Living with and Managing Clay Soils
Living with and Managing Clay Soils By Howard Drury
Clay soils usually cause problems for the gardener. In summer they form into hard lumps which are difficult to break up. During winter the ground is sticky, heavy and easily waterlogged. However despite these drawbacks, given suitable cultivation techniques, correct choice of plants and long term attention to soil management, a clay soil can be "tamed" that can often provide better results than most other soils.
WHAT IS CLAY SOIL ?
Soils consist of mineral rock particles of varying size and texture, organic materials, nutrients, water and air spaces. The coarser the mineral particles, the sandier the soil is considered to be and it will be more free draining. The finer the mineral particles, the more clayey the soil will be and it will also be more water retentive. Most soils contain a mixture of particle types and problems are only usually encountered when the clay content exceeds 35%. To test for this, knead a moist soil sample into a ball, roll it out into a sausage and bend it round to form a ring. Soils containing less than 35% clay will not stick together easily and cannot be rolled into a sausage shape. At 40% and more, the soil can easily be formed into a ring.
Mineral particles in a clay are so small and uniform they fit closely together leaving little or no space for air, but they do have a tremendous capillary attraction for water. This mass of tiny particles has a huge surface area (1oz or 28g. of pure clay has a surface of about 1 acre) which acts like a giant fly paper, attracting and holding dissolved chemical nutrients. Technically, clay is a colloid having an extensive capacity for ionic exchange. So, for plants trying to grow in this medium there are both advantages and disadvantages.
1) Clay soils retain nutrients well ‑ they are not 'hungry'.
2) They are less likely to become acidic because of the good retention of calcium. They do not need frequent liming.
3) Clay soils hold water and are less prone to drought.
1) It wet weather they hold too much water and become waterlogged. This creates problems for root growth and if walked upon, the soil is easily compacted making matters even worse.
2) In dry conditions when water is removed, the soil actually shrinks as the particles move closer together. Deep cracks are formed which can damage foundations, roots and disturb thesurface of lawns.
3) Poor air penetration and strong water retention mean that clay soils are slow to warm up in spring so delaying seed germination.
GROWING ON CLAY
To grow successfully in clay soils you must minimise the problems whilst taking full advantage of the benefits of this type of soil.
Flowers and Shrubs
Perhaps the simplest answer to the problem is to establish long term, low maintenance features with shrubs and herbaceous plants. During summer these will benefit from the water and nutrients stored in the clay. Once established, the only work required will be weeding, mulching and the application of a general fertilizer every two years. In general it is better the avoid annuals and flowers from seed which involve much digging, soil preparation and germination problems. Plants with fine, fibrous roots (many heathers and Ericaceae) cannot tolerate water logging and the lack of aeration associated with clay. Some bulbs (lily, tulip) suffer from poor drainage and may rot during winter months. In general plants that will thrive have tough roots (roses) or are used to moist conditions (willow). Prepare the ground in the autumn (see later) and plant in spring. Spring planting will avoid the cold and wet winter conditions that can adversely affect young plants planted in the autumn.
The problems of waterlogged lawns are all too well known to anyone with a clay soil. When laying lawns, avoid dips and hollows which will trap water. A sloping site will give natural drainage, but in severe cases a proper drainage system will have to be installed. Before constructing lawns, prepare and improve the soil conditions as explained later in this fact sheet. Established lawns can be improved by removing plugs of turf with a hollow tined fork and brushing coarse concreting sand into the holes. (Fact sheets 14, 21, 124, 204 & 207).
To grow vegetables on clay soils will inevitably involve you in regular digging and cultivation. Prepare beds 4ft. wide with an 18in. path between them wherever possible. It will then be possible to work from the path without treading on the soil. This will help to avoid compaction and ensure that the beds stay aerated. The soil structure can be improved over a period of time by adding well rotted, bulky organic manure or compost and concreting sand. Avoid fresh manure as this can produce toxic chemicals when it decomposes in the poorly aerated soil. All this will raise the general level of the bed and so improve drainage. It also means that the effort of soil improvement is invested entirely in the growing area.
The timing of digging is vital. Try to dig in the autumn before the weather becomes really wet. If this is not possible, wait until spring when the soil has dried out after winter. Avoid treading on and so compacting the ground when it is wet. When autumn digging, leave the soil in rough clods. The frost and winter weather will break them down. In spring fork and rake lightly to maintain the open structure of the soil. Liming can be carried out in autumn. Once a satisfactory pH level of 6‑7 is achieved no further liming will be necessary for several years. However if manure is incorporated during the autumn digging, do not lime until spring otherwise the nutrients in the manure will react chemically with the lime. (Fact sheets 11, 46, 82). The extra moisture associated with clay soils means that slugs can be a great problem, especially to potatoes. The vegetable gardener will therefore need to be extra vigilant and take all precautions to control the slugs. Fact sheet 26 has further details.
Germinating vegetable seed can be a problem in spring because the soil is slow to warm up. This can be overcome by raising plants in peat pots, polystyrene cells, paper pots etc. in the greenhouse and transplanting them later. Unfortunately root crops such as carrots and parsnips do not usually transplant well and the soil will need to be warmed before and after sowing by using cloches. Sow thickly to ensure an adequate number of seeds germinate and thin out later on. Alternatively, seeds can be chitted indoors (germinated just to the point where the root radicle starts to emerge from the seed case), dispersed in gel and squirted into the prepared ground using a plastic bag with the corner removed ‑ like a cake icing bag. For the gel, use wallpaper paste containing no fungicide.
