Module 02: Soil

Soil types and structure

Just as we cannot alter features such as climatic conditions and aspects, so we have to accept the soil that comes with a garden plot. You may be fortunate enough to have a garden site with soil naturally quite suitable for producing excellent crops, or one that has been made so through a history of careful cultivation; the ideal soil would be a fertile, well-drained loam 18in (45cm) deep. Often, however, we have to contend with cold, heavy, poorly drained clay soil or light, sandy soil prone to rapid drying and loss of fertility. Luckily, these less than ideal soils can almost always be improved; even where this cannot be done, at least some fruit and vegetables can be grown in raised beds or in containers.

Soil supports and sustains plants, providing anchorage and a source of nutrients and water. It is highly variable according to location, even between sites in close proximity. Soils differ in their physical characteristics, their levels of organic matter, and their depth and condition. All soils are a mixture of weathered rock and rotted plant and animal remains or waste, naturally supporting myriad organisms, whose lifecycles make the soil into a living medium. It is organic matter, and this soil fauna that gives the surface layers their range of brown colouring.

Soil texture

The average gardener does not need to learn or understand soil science in-depth, but it is necessary to be able to identify the essential type of the soil on which a kitchen garden is to be established. This is because the different soil types have different attributes, which affect plant growth and call for different ways of cultivating and maintaining the site. With a few specific preferences, most fruits and vegetables will grow reasonably well on a wide range of soils.

Soil texture is broadly classified into five main types: clay, silt, sand, chalk, and peat. The term loam is widely used to suggest fertility and soil of all-around excellence for growing, as in the recommendation to choose “a good medium loam.” The term really needs qualification, for there is clay loam, silt loam, or sandy loam, depending on the basic soil type; clay, silt, and sand describe soil textures based on the proportions of different sized mineral particles found in them. Although the basic mineral content of the soil is fixed by its origin, the structural nature is affected by the occurrence of stones or gravel and also by the addition of lime and organic matter.

Soil fertility and structure

The organic matter and fauna in the soil give it both its fertility and its structure; these two are closely linked. One essential component of fertile soil is humus. The term is usually taken to refer to the actual organic content of the soil, but this is a simplification. Humus is a complex mix of compounds that derive from the decay of that organic matter to form a dark coloured, sticky substance. It has a crucial influence on the retention and release of nutrients, the formation of good soil structure, and the soil’s ability to hold water.

Also essential to soil fertility is the wide range of living organisms that populate it, which we often take for granted. These include beneficial bacteria and fungi, and a range of microscopic worms and mites, besides the visible centipedes, earthworms, and beetles. All rely on organic matter, variously incorporated and applied annually in mulches, which they break down into humus.

A soil must have a good structure if plants are to thrive. Soil structure is measured by its tendency to form crumbs of mineral particles and organic matter held together by humus. Well-structured soil is easy to cultivate and has evenly distributed air spaces. This aeration allows plant roots to extend through the soil easily and facilitates the movement of water and nutrients. It also has the effect of warming the soil, and so promoting plant growth. Soil structure is influenced by the activities of soil organisms in creating humus and by the levels of organic matter present. Also important are the nutrients that the organisms need to form humus, and lime, which is vital to crumb formation.

Other factors helping the development of good soil structure include the action of frost and alternating wet and dry conditions. Ensure that the soil structure is not damaged by being cultivated in wet conditions or by excessive traffic over the dug surface. Walking or moving wheeled implements across the surface can compact soil at any time, but especially in wet conditions. Aeration can even be damaged by continual winter rain. This damage can be reduced by leaving a covering of organic matter on the surface during winter, which can be dug in to maintain soil fertility, or by planting an overwintering green manure or crop cover.

Soil profile

The horizontal bands that can be seen when digging down into a soil make up the profile. The most easily identifiable profile in gardens is topsoil, subsoil, and the parent rock material below them. Topsoil is the layer of most activity because this contains organic matter and organisms that live on it. The subsoil is usually paler in colour and is formed from weathered rock. The depth of topsoil and subsoil through which roots can penetrate has great importance to the growth of plants. It is worth digging a small test hole on your site to ascertain the profile.

Fruit trees do best where the total depth of well-drained, weathered soil is at least 24in (60cm); sweet cherries ideally need at least 3ft (90cm). Soft fruits require a soil depth of at least 18in (45cm), except for strawberries, which can succeed in a depth of 15in (38cm), as can vegetable crops. These are general limits and are based on the success of crops grown commercially; what is of paramount importance is that water can drain freely through any depth of profile.

Drainage of the soil

Roots require air to grow and to absorb nutrients and water. Waterlogged soil becomes cold and airless. This prevents nutrient uptake and leads to plant roots, becoming diseased or even dying. There are clues that indicate a poorly drained garden site. The most obvious is evidence of persistent puddling after rain. Others are the presence of plants that thrive in wet conditions, such as sedges, rushes, or moss, and poor growth of perennial plants.

Poor drainage arises in several ways. It can occur if the topsoil is infertile through poor cultivation or lack of humus: careful cultivation and dressing with organic matter will improve structure. The soil profile may naturally include an impervious layer known as a pan, which is often only 1–2in (2.5–5cm) thick. A pan can occur in the subsoil as a result of accumulated minerals, especially iron. Alternatively, there may be a barrier in the subsoil or the topsoil caused by compaction. Because free drainage is so important, it is worth digging a narrow inspection pit on suspect sites to examine the soil profile.

A natural hardpan is easily identified, as is compaction, which is often marked by a dark horizontal band. Deep double-digging will break up compacted layers, and a natural hardpan can be breached with a pickaxe. Other problems are more intractable: it may be that the garden lies in a hollow of natural drainage from surrounding land, or that the area has a naturally high water table. Both of these situations would be very difficult to remedy, and in such conditions, it may be necessary to grow crops in raised beds or in containers.

