This article is derived from a talk given by Geoff Hodge to the St Ives Gardening Club on 15th April 2015. Geoff is a freelance journalist who lives in Peterborough and has personal experience of heavy clay. First the good news. Clay is a very fertile soil BUT can be very difficult to work during wet or after a hot dry period.
In general St Ives has heavy boulder clay with, in some places, chalk content. To make it workable a large quantity of organic material is required. Manure, compost, soil improver, composted bark and leaf mould will all do. A minimum of four inches (102mm) is needed. It may not be possible to cover an allotment plot at one go or in one season so do not be tempted to spread what you do get over the whole plot. Peat is not recommended because once it dries out it is not much use and its use will be banned fairly soon. You will need to top up the organic material periodically. In addition, if possible, add sharp sand. Do not use builder’s sand. Finally add a flocculating agent such as lime, calcified seaweed or gypsum. The latter is used for plaster board so that material if broken into dust is effective.
IT IS IMPORTANT TO DIG ALL THIS IN TO GET THE FULL BENEFIT OF THE FERTILITY OF CLAY. ALSO RAINWATER AND ROOTS OF PLANTS CANNOT PENETRATE FAR UNLESS THE UNDERLYING CLAY HAS BEEN LOOSENED.
Most manures and composts have very limited fertility. However, if you can get it, mushroom compost does have fertility.
Geoff recommends using raised beds, as does Monty Don, for vegetables. This eliminates/reduces the need for digging and should minimise compaction caused by walking on the soil. Raised beds require more watering although the use of water retention crystals can help. Many plants can be grown in rows that are closer together than normally recommended. However, this benefit is also available on normal vegetable beds if they are organised so that hoeing can be done without walking on the beds. Spacing is the same between rows as it would be in a row. Suggestions for reducing the cost of constructing raised beds are given at the end of this article.
Despite the fertility of clay Geoff recommends the use of a continuous feed. Liquid feeds are quickly exhausted and slow release ones, such as bone meal, take too long. Very effective and low cost fertiliser can be made using stinging nettles or Russian comfrey.
Good quality tools will make life easier. He reckons that clay tends to slide off stainless steel ones. A good quality hoe is essential and regular use of a sharpener is strongly recommended. A two to three inch (50 to 75 mm) mulch will reduce the need for weed control and help conserve moisture. Growing green manure over the winter will add further organic material to the soil and keep the weeds down. Alternatively if you are not using raised beds you can loose dig your plot and let winter frosts act on the clay like farmers do but do some winter hoeing as well to keep the weeds down.
Everybody should know and check the PH level of their soil.
What are known as pallet caps or old scaffold boards can be used to help keep the cost down of raised beds. If you can get them, pallet caps can be partly dismantled and extended to so that they can be adapted to the required size. Quite a few plot holders on the Hill Rise site have managed to obtain them. Periodically the NSALG magazine have adverts from suppliers of various designs of raised bed materials some of which are quite innovative.