Knowing Your Garden Soil

Knowing Your Garden Soil

Below the surface of the garden there is as complex a living world as there is above it. The soil has been formed over thousands of years from the breakdown of rocks into mineral particles of sand, silt and clay. However, this is only its “skeleton”, usually making up about half the volume of a soil. The rest is made up of air, water, organic matter, plant roots and the all-important creatures that live in the soil.

The soil skeleton

Knowing Your Garden Soil A soil is determined by the different amounts of sand, silt and clay particles it contains. Those with a fairly equal mixture of particles are called loans; a clay loam contains a higher percentage of clay, a sandy loam more sand.

You can easily work out what type of soil you have in your garden by picking up half a handful of moist topsoil and feeling it. Knead the soil with your fingers — a sandy soil feels gritty, clay feels sticky and silt feels smooth and silky. Now roll the soil between your hands. Very sandy soils will not form a ball without crumbling. If the soil forms a cohesive ball, rub your finger over the surface: a clay-rich soil will take a shine.

Air and water

The mineral particles in the soil are held together in aggregates, or “crumbs”, with spaces, or “pores”, between them. This is the soil structure. The pores are very important: large pores allow drainage so that air can get into the soil and small pores hold water. The pores also form the living spaces for all the soil-dwelling creatures and a passage for plant roots. In a well-structured soil, plant roots can spread easily and exploit a large volume of soil for nutrients and water.

To assess the structure of your soil, crush a clod in your hand. A well-structured soil is friable whether it is wet or dry; it is not sticky or dusty. Take a look at the soil profile as well (see below).

The structure can be destroyed when the soil is compacted by walking on it or using machinery, by heavy rain or irrigation and by cultivating in the wrong conditions, such as when the soil is very wet or frozen.

Organic matter

Organic matter is material of living origin. In the garden this is usually plant debris, animal manures and the remains of its living inhabitants, including microscopic organisms as well as insects and small mammals. Organic matter therefore contains a complex range of ingredients: proteins, sugars, carbohydrates and all the other materials that make up plants and animals. It is continually decaying as it is worked on by micro-organisms and various small creatures, and simpler substances such as minerals and salts that plants can use as food are released into the soil.

Organic matter has many benefits, including helping soil particles adhere together into crumbs, thus improving the soil structure. This helps drainage on heavy soils, making them more workable and easily penetrated by plant roots. Because it holds water, organic matter makes light soils more moisture-retentive. In addition to this, it provides a food supply for soil inhabitants, helping them to thrive and multiply and, in turn, release plant nutrients. Organic matter also holds onto nutrient elements in the same way as do clay particles, preventing them from being washed out.

Finally, the addition of organic matter helps to prevent soil erosion and strengthens the soil against the effects of heavy rain and cultivation. The organic matter also makes the soil darker and therefore warmer.

The organic matter in the soil needs replenishing regularly, particularly when the soil is disturbed. This introduces more air and speeds up the decaying process.

Soil life

Even just a spadeful of garden soil contains thousands of living creatures. Although a few are pests or disease organisms, the vast majority are harmless, while some of them are positively beneficial.

One of the most useful is the familiar earthworm. These tunnel through the soil, partly by pushing between the soil particles and partly by eating through the soil and organic matter.

They leave channels, which help drainage and aeration, and make it easier for plant roots to penetrate. They also produce casts which contain more nutrients than the surrounding soil and are cemented together with gums, making them more stable.

Other soil inhabitants are much less noticeable and microscopic organisms are not visible to the naked eye. Many play an invaluable role in breaking down organic matter, controlling diseases and making the minerals in the soil more available to plants. One group of fungi, the mycorrhizas, form a close relationship with plant roots, helping them to absorb nutrients — particularly phosphorus — and protecting them from disease organisms.

You can see the extent of soil life in the top layer of the soil profile and encourage a healthy population by making sure that the soil is not too acid and by adding organic matter. Disturb the soil as little as possible and always make sure that the surface is covered, either by plants or a mulch.

Soil profile

For a cross-section or “profile” of the soil in your garden, dig a hole with vertical sides. It must be at least 1 m (3-1/4 ft) deep.

The dark topsoil is where plant roots feed and most of the soil life is found. Ideally, this should be approximately 60cm (2 ft) deep, but it is often much shallower, which has a bearing on the range of plants that will grow well. The lighter subsoil contains few plant roots and less soil life, and its nature, along with the bedrock beneath, affects the drainage of the topsoil.

Soil pH

The pH of the soil is a measure of its acidity or alkalinity, both of which affect the soil life, the availability of nutrients to plants and the range of plants that flourish. Either buy a soil pH testing kit or get a professional soil analysis.

Depending on the garden’s history, the pH of the soil may differ from area to area and you may need to test several samples. Even if the pH is at a satisfactory level, recheck it at least every two or three years (more often on sandy soils). Vegetables, fruit and most other plants will grow within a pH range of 5.5 to 7.5, although fruit tends to do better in acid soils around pH 6.0. Above and below this range nutrients become less available and more plants will begin to struggle. However, there are specific plants which need very acid soil in which to grow, and some which tolerate very alkaline soils.

If your soil is too acid for the use intended, you can raise the pH by adding ground limestone or dolomite limestone. Follow the instructions in the pH kit, or add approximately 200g/sq m (7oz/sq yd) annually until the soil reaches the required pH. Calcified seaweed will also raise the pH — add approximately 40g / sq m (1-2/5oz / sq yd) annually. Slaked lime, hydrated lime and caustic lime are very soluble and quick-acting and therefore should not be used in the organic garden.

If your soil is pH 6.5 or above, you should not add any of these materials. Some vegetable plots that have been limed every year as part of the rotation end up being far too alkaline. Be wary of using comfrey or mushroom compost, as these materials raise the pH of the soil.

Do not add any type of lime to the soil together with manure as the two can react and release ammonia gas, wasting valuable nitrogen. Apply lime in the autumn and manure in the spring, or dig in the manure and apply lime to the surface.

If your soil is too alkaline, add compost and manure every year and eventually the pH should become lower. Creating raised beds can help as these are not affected by seepage of lime-rich water from the surrounding land.

Nutrient analysis

A chemical analysis will indicate the amounts of important plant nutrients that are present in the soil. It will give the phosphorus and potassium levels and usually those of magnesium and calcium. Other minor or trace elements may sometimes be analysed. If all your plants are growing well a soil analysis is not essential. However, it is advisable if you are starting a new garden, particularly if you are growing fruit or other long-term plants. An, analysis is also advisable if you have soil problems that do not appear to be caused by poor drainage, poor structure or the wrong pH.

Advertisements for soil analysis services often appear in gardening magazines. Most of these do not take into account the contribution made by the organic matter in the soil and the reserves of nutrients that can be released by the action of micro-organisms. If possible, get an analysis that is intended specifically for organic growers. This will measure the organic matter and activity of soil organisms, so giving a better indication of the true potential of the soil in your garden. It will also make recommendations for correcting any deficiencies with organic fertilizers.

28. January 2011 by Dave Pinkney
Categories: Organic Gardening, Soil Cultivation | Tags: , , | Comments Off on Knowing Your Garden Soil


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