Why is my soil so compacted?

Hopefully, the ground in your yard has thawed by now (regrets to those who still have frozen ground!). So why is it that when you try to push your shovel into the soil, it doesn’t budge? If it’s hard for you to push that shovel, it’s even harder for plant roots to penetrate this type of soil.

Soil scientists refer to this as soil compaction, or compressed soil that is reduced in volume. Why does this happen and what can you do about it?

Let’s start with some soil basics. Soils are comprised of three major things – air, water, and solid materials. It might seem counter intuitive but a healthy soil should only have about half of its volume full of the solid materials. The other half should either be empty or consist of water. That empty air space is critical to ensure that gas exchange (carbon dioxide and oxygen for and from plant roots) and water can move throughout. Compaction occurs when there is less free (air) space. Soil is compressed or higher in density.

The solid materials of the soil include mineral and organic materials. Mineral portions are made of the remains of the underlying rock (parent) material. The organic portions are comprised of the remains of living things such as plant roots, plant leaves, or microorganisms.

To understand soil compaction, think of soil as made of bricks. In our example, tan and grey represent mineral materials. Dark brown represents organic matter. Blue represents water, and empty spaces are air. A healthy, uncompacted soil has enough space for water and air – and plant roots!

A useful way to visualize this is to think about soil like a pile of bricks — different soils in different areas, and different sized bricks. In addition, the bricks are oriented in many different ways. In a healthy soil, the bricks would be stacked or organized to make it easy for water and air to move in between the solid particles.

Organic matter is the other critical piece of the puzzle. Of the solid materials, the organic matter is the smaller fraction by volume. But organic matter is key to ensuring that those bricks or particles aggregate and associate together, kind of like a healthy glue.

Compacted soil is a problem for gardeners and farmers alike for a few reasons. It means that there may not be adequate space for roots to properly penetrate the soil, which might prevent plants from taking in nutrients and water. It could also mean that rainwater or irrigation water is less likely to infiltrate, or enter the soil. That means that rainwater and snowmelt can run over the surface of the soil and into stormwater systems – and even cause erosion.

Heavy construction equipment, other vehicles, and even foot traffic, can compact your soil. Source: Morguefile

How does soil get compacted? If you have new construction, all those construction vehicles on your soil caused compaction. Or perhaps you’ve been letting your garden area lie bare over the winter – or longer. Bare soil is prone to erosion of the “good stuff”. Loose, uncompacted soil can blow away with the wind, or be carried away with rainfall and snow melt.

What can you do to reduce compacted soil? Managing compacted soil requires both taking care of soil structure (remember the bricks being properly organized!) as well as increasing the organic portion.

Gardeners might have the instinct to use a rototiller to plow the soil and soften it. Limited and low intensity plowing in a garden may sometimes be necessary and beneficial. However, even if plowing softens soil in the short term, in the long term it can decrease the healthy structure of soil aggregates (again, the orientation of the hypothetical pile of bricks). Tilling also hurts fungi, bacteria and other life in soil. For these reasons, we don’t recommend that home gardeners till their gardens very often, if at all.

Experts agree that any activities that put pressure on the soil surface should be kept to a minimum. Excessive plowing in a garden can also reduce organic matter. So, too much plowing can dig your soil into a deeper hole – pun intended – where you have a negative cycle of declining soil structure and a smaller organic fraction.

Here a few other important tips:

• Compost is a great and fast way to get more organic matter into your soil. Using hand tools, work leaves, mulch or other organic materials into the top several inches of the soil. Organic matter is softer than mineral matter, and it helps increase the amount of air and water space. In addition, it adds needed nutrients for plants to grow – and plant roots reduce compaction.
• Cover the soil as much as possible with cover crops. This includes in the winter! Growing plants at the end of the season (or after you clear a section of the garden) increases the amount of time that plants work their magic and convert sunlight into carbon-containing substances. Cover crop plant roots also create pores in the soil, biologically reducing compaction, and add to the organic portion of your soil. You can buy radish seeds and let them grow early in the season – nothing like large tap roots to naturally break up the soil! Or, plant them late, and leave them in place over the winter.
• Avoid activities in your garden area during wet days. Soil is more susceptible to compaction when it is wet. This will prevent further compaction. Even foot traffic compacts soil.
• Create foot paths in your garden. Limit walking traffic to specified paths.

You might not see results overnight but trust that with minimum soil disturbance, and the addition of compost and cover crops, you will activate the biology of your soil. Expect earthworms and other critters to follow, which becomes a healthy cycle that long-term will lead to less compaction. Reducing the compaction in your soil will help with the long-term health of your soil, the animals that live in it, and your plants!

Answered by Andrea Basche, Union of Concerned Scientists

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To view a video about soil compaction in gardens and farms, watch this YouTube video, produced by the American Society of Agronomy: https://www.youtube.com/watch?v=epONiSWwaTw