IMPROVING CLAY SOILS
There is no quick and easy way to do this. Improvement is relatively straightforward, but it does take time and effort. The aim with all techniques is to break up the continuous and uniform structure of the clay into 'crumbs' or small lumps separated by spaces that allow air to enter the soil and the water to drain away.
Undoubtedly the best soil improver is humus, a highly complex gel‑like medium formed from decomposed plant and animal remains. It causes the soil to form into crumbs. Humus comes from the addition of large quantities of materials such as farmyard manure, garden compost, peat, composted bark (which has a more lasting effect than peat), seaweed, spent mushroom compost etc. These materials must all be in a well rotted state before digging in. Fresh manure will sour in clay soil and tends to be in big lumps rather than the easily dispersed form of mature manure.
Add these regularly (annually) rather then all at once. The general rule is ‑ the bulkier the better. So think in terms of trailer loads, not just a bag or two. In this respect concentrated manures are of little use. Their role is to provide nutrients with which clay soil is already well supplied. Frost action, worms and plant roots also help to form the clay particles into the small lumps and the presence of manure will encourage both worms and root growth.
Calcium is strongly attracted to the surface of the clay particles and causes them to clump together into aggregates. Calcium can be added either in the form of lime or gypsum. The lime will obviously change the pH level and so will be beneficial if the soil is acid. On alkaline clays or where lime hating plants are to be grown (rhododendrons etc.) use gypsum which does not affect the pH. It is better not to add the lime all at once, but do so annually at the rate of ½-1lb on the pH level. (See fact sheet 20).
If you have the time and energy, coarse sand and grit can be added to the top spit. These will improve the drainage by separating the clay mass. However, very large amounts will be needed to have any effect. Think in terms of a 2in. layer spread over the plot. This will then need to be well mixed into the soil ‑ no easy or light task ! Expanded rock (perlite etc.) will have the same effect. These are much lighter materials, but far more expensive. As with all materials mentioned so far, it is better to concentrate on improving just one area of the garden (a seed bed or vegetable plot) rather then trying to treat the whole site at once.
These are often based on some form of calcium or organic material. A product review appears in 'Gardening Which', December 1984. The conclusion reached is that bulky materials, (manure, compost etc.) produce the best results and that materials from a packet which are simply applied to the surface are unlikely to have a dramatic effect. However newer and improved products have since come on the market and from brief trials carried out at Kings Heath Park it would appear that polymers do have a beneficial effect on clay soils. See fact sheet 98 for further information.
All the above techniques will improve the top soil. In cases of severe waterlogging, additional drainage will be required. This can be a major undertaking and is extremely disruptive in an established garden. Consider doing it before laying a new garden. It will involve digging trenches 2‑3ft. deep with a slight gradient to carry the water away. If the site has a natural slope, this is an added bonus. Consider where the water will drain to ‑ a ditch needs to be located or a soakaway constructed. Over very large areas a herring bone system of side trenches can be dug at 15ft intervals feeding into the main trench. Half fill the trench with broken bricks and rubble. Cap with a layer of gravel and then replace the top soil.
Alternatively perforated plastic or clay pipes can be laid on 2ins. of gravel in the bottom of the trench. Add more gravel before replacing the top soil. If possible lead the drains to a surface ditch. If not, construct a soakaway by digging a large hole deep enough to reach free draining subsoil and fill it with rubble. On very deep clay soils, soakaways are no use because there is nowhere for the water to go and they simply act as reservoirs. In these cases surface ditches are the only solution.
PLANTS FOR CLAY SOILS
Obviously there are other factors such as pH, wind, shade etc. which will affect the final choice of plants, but the following is a brief list of plants known to succeed in clay soil. It is not exhaustive and does not imply that these are the only plants which will grow in this situation.
Trees and Shrubs
Berberis Buddleia davidii Cornus alba (dogwood)
Cotoneaster Cotinus (smoke tree) Cytisus (broom)
Deutzia Elaeagnus pungens Escallonia
Forsythia Genista Hamamelis (witch hazel)
Hebe Hydrangea Hypericum
Ilex Ligustrum (privet) Lonicera (honeysuckle)
Mahonia Magnolia Philadelphus
Potentilla Prunus laurocerasus Pyracantha
Ribes Roses Rhododendrons (some)
Salix Senecio greyi Sorbus
Spirea Syringa (lilac) Viburnum
Perennials ‑ moisture loving varieties
Aconitum Ajuga Astilbe
Bergenia Daffodil Digitalis
Hosta Helleborus Hemerocallis
Iris Lythrum Mimulus
Meconopsis Osmunda regalis Pachysandra
Polypodium Pulmonaria Primula
Polygonum Rheum Ranunculus
Snowdrop Trollius Vinca
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Howard Drury, Horticultural Broadcaster, Speaker, Lecturer, Writer, Adviser and Consultant
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The information given in this Fact Sheet is provided in good faith. It is however of necessity general information and advice on the topic. Howard Drury will not be under any liability in respect of the provision of such advice and information and you are strongly advised to seek independent advice on any particular gardening problems or queries you may have, preferably from experts who can (when appropriate) inspect the problem before providing advice.
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