What The Heck Does Well-Drained Soil Actually Mean?

We get to the bottom of this common gardening term.

November 25, 2015
man digging into soil
PHOTOGRAPH BY JUSTIN SULLIVAN/GETTY

"Plant in well-drained soil” is a gardening axiom that’s commonplace in every planting manual. But what does it mean? How do you know if your soil drains well, and if it doesn’t, what can you do about it?

Good drainage starts with good soil structure, because well-structured soils contain enough pores—the gaps between soil particles—to allow air and water to flow freely. Ideally, the sand, silt, and clay particles are arranged so they occupy only half of the space, leaving the other half as pore space. Most plants grow best when about 50 percent of the pore space holds water.

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When it rains, precipitation percolates easily through well-drained soil. Plant roots can obtain both enough air and enough water in these conditions. But when the balance tips and soil retains more water than air and remains waterlogged for sustained periods, plant roots suffer from a lack of oxygen and will sooner or later suffocate. (Wetland plants are the exceptions. They have adapted to swampy conditions and prefer to have their roots wet.)

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An easy way to gauge how well your soil drains is to dig a square hole about 1-foot deep by 1-foot wide, fill it with water, and allow it to drain completely. Immediately refill the hole with water and place a ruler in it to measure the water’s depth. After 15 minutes, measure how much the water has fallen; multiply this by four to calculate how much the water will drain in one hour. An hourly drop of less than an inch indicates poor drainage. In well-drained soil, the drop in water level will be 1 to 6 inches per hour. Soils that drain more than 6 inches per hour are considered dry and droughty.

Soil can drain poorly for several reasons, manmade or natural. Soils with a high proportion of clay contain smaller pore spaces that release water very slowly, so clay-heavy soils are often dense and wet. Water tends to collect and pool in soil that is in a low-lying spot, underlaid by rock or an impervious hardpan layer, or sits above a high water table.

Improper landscaping can create low areas where water pools in the yard or around the foundation, and slow drainage is compounded by runoff from roofs, downspouts, and streets. Soils can also lose their natural porosity because of construction activities. Contractors may remove topsoil during construction, leaving only subsoil, and heavy equipment and traffic can compact soil, reducing pore space and creating a manmade hardpan. Tilling exposes soil particles to wind and water, so tilled soil is more easily compacted and eroded and more likely to lose organic matter. In compacted soil, the proportion of mineral solids rises to 60 to 80 percent of the total; the remaining pores are so small that water does not filter through them easily.

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Of course, the easiest solution to poor drainage is to accept soggy soil and choose plants that tolerate or require wet conditions. But gardeners who wish to grow more than bog plants have other options. One effective approach for dealing with slow drainage is to create raised beds filled with porous topsoil, thus keeping plant roots above the soggy soil.

Some drainage problems can be solved by shaping the ground surface with swales, or contoured depressions that divert excess water away from planting beds. Rain gardens—shallow catch basins planted as flowerbeds—collect water during rainstorms, allowing it to slowly percolate into the soil. A French drain, a rock- or gravel-filled trench, can also carry water away from planting areas to a catch basin or other outlet.

Organic matter amendments can improve soil’s water-holding capacity by aggregating soil particles and creating larger pores. Cover crops such as vetch or clover improve drainage both by breaking up heavy, compacted soil with their roots and by increasing soil’s organic matter content as they decompose.

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