Many plants grow best in moist, well-drained soils. But what is “well-drained” soil? And if we don’t have it, how do we get it? The rate at which water moves down through the soil profile is influenced by pore size in the soil. When soil is not well-drained, it has smaller pores and retains water for longer periods of time. Excess water in the soil displaces oxygen. Low levels of oxygen in the soil will negatively affect the root health of plants.
Soil Drainage is Affected by Soil Structure
Soil is composed of particles of sand, silt, and clay that vary in size. Sand has the largest particle size and clay has the smallest. Soils are typically a combination of all three particle sizes. When soils have a large percentage of sand, they tend to have more pore space and are more sharply drained. Soils with a high percentage of clay tend to have much less pore space. This reduced pore space can impede water drainage and hold water for longer periods of time.
Compaction Heavily Influences Soil Drainage
When soils are not managed well, they can become compacted. This is particularly common during construction. Frequent walking or driving over soil compresses the soil pores and limits the oxygen and water that can move into the soil. Tilling too much or too frequently can break down soil aggregates reducing its ability to drain well. Improper soil management can also lead to the formation of hard pans or impermeable layers that are impervious to water.
Depressions, low areas, sites with high water tables, and those that have underlying rock layers can also be slow to drain. Conversely, sloping sites tend to be more well-drained. However, don’t assume that sloping sites are always well-drained. Locations with high clay percentages or those with hard pans can still be poorly drained, even when sloped.
How to Test Soil Drainage
A percolation test of the soil (perk test) is the best way to determine how well-drained the soil is, even on the side of a slope. To perform a perk test, use a shovel or post-hole digger to dig a hole at least 12 inches down and 4 to 12 inches wide. It can be deep as 18 inches, especially if the rootballs of the plants you are planting are taller than 12 inches. Avoid working in overly wet soils as it may glaze the side of the test hole, which can give false test results. A planting hole can be used for this test to avoid excess digging.
Fill the hole with water and let it drain. This could take just a short period or an entire day. This step saturates the soil. Immediately after the water drains for the first time, refill the hole with water. Measure the depth of the water with a ruler and after 15 minutes, measure the drop in water in inches. Multiply this number by 4 to calculate how much water drains in an hour.
Analyze the Results
Soils that drain 1 to 3 inches an hour are desirable for most plants. Soils that drain less than one inch per hour have poor drainage. You will want to improve drainage or select plants that tolerate wet growing conditions for these locations. Soils that drain more than 4 inches of water per hour are very well-drained. These soils can be amended or planted with species that tolerate drier soil conditions.
What to do if Soil Drainage is not Ideal
There are several options home gardeners can take to address soils with less-than-ideal drainage.
Select the Right Plants
Different species of plants have different levels of tolerance for either wet or dry soils. If drainage is poor, plant species that tolerate wet conditions. Trees that tolerate wet soils include bald cypress (Taxodium distichum) and black gum (Nyssa sylvatica). Shrubs that don’t mind wet conditions include buttonbush (Cephalanthus occidentalis) and redosier dogwood (Cornus sericea). Perennials that grow in wet soils include rose mallow (Hibiscus moscheutos) and queen of the prairie (Filipendula rubra).
Amend the Soil with Compost
Add organic matter (not sand) to improve drainage in poorly drained soils. Incorporating organic matter, such as compost, improves soil structure in poorly drained soils by increasing soil aggregation. Soil aggregates are groups of soil particles. The space between the aggregates provides pore space for holding and exchanging air and water. Adding organic matter is also ideal for improving very well-drained soils. Incorporating organic matter into sharply drained soils improves the water, air, and nutrient holding capacity.
Spread compost 2 to 4 inches thick over the soil surface and work it into the top 6 to 12 inches of soil by spading with a shovel or garden fork or through tillage. Do not over till. Using a rototiller too much or too frequently breaks down soil aggregates.
Physically Break-up Soil and Prevent Compaction
Whenever possible, avoid soil compaction. Reduce walking or driving on soil and never work in wet soils. Tilling, spading, trenching, or aeration can be used to physically break up hardpans or impermeable layers. Incorporating organic matter during tilling or after core-aeration can further improve soil conditions.
Plant Cover Crops
The use of cover crops, especially in vegetable gardens or annual planting beds, can help improve soil drainage in those garden areas. Cover crops increase soil organic matter, and their growing roots can help break apart compacted soils.
Redirect Water and Build a Rain Garden
Water either moves across the soil surface to a lower elevation or down through the soil profile. Change the soil elevation to remove depressions or redirect water from uphill or downspouts to reduce the amount of water in the plant’s root zone. Don’t just redirect the water to someone else’s yard, however. Install a rain garden to collect redirected water to allow more time to infiltrate the soil. The plants in a rain garden are selected specifically to tolerate these conditions.
Build a Raised Bed
Constructing a raised bed or berm is a great way to create soil conditions that are just right for the plants you wish to grow. The raised bed must be large enough to contain the entire root system of the plants you’re growing. For that reason, raised beds work best for smaller plants like vegetables, perennials, or small shrubs.