. Then in the spring (and every spring after that) add more compost to the surface of your garden, and again let nature take its course to work it into your soil.
The second approach is one that we usually never consider, which is to till your garden bed, but only do this once as part of its initial creation. Tilling can take the form of digging and turning the soil or rototilling. Digging and turning damages the soil structure, while rototilling destroys it and can cause the development of a hard pan (which will prevent the movement of air and water and contribute to soil erosion) at the level in the soil just below where rototilling is done. Rototilling also causes compaction of the soil. In your case however, you might want to consider tilling, just once. Your soil structure is likely pretty much non-existent because of the severe compaction during construction, and tilling, whether it’s digging and turning the soil or rototilling, might be the only way to break up your severely compacted soil and allow removal of the garbage. After tilling, add compost to the surface of your soil as described above. In the spring, do not till your garden, just add compost to the surface of your garden. Tilling of any kind should never be done on a regular basis as part of routine garden maintenance Digging should only be done as part of planting, and it should be kept to a minimum.
Good soil for planting is at least 8” deep, and great soil is 12” deep. Grass needs at least 4” of good soil, 8” for great soil. You can get a reasonably good idea of the depth of your good soil with a wire like a coat hanger. Push this into the soil until it stops going in without bending (this will be the level at which soil compaction starts), and measure how far it went in with a ruler.
Your lawn is likely growing well now in the topsoil that was put down by your contractor, but I think it will likely struggle in the longer term as this topsoil breaks down and the grass roots grow and run into the compacted soil below. Since the sod is already laid, I think your best approach is to do core aeration (removal of small plugs of soil) now, before the ground gets hard, and then top dress with compost. Core aeration will help to loosen compacted soil, and compost will improve the soil structure and feed the soil, as described above. Core aeration is usually done annually in either the fall or spring, but can be done twice annually for heavily compacted soil. I suggest that you do this aeration followed by top dressing with compost again in the spring, and then in the fall, measure your good soil depth to determine whether to continue aerating and top dressing twice annually or just once annually, in the spring.
Best wishes for a thriving healthy garden !