Urban Farming: How to Check Soil Drainage

The soil particle size determines how well your urban garden will drain water. The microbes and plant roots need a balance of air and water in the soil to thrive, which is why proper soil water drainage is essential.

While some plants, such as cactuses, can survive on dry soil that drains water fast, and other plants, such as willows, can survive in temporary standing water, most plants need a well-drained soil to grow their best.

Brown patches on your lawn in midsummer while the rest of the lawn is green may be a sign that the soil under the grass is predominantly sand. If, after a rain, you have puddles of water in certain spots in your yard that last longer than in other sections of the yard, the soil in those spots is likely to be mostly clay.

If you want to be a little more exact about how well your soil drains, particularly where you want to place your garden, you can conduct a percolation test or a metal rod test. Read on for more.

How to perform a percolation test in your garden soil

To find out whether the spot where you plan to grow a garden or plant a tree is adequately drained, you can do a percolation test. Here are the steps to follow:

  1. Dig 1-foot by 1-foot holes in several places on your planting site.

  2. Let the soil dry out for a few days.

    Cover the holes to keep water out (and to make sure no one — human or beast — falls in).

  3. After the holes dry out, fill them with water and determine how long the holes take to drain completely.

    Use a timer so you don't forget what time you started. Then use the following list to determine you soil's likely drainage pattern:

    • If the water drains out within 10 minutes of filling it, the soil drains too well. It will probably dry out too fast for most plants.

    • If the water drains out within 30 minutes of filling it, the soil is still draining fast, but it's probably okay for plants that like well-drained soils.

    • If the water drains within 30 minutes to 4 hours of filling it, you have ideal drainage. Most plants thrive in this type of drainage situation.

    • If the water takes longer than 4 hours to drain, the soil is poorly drained and probably won't be good for most plants. It's best suited to plants that are adapted to wet soils, such as cattails and certain irises.

You can improve whatever drainage conditions you have by adding organic matter. Adding organic matter to the soil helps fast-draining soils retain more water and poorly drained ones to dry out faster. Organic matter really is the miracle additive for soil. And, of course, selecting plants that are adapted to your existing drainage conditions makes it more likely that they will grow and thrive.

How to perform a metal rod test in your garden soil

The metal-rod test, which helps you determine how well your soil drains, is particularly important in urban areas because you never know what's been buried under the soil you're trying to grow in.

Some garden areas have an impervious layer of soil called a hardpan, which is made of solid materials like asphalt, packed clay, or concrete. This layer can prevent soil water from draining, creating a wet environment for plants to grow in.

Knowing whether you have these materials under your garden and how deep down they are can help you decide whether to move your garden to a different spot or build raised beds on top of the soil instead.

The metal rod test couldn't be simpler. Just take a 1/2-inch-diameter metal rod and push it into the soil in different places around your garden. If you can push the rod down 6 to 8 inches without meeting any firm resistance, your soil doesn't have an impervious layer and it's okay to garden or plant in.

If you do find an impervious layer, you can dig down to see what it is, or you can simply build your garden up.

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