Healthy soil is essential to successful plant growth; it physically supports plants and supplies them with water, air, and a range of mineral nutrients. If the soil in your garden is not ideal it can be improved with a little time and effort. It is possible to improve it’s structure with the addition of bulky compost or well-rotted manure. Extra nutrients are easily added to the soil with fertilizers, and lime may be added to make acidic soils more alkaline. Mulches and top-dressings improve plant growth by preventing weed seeds from germinating and reducing water loss from the soil during the summer months. For container-grown plants there is a wide range of potting composts to suit every purpose.
Most soil is classified according to its clay, silt, and sand content. The size and proportion of these mineral particles affect the chemical and physical behaviour of the soil. Loam soils have the ideal balance of mineral particle sizes, with between 8 and 25 per cent clay, which results in good drainage and water retention, combined with high fertility.
Clay – Clay soils have more than 25% clay particles and are characteristically wet and sticky. Generally heavy, slow draining, and slow to warm up in spring, but often highly fertile. They are easily compacted, however, and may bake hard in summer.
Soils containing less than 8% clay are classified as either silt or sandy soil, depending on whether silt or sand particles predominate. Both sandy and silt soils have a low proportion of clay particles, making them much less moisture-retentive than clay.
Sandy soil – A dry, light, free-draining soil, easy to work but relatively infertile. Sandy soils are particularly light and free-draining and need frequent irrigation and feeding; however, they warm up quickly in spring and are easily improved with organic matter.
Silt – Silt is reasonably moisture-retentive and fertile but compacts easily.
Peat – Rich in organic matter, dark and moisture-retentive. Peat is formed where wet, acid conditions prevent full decomposition of organic matter, which remains on or near the soil surface. Organic or peat soils are wet and acidic; they support excellent plant growth, however, if drained, fertilized and limed.
Chalk – Pale, shallow, and stony, chalk is free-draining and moderately fertile. Chalky soil, however, is alkaline and free-draining, and allows organic matter to decompose rapidly; they have moderate fertility.
Soil is usually made up of topsoil and subsoil, and a lower layer derived from underlying rock. The depth of each layer can vary. Topsoil contains most soil organisms and many of the nutrients. It is generally dark, because it contains organic matter that is added artificially or naturally by leaf fall. Subsoil is usually lighter in colour; if it is white, the parent rock is probably chalk or limestone. If there is little or no colour change beteween topsoil and subsoil, the topsoil may be deficient in organic matter.
Identifying your soil
Rub a small amount of moist soil between your fingers. A sandy soil feels quite gritty and will not stick together or form a ball, although sandy loam is slightly more cohesive. A silt soil feels silky or soapy to the touch. A silty loam may show imprints when pressed with a finger. A loamy clay soil holds together well and may be rolled into a cylindrical shape. Heavy clay soil may be rolled even more thinly, and develops a shiny streak when smoothed. All clay soils feel sticky and slightly heavy.
Acidity and alkalinity
Soil pH is a measurement of acidity or alkalinity – the scale ranges from 1-14. A pH below 7 indicates an acid soil, while a pH higher than 7 indicates an alkaline soil. Neutral soil has a pH of 7. The pH of soil is usually controlled by its calcium level. Calcium is an alkaline element that almost all soils tend to lose through leaching (meaning that it is washed through the soil by water). Soils over chalk or limestone, which are rich in calcium, are more or less unaffected; other soils, especially sands, gradually turn more acidic. Alkalinity may be increased, if necessary, by liming.
Electric pH meters and soil test kits may be used to measure soil pH. Make several tests across the garden, as the pH often varies; readings are particularly unreliable after liming. Soil tests kits use a chemical solution that changes colour when mixed with soil in a small test tube. The colour is then matched against a chart that indicates the pH level of the soil sample. A yellow or orange colour indicates acid soil, bright green indicates neutral soil and dark green indicates alkaline soil.
The effects of pH
Above all, pH affects the solubility of soil minerals and hence their availability to plants. Acid soils tend to be deficient in phosphorus and sometimes contain excess manganese and aluminium. Alkaline soils tend to lack manganese, boron, and phosphorus.
Soil pH also affects the number and type of beneficial soil organisms, as well as pests and diseases. For example, worms dislike acid soils, but clubroot, leatherjackets, and wireworms are common in acid conditions. On alkaline soils, potato scab occurs more frequently.
The pH range for good plant growth is between 5.5 and 7,5. A pH of 6.5 is usually optimum, depending on the plants to be grown. Peaty soils have an optimum pH of 5.8, however. The highest vegetable yields are generally obtained from neutral soils but most ornimentals tolerate a wide pH range. Some are more sensitive; calcicoles (lime lovers) and calcifuges (lime haters) are adapted to extreme pH ranges and their growth suffer if they are planted in soil with the wrong pH level.
Certain soil organisms are essential to maintian soil fertility. Beneficial bacteria and fungi prefer well-aerated soil and generally tolerate a wide pH, although most fungi prefer acidic conditions. One group of fungi (mycorrhizae) live in association with plant roots, and improve the take-up of nutrients from the soil. Healthy soil contains a teeming community of earthworms and other organisms, which help to aerate the soil and break down organic matter.