We cannot all achieve the perfect loam treasured by gardeners, but we can go a long way towards improving our soil, provided we understand it and know how to treat it.
Soil Type and Texture
Soil is the raw material of a garden, made up of four basic components: sand, silt, clay and organic material. The varying proportions of these will determine soil texture and how well plants grow.
You need to know what type of soil you are dealing with and its pH value (acidity/ alkalinity), as this will influence how you cultivate the soil and the time of year you do it. Heavy clay soils are best cultivated in autumn before they become too wet, and sandy soils in late winter and spring. The pH will to an extent dictate the plants you can grow -- you should avoid trying to grow acid-loving plants on chalk soil, for example. You can test the pH using a kit; it is a good idea to take samples from different areas of the garden.
Sandy soils are at least 70 per cent sand and gravel and no more than 15 per cent clay. The color varies, depending on their organic matter content. They are very free-draining and often lack fertility, but do have the advantage of warming up quickly in spring. They feel gritty when rubbed between finger and thumb.
Clay soils contain at least 45 per cent clay, and less than 40 per cent sand. Their high water-holding capacity means they drain slowly; some are prone to waterlogging. Most are quite fertile and good at holding plant nutrients, but they can be difficult to cultivate and are slow to warm up in spring. They have a smooth, soapy texture when moistened and rubbed between finger and thumb.
Chalky soils have a high concentration of chalk or limestone (it may be visible as pieces of rock) and are often shallow, with soil depth less than 30cm (12in) over the rock below. They can be very
fertile and are usually biologically active, with high populations of worms and beneficial bacteria. They tend to be free-draining, but high pH limits the range of plants they can support.
Peaty soils are correctly termed 'organic' as a relatively high level of organic matter (minimum 15 per cent) influences their characteristics. They are good at retaining water and can be very fertile (unless they are pure peat). Although this type of soil is usually h plants that love acid conditions, there are many alkaline organic soils. They are crumbly and fibrous when handled.
Silty soils, often referred to as loams, contain at least 70 per cent silt with a clay content below 12 per cent. Many gardeners view these as ideal soils. They are usually good at holding water and are also free-draining, fertile, productive and easier to work than other types. They are smooth but slightly gritty when moistened and rubbed between finger and thumb.
Understanding pH Values
Whether soil is acid, alkaline or neutral will determine the range of plants that can be grown in it. Acidity and alkalinity are measured using the pH scale, which ranges from 0-14; pH 7 is neutral, neither acid nor alkaline
Most natural soils range from pH 4 to pH 9; few plants grow in soil with pH above 9 or below 4. Gardeners usually aim at a soil of pH 6.5-7 to ensure they can grow the maximum range of plants.
TESTING TIP: Avoid handling soil samples with your bare hands as the pH of your skin may affect the final reading.
Adding Lime
Lime raises the pH and neutralizes soil acidity. This often benefits plant growth because many plant nutrients are more readily available when there is lime present in the soil, and many soil organisms can only function well in a soil where there is lime. Ideally, lime should be added to the soil after digging but before it is cultivated down to a finer tilth -- this ensures that the lime is evenly distributed. Apply lime in small quantities rather than run the risk of
Single Digging
The most commonly practiced form of digging cultivates the soil to the depth of a spade blade (called a spit), usually about 25-30cm (10-12in) deep. Cultivation is concentrated in the area where most plant roots naturally grow, in the top 10-20cm (4-8in) of soil.
If the plot is large, mark out the extent to be dug using canes and a garden line, then dig a trench at one end. A new trench is created each time a section of the plot is dug. Repeat this process until the entire plot has been dug. Keep your back as straight as possible when lifting spadefuls of soil.
Improving Soil Structure
Structure is the term given to the way the individual particles of soil bond together in clusters. If soil particles are fine, they are packed together so that water drains through slowly and the soil stays wet. By adding organic materials the structure can be improved by 'opening' the soil, allowing air and water to penetrate. Conversely, if a soil is too open (drains too quickly), adding organic matter will help the soil particles to stick together more closely, allowing the soil to hold more water, why add organic matter? Animal manures and fresh green plant waste provide small amounts of nutrients quite quickly, mainly nitrogen (N), phosphate (P) and potash (K). However, fibrous and woody materials are much better for improving soil structure and 'opening' heavy soils, while on lighter, free-draining soils, they improve moisture retention. The ideal garden compost is a mixture of the two.
All bulky organic materials have low levels of nutrients when compared with inorganic concentrated fertilizers, but as the organic matter rots, it produces organic acids that dissolve nutrients already in the soil, making them available to plants.
For green manure, grow borage, comfrey, mustard, red clover or ryegrass and dig into the top soil when the plants are six to eight weeks old. This will improve organic matter and nutrient levels, particularly nitrogen content, and smother germinating weeds.
ORGANIC MATTER TIP: For maximum nutrient benefit from bulky organic materials such as manures, incorporate them into the soil when they have only partly rotted. The longer they are stored, the lower their nutritional value, because nutrients can leech away.
improving soil fertility
A healthy fertile soil must have a biologically active community of different organisms, capable of releasing and recycling nutrients so that plants can feed. For this activity to take place, there must be a balance between the amount of air in the soil (so that the beneficial organisms can live) and water (so that chemical changes can take place).
Time the application of manures, organic mulches or fertilizers so plants gain maximum benefit, applying them either just as growth starts as a base dressing, or part-way through the growing cycle as a top dressing. Apply dry fertilizers to moist soil, as plants absorb nutrients in soluble form.
Bear in mind that over-feeding with high concentrations of fertilizers and manures can severely damage or kill plants by chemically burning their roots.
The Benefits of Digging
Digging over vacant ground within flower borders and vegetable plots at least once each year is a good way of incorporating organic matter into the soil, improving drainage and root penetration. It is the most effective way of preparing the soil for the next growing season and also keeps the garden tidy by burying unwanted plant waste and weeds. However, especially when soil is being brought back into cultivation from a neglected state, disturbing the soil in this way will lead to the emergence of weeds, as most of their dormant seeds will start to germinate once exposed to daylight.
For shallow-rooted plants, it may not be necessary to dig to a greater depth than about 30 cm (12 in) unless the soil is compacted or badly drained.
Double Digging
This technique is often used where a hard sub-surface layer of soil has formed, or on land that is being cultivated for the first time. It involves digging to the equivalent depth of two spade blades (see opposite). The aim is to improve the crumbliness of the subsoil without bringing it up to the surface, while keeping the most biologically active layer of soil (the topsoil) close to the young roots of the plants. This makes it a useful technique for an area where long-term, deeper-rooted plants, such as roses, shrubs, trees or fruit bushes, are to be grown. The benefits of double digging a plot can last for up to 15 years.
DOUBLE DIGGING TIP: Avoid mixing the subsoil with the topsoil. If the two are mixed together, the fertility of the topsoil is diluted, rather than the fertility of the subsoil improved.
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