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Be a Better Gardener: Soil know-how

Dorthe Hviid, Berkshire Botanical Garden’s Director of Horticulture, examines soil in one of the Garden’s 24 beds in Stockbridge, Mass.
August 30, 2018 11:34 am Updated: August 30, 2018 11:44 am


Once, when I was a student potting up seedlings for an elderly gardener, I made the mistake of referring to what I was working with as “dirt.”

My boss grabbed a pinch of the potting mix, held it up to my face and barked, “This is soil.” He threw it on the floor and said: “Now it’s dirt.”

The point he was making was that soil is, to the gardener, a precious commodity that must be understood and handled with respect — not treated like dirt.

The basis for treating the soil with respect is understanding it, for only then can you appreciate its needs and proper management. Fortunately, it’s not hard to develop a fundamental appreciation of this sort.

The first step is to have your soil tested. I’ve advocated this before as a matter of thrift and environmental responsibility. Knowing just what nutrients your soil needs saves you from over-dosing with fertilizers, a mistake that wastes money and promotes water pollution.

But the really important part of the entry-level soil test is the knowledge it will give you about your soil’s pH, its level of acidity or alkalinity. Knowing this will help you to choose plants that are naturally adapted to your soil. If your soil is markedly acidic, for example, you would be better advised to plant acid-loving rhododendrons, azaleas or hollies, for example, rather than lilacs (which prefer alkaline soils) or yews (which prefer a neutral pH). You could try to change the pH of your soil by adding lime, but that’s at best a temporary fix, as the conditions that made your soil acidic in the first place are almost certain to re-assert themselves. It’s much smarter to match your planting to the natural pH of your soil than to try to fight nature.

Another characteristic of your soil that is important for a gardener to investigate is its texture. “The definition of texture,” explains Dorthe Hviid, director of horticulture at Berkshire Botanical Garden, “is the amount of the various sizes of particles of sand, silt and clay contained in the soil.” To determine what your soil consists of, wet a pinch of soil in the palm of your hand, she suggests, and rub it with your forefinger to determine the amount of sand, silt and clay in it. If it feels gritty, the soil is of sandy texture; if it feels smooth like flour, it is silty, and if it has a sticky feel it would be classified as a clay soil.

There are pros and cons to each of these textures depending on what you are planning on growing, Hviid points out. Sand has the largest particles.This means that sandy soil drains well and promotes root growth, but it also dries out more quickly and is not as fertile as nutrients move through the large particles of sand faster. If you need to fertilize sandy soil fertilize at a reduced rate, but more often.

Silty soils consist of medium-sized particles and hold nutrients and water better than sandy soils and are more prone to compaction if they are walked on or cultivated while they are wet.

The smallest particles are found in clay. This makes clay soils even more vulnerable to soil compaction, Hviid warns. Clay soils hold water and nutrients best of all, but because the particles are so small they may lock together and prevent water from passing. This will cause the water to run off or not drain properly, which can make the soil water-logged.

Adding organic material such as compost helps improve most soils.The compost acts like little sponges holding moisture and nutrients, Hviid explains, and makes the soil more friable and airy. This is especially important in dense clay soils, as the organic matter improves the infiltration rate.

Most soils are a mixture of sand, silt and clay. Determining which predominates in your soil will give you valuable information about how to create an ideal soil for the plants you intend on growing.

Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, Mass. Its mission to provide knowledge of gardening and the environment through 25 display gardens and a diverse range of classes informs and inspires thousands of students and visitors on horticultural topics every year. Thomas Christopher is the co-author of Garden Revolution and is a volunteer at Berkshire Botanical Garden.