Factors in Improving clay soil
There are three keys to improving clay soil: the weather, organic matter and time. Time is the key, because nothing has ever been achieved quickly with clay. You could get some sort of immediate result by raking in peat, pulverised bark or an artificial soil conditioner, but the effect is likely to be short lived. To make a lasting improvement you need time which, depending on the severity of the problem, could mean anything from about three to ten years.
The weather’s contribution towards a reasonable soil lies in its shattering effect on the clay’s structure. The two principal agents are alternate freezing and thawing. When the ground freezes, the water in it expands and breaks up the clay. In the subsequent thaw, cracks and fissures remain to collect water for the next freeze. And so on. This is a continual process in winter, and the effect reaches down as far as the limit of the frost. So dig deep in autumn, most usefully with a fork that has broad, flat tines. Leave the ground in heavy clods to give larger surfaces for the frosts to work upon. In spring, rake what remains of the clods into an even tilth for sowing and planting.
The third essential ingredient when putting together a workable soil is organic matter. Use plenty of it, and the bulkier the better – farmyard or stable manure, or garden compost. This begins the long-term build-up of the humus on which plants feed. Fork the manure or compost into the ground in autumn, to mingle and break down with the top 6in of soil, ready for planting or sowing in the spring.
Best time for digging
Always try to do your digging in the autumn, when the ground will lie vacant for several months to come. This will give the frost time to work on it. By digging deep immediately after harvesting a crop, you have brought to the surface raw and unweathered clay which will be compacted by the rain. If you have to prepare land in spring or summer for another crop, keep the digging as shallow as possible so that the surface tilth is retained.
Hard clay in summer
If your garden clay soil gets very hard in summer then adding lots of compost or manure is the solution. Any soil which dries out and ‘caps’ to this extent is clearly short on organic matter. If you are already digging it in regularly, keep at it. If not, then start the next time you dig. Depending on the heaviness of the clay, it could take several years to overcome this fault – but you will win in the end.
To speed the process, it would also pay you to work peat or pulverised bark into the surface before sowing or planting. In addition, spread a 1-2 in thick layer of manure or compost in spring around the bases of your roses, trees and shrubs.
Soil conditioners based on composted seaweed are highly successful because they contain naturally occurring gluey substances called colloids which gather together the individual clay particles into crumbs or blocks so that the ground drains better. Surprisingly, colloids also help sandy soils to retain more water and nutrients. Calcified seaweed conditioners act in much the same way as ground chalk or garden lime which open up clay soils and improve their drainage. These two types of conditioner also contain trace elements beneficial to plants.
There is another group that is purely chemical. These products are more expensive, and are usually based on compounds called polymers. They work well for a short time so that sowing or planting can be carried out in the improved tilth. But they contribute nothing else to the soil.
None of the concentrated conditioners has the same permanent value as bulky organic matter, but they can be used to get you off to a good start.
Most clay soils, but not all, are acidic. Your first step is to buy a simple test kit from a garden shop or centre. Follow the instructions to ascertain the pH level of your soil. It is likely to show a reading between 4.5 and 6.5 .
The application of lime on an acidic soil will neutralise the acidity and so encourage worms and beneficial bacteria. It also improves the soil by assisting the clay particles to collect into crumbs, which makes for quicker drainage and better rooting.
Lime contains calcium which is essential for plant growth and acts as a catalyst that releases other nutrients bound up in the acid clay. Another great advantage is that it seems to discourage pests such as wire-worms, slugs and leatherjackets, and it certainly checks club root, a disease often found in acid soils. How much lime to add to your soil will depend on the results of your soil test. The more acid your soil, the more lime it’ll need to bring it up to the level that most plants prefer.
Generally, test again a year after giving a first lime treatment, before applying the next lot. As your soil improves with regular manuring and liming, it’s very likely that you will need to lime only once in every two or three years. The best time to add the lime is in early winter, after digging over the ground.
Do not mix lime with other soil dressings, most especially not sulphate of ammonia or general chemical fertilisers. Do not use lime until a month after chemical fertilisers have been applied, and allow three months to elapse after manuring.
Clay soil is good for roses
The conditions associated with clay soils suit roses. Clay is a good provider of plant foods, especially potash which encourages flowering. It is also moisture-retentive so that roses grow well without check, even during very dry summers. The heaviness of the soil, too, makes a solid anchorage for the roots and minimises wind-rock.
Vegetables for clay soil
The traditional crop to break up a clay soil has always been potatoes. They don’t need a fine tilth, and the action of earthing up the rows two or three times during the early summer helps to break up the soil, as well as keeping the weeds down.
Well-cultivated heavy soils are excellent for growing runner, french and broad beans, peas, cabbages, brussels sprouts and kale (though not cauliflowers). Root crops such as carrots and parsnips do not like clay. They
tend to fork or branch underground, making them useless for the kitchen. Most soft fruits do well on clay and, provided the ground does not get waterlogged, so do many fruit trees including apples, pears and plums.