Far from being mere “dirt”, garden soil is the raw material with amazing properties out of which you create your gardening dreams. Plant roots find moisture and food in the spaces between soil particles, and the soil’s job is to hold enough moisture to sustain the garden in dry spells, but allow surplus water to drain away in wet weather. It also protects roots from heat and cold. Soil is just a thin skin on the planet’s surface, but all life on land depends on it.
Assessing your soil is essential for successful planting. While some plants tolerate a range of soil types, many have definite preferences. Dig an inspection pit; it will help you evaluate your soil in terms of the following factors.
- Texture: This can vary from sandy through to loam, to clay, and all types in between.
- Depth of topsoil or uppermost, fertile layer—the deeper, the better.
- Type and depth of subsoil: Your garden might reveal surprising combinations, such as gravelly soil over clay, or even acidic loam over limestone. If your topsoil is shallow, the subsoil is more important, particularly for trees and larger shrubs that need deep roots for stability.
- Moisture content: Different soil types have different water-holding capacities; a little rain may drain through sand quickly, while the same amount can make a clay soil quite wet.
Clay Soil or Sandy Soil?
The size of soil particles determines its texture and what grows best in it. Take a handful of moist topsoil, and shape it into a ball. Try to work it into a sausage shape, and rub it between your fingers. The “ideal” soil is loam, which will hold together in a ball and show finger impressions, without being sticky. Most soils, however, tend toward either clay or sand.
Acidic Soil or Alkaline Soil?
Your soil’s acidity or alkalinity affects what plants you can grow. Some, like rhododendrons, do not grow well in an alkaline soil, while others, such as clematis, thrive on alkaline, or “limy,” sites. The term pH is a way of expressing how acidic or alkaline a substance is: the lower the pH number, the more acidic the soil. Acidity and alkalinity depend on how much calcium is in the soil. Acidic soil lacks calcium and alkaline soil has it in excess, while neutral soil (7 on the pH scale) has just enough calcium to mop up acidity.
- Alkaline soils have a pH of over 7. You cannot make them acidic in the long-term, and acid-loving plants will certainly fail. However, limy soil is often well-drained and quick to warm up in spring. Incorporate plenty of organic matter and a wide range of plants will thrive, but don’t overfertilize—this results in tall, floppy plants.
- Acidic soils have a pH of 1–6. They are ideal for a wide range of plants, but acid-loving or “ericaceous” plants are almost impossible to grow elsewhere. If your soil is not acidic, you can grow them in containers filled with ericaceous compost. You can make acidic soil more alkaline by adding lime, but think carefully before using it. Most plants will put up with some acidity, and you can grow a wider range in mildly acidic soils. In theory, you can add acidic material to limy soils to lower the pH, but acidifying materials, such as sulfur dust, act slowly, are needed in very large amounts, and are expensive. Clay soil feels smooth and sticky, will roll into a sausage and bend into a loop, and is shiny when rubbed. The tiny particles pack together so water does not easily drain, and air spaces are minute. It is slow to warm up in spring and sticky to work.