Soils rich in fine clay particles are called ‘heavy soils’ and, although hard to manage, are also potentially very fertile when treated in the right way.
Gardening with clay soil
Where to see clay soils:
- RHS Garden Rosemoor has a very sticky clay soil, whose management is not helped by the high rainfall in Devon
- RHS Garden Hyde Hall has a heavy clay soil, but the low rainfall in Essex makes handling this soil very much easier than in Devon
Clay soils are:
- Heavy to dig and cultivate
- Drain slowly after rain
- Warm up slowly as summer approaches, leading to delayed plant growth and ‘workability’
- However, against this, they hold water well
- Are usually rich in plant nutrients
Warning: if worked or walked on when wet they lose their structure, and become puddled and compacted. Remedying this is slow and laborious; so damage should be avoided at all costs.
Techniques and tips if you garden on clay soil:
- Dig in autumn and early winter when relatively dry. Once wetted by winter rains, clay soils often cannot be worked or walked on until mid-spring
- Allow winter frosts to work on clay and break it down
- Where digging is required, it is traditional in wet regions to dig clay into narrow ridges to allow more frost activity and better drainage
- There is often only a brief period when clay soils are workable between waterlogged soils after winter and baked hard clay from late spring
- Avoid early planting or sowing unless drainage can be improved by making raised beds or the ground dried and warmed in advance (for at least six weeks) by covering with cloches or clear polythene sheets
- Trees, shrubs (especially roses), climbers and many bulbs are easier to grow on clay soils than plants that require frequent sowing, planting or dividing; annuals or bedding plants, for example
- Planting woody plants on slight (25cm/10in) mounds can help avoid root damage from waterlogging
- Planting of trees, shrubs and climbers is best done when soils are dry and workable in early autumn
- Although early vegetables are a challenge on clay soils, maincrop vegetables are usually heavy yielding and potentially of outstanding quality
- Tree fruits generally thrive on clay soils, although some soft fruits, raspberries and strawberries for example, are more trouble
- Ample paths and stepping stones are usually required where lawns are laid on clay soils to avoid trampling wet clay soils and damaging them
- Lawns are often too wet to walk on for prolonged periods in wet weather and are frequently very prone to worm casts. Golf courses lay turf on a 7.5-15cm (3-6in) layer of sharp sand to overcome this and this technique can be used in gardens as well
How to improve clay soils
Five steps to improving clay soils:
- Make raised beds to assist drainage and to reduce trampling of the soil
- Consider adopting a ‘no-dig’ regime, especially in raised beds, as these suit clay soils well
- Some, but not all, clay soils respond to extra calcium, which causes the soil particles to flocculate (clump together). Where the soil is acid, lime can be applied, but elsewhere it is better to add gypsum. Gypsum is the active ingredient of many commercial ‘clay improvers’. Test on a small area in the first instance to ensure it is effective on your type of clay
- Dig in plenty of bulky organic matter such as manure or, ideally, composted bark, as this can make a noticeable improvement to the working properties of clay
- Apply organic mulches around trees, shrubs and other permanent plants as these will reduce summer cracking and help conserve moisture
Adding grit, sand or gravel to clay soils:
Clay particles are amazingly dominant in a soil. This is explained by the relative size of the different particles (clay, sand and silt) that soil contains. Clay particles are very small but, because this allows more particles to fit in any given space (say 1cm cubed), they have huge surface area that dominates the physical properties of soil. In comparison, sand and silt particles are larger, so fewer particles are needed to fill a space (say 1cm cubed again). As a result, the overall surface area of sand and silt is smaller and so much less influential on determining the characteristics of a soil than the clay particles.
In practice what this means is: to dilute the proportion of clay in a heavy soil requires very large volumes of grit or other material. It is seldom feasible to do this on anything but a small scale and, for most gardeners, other options such as raised beds, adding organic matter and choosing plants that thrive in clays are more practical.
Even where a clay soil contains for example 40 percent clay particles (a relatively modest content compared to heavy clay soils), the proportion of clay in the top cultivated part of the soil would have to be reduced by half to make the soil easy to work. This would require 250kg per sq m (460lbs per sq yd) of grit or gravel. Adding materials to clay can also make the clay less stable, so the soil becomes harder to manage. Experimenting on a small scale at first is recommended to be sure that any additions are worthwhile and won't have damaging effects on workability of the soil.