Marin Master Gardeners
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Providing Drainage – the Spaces in Between

February 19, 2007
Barbara Euser
by Barbara J. Euser
A friend of mine has devised a product for use with potted plants that holds moisture and promotes drainage at the same time. What an interesting idea, I thought. I began reflecting on soil and what it really is—the particles that compose soil and the importance of the spaces in between. The particles of soil are referred to as the solid phase; the spaces in between are called the pore space.
The mineral particles that make up soil can be categorized by size. The tiniest particles are called clay. Medium-size particles are called silt. Large particles are called sand. When clay soil is moist, you can form it into a ball and then squeeze it into a ribbon between your fingers and it will retain its shape. By contrast, if you try to form a ribbon of sand in your hand, it will fall apart. Silt is somewhere in between.
The size of the soil particles determines how much pore space there is for air and water. Tiny clay particles get so packed together that water cannot flow through them. Sand particles have so much space between them that water flows through practically unimpeded. The roots of plants planted in clay soil may suffer from over watering and rot because water remains caught in the soil, whereas plants planted in sandy soil may dry out because they cannot capture sufficient water as it passes through the soil.
The ideal soil for many plants is loam. That is the dark, crumbly soil that contains not only mineral particles but also particles of organic matter, such as decaying leaves. By mixing compost—decaying leaves and other organic matter—into soil, whether clay or silt or sand, we can moderate the rate at which soil drains. Adding compost to clay soil will cause it to drain more quickly, whereas adding compost to sandy soil will help the sandy soil retain moisture and drain more slowly.
The space in between the mineral particles of soil—the pore space—also controls the level of air flow to plants’ roots. Plants require air to grow. Unlike animals, plants use the carbon dioxide in the air to create carbohydrates and create oxygen as their respiratory waste product.
Ideally, soil also provides nourishment for organisms such as earthworms that reside there. Organic matter, such as compost, provides nutrients for the animals and bacteria and fungus that live in the soil.
According to the California Master Gardener Handbook, the four principal components of soil with ideal moisture content for plant growth are: pore space of 25 percent water and 25 percent air, and solid phase of 45 percent minerals and 5 percent organic matter.
Even after we conscientiously mix organic matter into the soil in our gardens, we will still have either a basically clay or sandy soil. To develop a thriving garden, we must choose plants compatible with the type of soil we have.
When gardening in containers, it is easier to control the quality of the soil. Commercially available potting soils contain bark or wood chips to hold water and create air pockets, and sand or vermiculite to provide structure. Potting soil will become worn out over time, that is, the minerals will have been used by the plant or have been washed out by continued watering. A soluble fertilizer will help replace missing minerals. Plants should be repotted with new soil when they outgrow their containers. When potting or repotting, if soil falls out of the drainage hole in the bottom of the container, a few broken pot shards can be placed over drainage holes to prevent soil loss. Any container in which plants are grown must have drainage holes. Soil should be as close as possible to the ideal mixture given above in order to drain properly.
Which brings us back to my friend Chris Burge and her “PotHoles.” Chris devised drainage discs to put in the bottom of containers to prevent soil loss and permit drainage. The pellets in the plastic mesh cover are hydrophilic material that retains moisture. The discs can be cleaned and reused.
Gardening was an important part of Chris’ life. Working in her garden comforted her and sustained her, even as she suffered from the cancer that recently ended her life. Developing PotHoles to make gardening easier, especially for city dwellers, was one of her final endeavors. A portion of the profits from the sales of PotHoles goes to cancer research. PotHoles may be obtained through the website
Whether you are gardening in a plot of ground or in containers on the balcony, the quality of the soil provides the basis for the health of our plants. Think about the type of particles that make up your soil, and remember the importance of the spaces in between.

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