By Erin Marissa Russell
Want to know how to improve soil drainage in your garden? One thing you’ll see on the care instructions for almost any plant is good drainage. Let’s take a look at why soil drainage is important and how to test your soil’s drainage as well as some ways to improve your own soil drainage.
Soil that offers good drainage is not compacted, so it allows air and water to filter through the soil where it becomes available to your plants’ roots. Both air and water are crucial for the health of your plants. However, there must not be too much moisture waterlogging the soil.
Where there is too much moisture, oxygen is not available to your plants’ roots. Soil that has more drainage filters the water down through the soil, keeping your plants from becoming overly saturated.
Where there is plenty of soil drainage, the root health of plants growing there is improved. One reason root health is important is that roots protect plants from erosion by holding on to the soil.
The fine root hairs, called feeder roots, growing at the tips of root systems are responsible for most of the work of absorbing water and nutrition. New feeder roots replace the older ones after they fade away. But if oxygen is not available to the feeder roots, they can stop working or die off at a faster rate than normal. Soils that offer good drainage keep this from happening by filtering away the water so it can be replaced by oxygen.
You may already suspect that your soil has poor drainage, especially if you see standing water in your garden after rainfall or irrigation. Clay soil is especially notorious for having poor drainage, while sandy soil has very good drainage. But you don’t have to rely on your suspicion that your drainage is poor; there’s an easy way to test drainage in your garden. If you’re digging planting holes of the right size already, you won’t even have to do any extra digging.
You can test your soil drainage with a percolation test, also called a perk test. This is an easy test to perform, and you can do it on your own. Dig a hole at least a foot deep and four inches to a foot wide using a shovel, hand auger, or post hole digger. (If the root balls of the plants growing where you are testing are more than a foot deep, you can extend the depth of your hole up to 18 inches.)
Start by filling the hole you have dug with water, then letting it drain away. This can be a quick process, or it can take up to 24 hours. Once the water has drained, fill the hole with water again. Measure the depth of the water with a ruler. After 15 minutes, measure the water again and subtract the second reading from the first to get the drop of the water line in inches. Then multiply the drop of the water line by four to determine inches dropped per hour.
Most plants perform well in water that drains one to three inches per hour. If your soil drains less than an inch per hour, you have poor drainage. If your soil scored less than an inch per hour, you will either want to follow the improvement tips in this article or choose plants that do well in wet soil. If your soil scored more than four inches per hour, you have very good drainage and can either use soil amendments or choose plants that thrive in dry soil.
- You can amend your soil to improve its drainage by adding organic material, such as compost, manure, or shredded leaves. This technique will work whether your soil has poor drainage or very good drainage. It works by increasing the number of soil aggregates, which are groups of soil particles with lots of space between them for oxygen or water. Organic materials will work better and add more nutrition than other substances, like sand or perlite.
To amend your soil with compost, start by spreading a layer of compost two to four inches thick. Work the compost into the soil down to the top six inches to one foot by hand with a garden fork or a spade, or you can use a tiller. (Using a tiller too often will actually impair soil drainage by breaking up soil aggregates.)
Organic materials like compost are recommended for amending clay soils. You will often see references to amending clay soil with sand, but we do not recommend it. Amending clay soil with sand only works at a ratio much higher than is practical for most gardeners; otherwise, it creates a mixture similar to concrete.
- An alternative to soil amendment is the use of green manures or cover crops. These crops are planted in the fall and killed in early spring before they go to seed, either by mowing or by glyphosate. The green manure provides nutrition and aeration to the soil. For more information, check out our article How to Use Cover Crops with a Home Garden.
- If you’re growing just a couple of different types of plants in areas where soil drainage is a problem, try adding more diversity with new plants. As a result, you’ll have a more diverse range of microbes in the soil, which contributes to soil health.
- Break up soil manually using spading, trenching, or aeration techniques. Although tilling too frequently can cause soil to become compacted, you can till once to improve aeration, though it will disturb your earthworm population. Major problems with tilling come when people overtill their soil.
- To keep soil healthy, you should have plants growing there when the weather is cool as well as when it is warm. The roots of dormant plants are part of an ecosystem that contributes to the health of beneficial microbes in the soil.
- Soil particles are pressed together if compacted, impairing the healthy movement of oxygen and water through the soil. Avoid activities that cause soil compaction, including walking or driving on the soil or working in it while it is wet. Tilling your soil too frequently can also cause soil compaction. In general, you want to avoid interfering with the soil in any way so that natural processes like earthworm burrows and the spaces between soil aggregates can remain untouched. Disturb the soil as infrequently as you can.
- Redirecting water away from your garden can reduce the amount of water that comes into contact with your plants’ root zone. Different techniques can change the elevation in your garden to prevent it being a low-lying area. If your property is at a downspout or in a depression, redirecting water may be the solution for you.
- Hilling is traditionally used with potato plants, but you can apply it in areas with poor drainage no matter what you’re growing. Hilling involves creating a small hill (also called a berm) around each plant in the area with poor drainage. Hilling is really a method of redirecting water away from your plants, as some of it will soak into the hill and the rest will run off the sides away from the plants’ root zones.
- Installing a drain is another way to redirect water to improve soil drainage. You can use a French drain, the most common type of underground drain. A French drain is basically a ditch filled with gravel and covered with soil. You can also use subsurface tile drains, which are sections of perforated pipe laid 12 to 18 inches deep. Vertical drains can be used where you are planting trees or shrubs. They consist of three-foot to five-foot holes that have a diameter between 4 inches and 6 inches, dug next to the roots with an auger or post hole digger. These holes are filled with gravel, which maintains the structure of the soil while still allowing water to filter down and away from the root zone of your plants.
You don’t have to implement all of the tips we’ve provided here. Just choose a few that fit into your garden and your lifestyle. Before you know it, you’ll be seeing the signs of healthy soil, including flourishing plant life, a busy population of ants, worms, and beetles in the top six inches of soil, and a loose, deep soil texture that crumbles when moist. Your plants will look their best when they’re growing in healthy soil that offers plenty of drainage.