How to Garden in Compacted Soil
Compacted soil may be starving your plants of the nutrients they need. Here’s what to do about it.Posted May 10, 2016
Whether it’s from the process of constructing our houses or from vehicle or foot traffic, many of our yards suffer from compacted soil. Whatever we plant there doesn’t thrive, and the weeds that can deal with the challenges of compacted soil eventually take over. Quack grass, anyone?
My yard is still in recovery from some major house projects. Many years ago, we moved an old house slated for demolition to an empty lot, and several years later we added five wells for a geothermal system. We’ve had a lot of big machines in our yard — backhoes, a truck carrying our 57-ton house, skid loaders, bobcats, well-digging equipment, and a bunch of other gizmos I don’t even know the names of.
When work on our house was completed the contractors did what they tend to do at the end of construction jobs: they brought in a few inches of topsoil, spread it around with a skid loader, and told me I was all set to plant. Note to the wise: best not to take gardening advice from building contractors.
I planted some grass seed to secure the soil for the winter, and spent many seasons afterwards struggling to make my shrubs and perennials happy. It was only when I learned techniques for reinvigorating our inhospitable soil that my plants started thriving.
What is soil compaction?
Healthy soil is about 50% air space where water, air, and nutrients can circulate. Plants need these spaces in the soil so their roots can reach out to get the nutrients and water they require. When soil particles get compressed, the space between them diminishes, making it hard for roots to penetrate. Water and air can’t infiltrate or move through soils without those pore spaces. Nina Bassuk of Cornell’s Urban Horticulture Institute says, “Soil compaction is the single most difficult and harmful… condition that a tree or shrub can experience.”
Beneficial microorganisms also need oxygen, and they can’t do their important work of decomposing organic matter without those air spaces in the soil.
What Causes Soil Compaction?
Soil can become compacted in a variety of ways:
- Heavy machinery — from the big machines used in new construction to riding lawn mowers and cars parked in the yard
- Foot traffic, which is why you don’t want to walk on your growing space
- Tilling soil when it’s too wet or dry
- Leaving soil bare so rain water forms a crust on the surface
Heavy clay soil is especially prone to compaction, as is soil without enough organic matter.
Symptoms of soil compaction
How to tell if your soil is compacted:
- Soil is hard to dig, whether it’s wet or dry
- Standing water puddles up, indicating a drainage problem
- Plants’ growth is stunted or leaves are discolored, implying nutrient deficiencies
Certain weeds tend to take over areas with compacted soil, where other plants can’t survive. If you have quack grass, bindweed, knotweed, chicory, mustards or plantain, your yard might be telling you your soil needs attention.
You can use a soil probe or hollow metal pipe to test your soil for compaction. If you can’t get the probe to enter the soil easily, you have compacted soil.
How to Fix Compaction
An ounce of prevention is your first line of defense. Try not to till when the soil’s too wet or dry, and avoid using heavy machinery (including riding mowers) or walking on planting beds when possible. Mulching well can help protect the soil against compaction, while helping to retain moisture and reduce weed competition.
Because bare soil is prone to surface compaction when rain falls and creates a crust at the surface, covering soil with mulch or growing groundcovers can help as well.
If the damage is already done, there are a few methods you can use to improve your soil’s structure:
If you’re trying to fix a compacted lawn, an aerator will help. Aerating the lawn around trees can help loosen the soil so their roots can penetrate further.
In smaller areas, you may also bury your compacted soil or replace it entirely, but be sure to add 1 ½ to 3 feet in depth or your plants’ roots won’t be able to penetrate deeply enough to get available nutrients.
Put Nature to Work:
- If your soil compaction doesn’t go too deep, natural freeze-thaw cycles may help loosen it. If your compaction is from heavy machinery, freeze-thaw cycles probably won’t be enough.
- Put those beneficial organisms to work for you. Working organic matter like compost into the soil is the most effective way to treat compacted soils. The soil organisms that break down organic matter aerate the soil in the process.
If your soil is badly compacted, you may need to add up to 50% compost to existing soil (for clay soils; it may be closer to 25% for sandier soils. Incorporate compost to a depth of 18 inches.
Some gardeners recommend adding sand to amend clay soils, but numerous university extension experts advise against this, as adding too little can actually make the problem worse. Since inorganic amendments may need to be added at up to 75% of existing soil by weight, this method is considered a far less efficient and reliable — and more expensive — way to treat soil compaction than with organic materials.
Agroecologist and permaculture designer Paula Westmoreland uses long-term strategies to address soil health and remedy compaction. When she prepares an area for planting, she lays down thick layers of soil and mulch and adds compost tea full of beneficial microorganisms. She waters the area slowly to help break up the soil and encourage microbes over a period of many months. Come planting time, the soil has been loosened by microorganisms that will also help plants thrive.
Do you have an area you think has compacted soil? Apply some of the simple checks above and start taking steps to restore its health. You’ll notice a striking difference in the growth and vitality of your plants once compacted soil is restored to its natural condition. Your plants will thank you for it.
Further resources for optimizing soil:
Susannah Shmurak is an enthusiastic advocate for healthier, more sustainable lifestyles. She shares practical tips about gardening, food, and low-impact living at HealthyGreenSavvy.com.