The problems with chalky clay soil By Simon Benjamin
January 2010
We have a soil I would describe as Chalky Clay around Lyminge, Kent. It has small clay particles and is fertile on the fairly thin top soil, no more than 12” deep. The sub soil is white and appears to have a high chalk content. The best way to improve this soil type is to add organic matter, use green manures or cover crops, and to avoid digging it when it’s too wet. Adding grit is pointless as it is quite stony anyway.
Organic additions
The best “bulking agent” to use is horse manure with this soil as it adds “body”, it increases its ability to hold water but also allows more air into the soil. Which in turn helps roots to grow quicker and easier. Leaves that have been kept damp throughout the winter and therefore semi rotted are also very handy and are best used as a mulch. This is great around fruit trees and bushes, but also useful around flowers and vegetables as long as you allow the plants to grow big before application. Home-made compost and spent potting compost are also great additions to the soil and are best used as a surface mulch; you could sow your seeds into this but first wait for a week or so to see if any weed seeds germinate. Hoe or pull these up before seed sowing. Additions such as concentrated chicken manure, don’t add body directly to the soil, but will make your plants grow faster and stronger, therefore making your soil richer over the long haul.
Green Manures
Green manures are simply plants that are sown in order to improve the soils structure, and some types also increase the nitrogen in the soil, these are known as leguminous plants. Typical green manures include rye grass, clover, and mustard. They can be in the ground for a few weeks (as a catch crop, often over winter) to several years. It is even possible to plant shrubs and trees as green manure, for instance Alder trees or elaeagnus shrubs fix nitrogen. This is obviously a long term solution, but can work well particularly if coppiced every other winter. This keeps them small and easy to remove once no longer desired. It has also recently been learnt that cutting leguminous plants causes them to drop some of their roots which are covered in nitrogen molecules. So you get more fertility and the plants don’t overwhelm your garden. However for smaller gardens red or white clover in ideal as it can stay in place for many years, you can plant straight into it, and it reduces the need for weeding.
Onion sets in particular enjoy this treatment and as they tend to root deeper than the clover there isn’t direct completion for water. This technique also works well for a perennial flower garden as the clover particularly the red form is a nice ground cover, and a good contract to cottage garden plants such as Lupins, Delphiniums, or Marigolds.
Double digging or how to break your back.
This technique involves removing all the topsoil, forking organic matter into the sub soil, then adding more organic matter into the soil as it’s returned on top of the sub soil. This is best down in stages, and coffee breaks should be taken often as its well earned. This technique is only necessary if you have a shallow soil or a hardpan. Luckily it only needs doing once, as it significantly improves the patch you do. It can deepen the top soil by many inches, but it requires copious amounts of organic matter. On chalky clay even a wheel barrow full every square metre wouldn’t be excessive.
Raised beds or how not to break your back.
A favourable alternative to heavy digging is raised beds; these are particularly good for vegetables. Just cover an area with cardboard or old newspaper, and then cover it with alternating layers of straw and manure or home-made compost, make each layer an inch or two deep. Keep layering until you have a foot depth or so. You can then cover this in garden soil or old potting compost and sow into it. It is likely to sink somewhat in the first year, this is normal and it means that some of the goodness is getting deeper into the soil. If you like a neat garden you probably want to use wooden boards to keep the raised bed from spilling around. Decking boards from a DIY store are fine for this and quite cheap. Remember thought to use proper fence posts cut down into 2 ft lengths to screw them to. This will anchor the board in place and will not rot quickly. Some post wood is guaranteed to last 20 years!! Although hopefully by then all your soil is dark and deep and fertile. You can make the beds any shape, but squares, rectangles and triangles are the easiest. I have a design in my head for a hexagram design for a herb bed, but that an idea for another day.
Conclusion
The chalky clay soil we have in this part of Kent is not the most perfect soil on its own. It can often create a surface pan. It’s hard to work, and fairly heavy. It also loses water quicker than some soil types as the subsoil is very porous. However you can walk around Kent and see fields and woodlands and orchards all growing away well. This proves that with the right adjustments and/or the right planting combinations you too can grow plants well it this potentially tricky soil. I hope this article gives you some ideas to get you started on that path.
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