Lime and soil acidity

Lime is a vital soil ingredient in the kitchen garden because it affects the fertility of the soil in several ways. Lime is itself a source of calcium, a major essential element for healthy plant growth, and is also vital to the production of a good soil structure.

The effects of lime

The presence of lime governs the acidity of the soil: the more lime present, the less acidic the soil. This affects the microorganisms that break down organic matter, which mostly cannot survive in a soil that is very acidic. Soil acidity is also important in the uptake of nutrients, because some of them, such as potassium compounds, become unavailable to plants in soils that are very acid, while others may accumulate to concentrations that are toxic to plants. The incidence of some diseases is influenced by soil acidity; clubroot in cabbages thrives on acid soils and scab in potatoes on alkaline ones.

Lime has a valuable beneficial effect on the structure of clay soils, because its presence initiates a chemical process that aggregates soil particles into stable crumbs, which are clusters of mineral and organic matter. A good crumb structure is important both for proper aeration of the soil and for effective water and nutrient retention. The level of lime also influences the living inhabitants of soil. The activity of earthworms and the microorganisms, especially bacteria, that reduce bulky organic matter to a constituent of humus decreases as the acidity of the soil increases.

Knowing your soil pH

To gain the maximum benefits of lime in soil and avoid the problems resulting from its absence or excess, it is necessary to understand first how to measure the acidity of the soil, and second how to adjust the level. The degree of acidity of a substance is measured on a graduated system known as the pH scale. At the middle of this scale, pH 7 represents the neutral condition: values lower than this, down to pH 0, indicate increasing acidity, and values above it, up to pH 14, indicate increasing alkalinity. Garden soils usually have a pH of between 4.5 and 7.5, and most of the crops to be grown in a fruit and vegetable garden will do best on soils of around pH 6.5; this standard remains constant, and is the level for which you should aim in treating your soil; the pH level directly or indirectly affects everything else that you do.

There are accessible and inexpensive means for the gardener to determine the level of soil acidity and the need for lime. Simple pH measurement kits are available from good garden centres and mail-order retailers; their results are easy to read and sufficiently reliable. Use a kit in the first stages of planning, and rectify any imbalance before planting. It is also advisable to use them for checking every few seasons, as levels will change, and it may be necessary to make adjustments. On a large site, take small samples from across the area, then mix them before testing a small quantity as a representative sample. Alternatively, perform several tests at various points.

Extreme pH problems

The acidity of the soil affects the availability of nutrients, and modifying soil acidity by applying lime is an effective means of influencing nutrient availability. Some liming products can also be sources of the main nutrients; for example, ammonium nitrate formulated with lime contributes nitrogen, and dolomitic limestone also contains magnesium.

A soil with an excessively high lime content is just as unsatisfactory for fruit and vegetable growing as a soil with too little. In very alkaline conditions, most of the essential nutrients that are required in only small quantities—trace elements such as iron, manganese, and copper—can not be readily absorbed by the plant. On overly lime-rich soil, apples and pears often show marked yellowing (chlorosis) between the leaf veins, due to iron or manganese having become unavailable, and similar chlorosis is found in some vegetables, for example, beets.

Applying lime

It is much easier to reduce soil acidity than to raise it, so take care in deciding how to apply lime. Three forms of lime are generally available: quicklime, which is caustic and dangerous; hydrated lime, which is faster acting but can damage foliage; and crushed chalk or ground limestone, the least hazardous and usually the least expensive form of lime for garden use.

Rates of application to bring an acidic soil to pH 6.5 vary not only according to the existing pH value but also to the soil type: the chemistry of a clay soil makes it far more resistant to the effects of liming than sandy soil. The pH of very acidic soil can be raised only gradually, so regular pH checking is needed in the early development of the garden. Dressing requirements of more than 12oz per sq yd (400g per sq m) need to be applied over several seasons.

Whatever the quantity, the best effects will be obtained where the application is made well before planting, ideally on two or more occasions in the fall and winter; this will allow the lime to initiate changes in the soil gradually and more effectively. Apply lime after incorporating animal manures, but not less than three or four weeks later, to avoid chemical reactions that will release ammonia and allow nitrogen to escape, and aim to work in the dressing to a depth of 6in (15cm).

Liming the soil

1. Weigh out enough lime 1sq yd (1sq m) of your plot. Put it to treat into a pot and mark the level, so that you can measure out the rest of the lime using the pot. Mark out your site into a square yard (square meter) grid.
2. Put the lime on and scatter it gently and evenly over the blade of a shovel your marked-out square of soil. It is important that liming is done on a still day, so that the lime is not blown onto other areas of the garden, causing scorching of plants.
3. Work across the plot square of the grid in the same way. Treating each Rake over the surface to distribute the lime evenly and incorporate it into the soil, or dig it into a depth of 6in (15cm).

Safety tips

• Store with care, keeping lime or sulfur in a clearly labelled, closed container, securely placed out of reach of children.
• Choose your time and only ever apply lime or sulfur on a clear day.
• Cover your eyes with protective goggles that offer protection around the sides and fit closely to the face.
• Protect your skin by wearing pants, long sleeves, and close-fitting gloves.
• Wear a simple cloth mask over your mouth and nose, since lime is easily inhaled.
• Be sensible: the operation of liming is a perfectly safe practice if these simple precautions are followed.

Reducing alkalinity

Rainfall, continuous cultivation, and cropping will gradually increase soil acidity. The application of flowers of sulphur also has an effect, but this is slow and dependent on relatively warm soil for bacterial activity. For general guidance, apply 270g per sq. m on clay soils and half this quantity on sandy soils. Repeat pH testing and application as necessary; the process is only worth considering in extreme conditions.