What is soil?

The organic matter we think of as compost is present in all good soils, but the soil itself is much more complex. Rocks, roots, decomposed plant material, soil-dwelling animals, and microorganisms are all found in a typical soil, but not all soils are the same. Learning more about the type in your garden will help you to choose plants that will thrive there and to know how your compost can improve it.

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The type of soil you have in your garden will determine which plants and crops will thrive there.

Plumbing the depths

Most people think of soil as the brown earthy material that plants grow in, and that is partly true. The crumbly or muddy material that comprises this upper layer, which is usually about 12–16in (30–40cm) deep, is known as “topsoil.” Below that is “subsoil,” which forms a deeper layer, reaching down 4ft (1.2m) or more into the ground. Beneath it is the bedrock on which soils form. Gardeners are mainly concerned with the fertile topsoil, where most plant growth occurs, but the subsoil also plays its part; it helps to store water and air for the long roots of larger plants, including trees and shrubs, to tap into. The bedrock influences the size of the soil particles. Known as its “texture,” these determine whether the soil is free-draining or water-absorbent. The bedrock also affects a soil’s acidity, alkalinity, or neutrality.

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Your garden soil is influenced by the bedrock and its organic matter content.

How compost affects the soil

Compost is the magic ingredient that will improve the growing conditions in your garden, whatever the soil type. The sticky black substance in compost known as “humus” coats sand and silt particles, helping them to retain more water and nutrients. In clay soils, it binds the tiny particles together to form larger aggregates with wider channels between them that allow air and water to pass through, thereby improving the soil’s drainage capacity. Compost also attracts soil-borne creatures that feed on it, and they also open up passageways for water and air, as well as releasing plant nutrients via their faeces. Worms in particular are key to this process. In addition, compost introduces microbes that increase fertility and support the growth of healthy plant roots, which also helps to bind soils together, preventing erosion. All of these factors combine to create soils that hold water and nutrients, yet also drain well, providing the perfect conditions for plant growth.

testing your soil

To find out which type of soil you have, take a sample about the size of a golf ball from just below the surface a few days after it has rained. Let it dry out until it is just damp, then rub it between your fingers. Sandy soil will feel gritty and when you try to roll it into a ball, it will fall apart. Silty soil feels slippery and will not form a solid ball either. Clay soil is sticky and smooth to the touch and will keep its shape, with a shiny surface, when rolled into a ball.

THE ACID TEST As well as knowing your soil’s texture and structure, it is also important to establish its pH value, which is a measurement of its acidity, neutrality, or alkalinity. The pH value will influence your plant choices and it is easy to check using a soil-testing kit. Test a few samples taken from around the garden—you may find their values differ slightly in a large space. Dig up a sample from just below the surface and follow the instructions on the kit.

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A soil-testing kit is an easy way to find out if your soil is acid, neutral, or alkaline.

Why Compost? | WHAT IS SOIL?

SOIL TYPES

Soils are divided into three main groups: sand, silt, and clay. There are other soils that are not as widespread, such as chalk and peat, but these are rarely found in gardens. While all soils contain a combination of different ingredients, one sediment is often dominant. Soil that contains almost equal measures of sand and clay particles is known as “loam,” which provides the perfect conditions for growing crops and many ornamental plants. However, there are plants that have adapted to all soil types, so you can still have a garden full of flowers and foliage if you do not have loam. You can also improve your soil by adding homemade compost, which will widen your planting choices.

SANDY SOILS These are made up of relatively large particles, which you can often see with the naked eye. The particles do not absorb moisture and there are comparatively wide gaps between them that allow water to drain through. This is why sandy soils dry out quickly, and because plant nutrients are dissolved in water, they also drain away, which makes these soils relatively infertile. Sandy soils also warm up quickly in spring but cool down just as fast in winter. They are easy to cultivate and are described as “light” for this reason, not because they are lighter in weight than other soils.

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Drought-tolerant plants such as lavender, salvia, and thyme thrive in sandy soils.

SILTY SOILS The particles in silt are smaller than sand but larger than clay. The gaps between them allow good drainage, but are small enough to hold some water and nutrients in the soil to boost fertility. Silty soils are not common in gardens and are generally found near rivers or in areas where rivers or seas previously existed.

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Many fruit and vegetable crops will grow well in silty soils as long as they are free-draining.

CLAY SOILS These soils contain minute clay particles that can only be seen through a microscope. They are absorbent and the tiny gaps between them hold water in suspension rather than allowing it to drain through, which makes clay soils prone to waterlogging. They tend to be very fertile, but take longer than either sand or silt to warm up in spring. They also dry out and crack during the summer. They are described as “heavy” soils because they are difficult to dig.

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Clay soil holds enough water to form a sticky ball when it is rolled in your hands.

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