Advice
RHS Help & Advice
Clay soils
Although clay soils are often seen as being problematical - soggy and wet in winter and like concrete in summer - a good clay soil is one of the best soils you can have as it will grow just about any plant well.
Clay particles are extremely small, with 1,000 times the surface area in a given volume of soil compared with larger sandy soil particles. As a result, clay packs closely and binds water tightly by surface tension preventing it draining away.
Sticky wet soils are likely where the clay content is greater than 30%. Fortunately, clay soils naturally break into small lumps when they dry and shrink in summer, allowing roots, water and air access. Adding organic matter, especially composted bark, as a mulch, or dug in, helps promote this texture, without which clay soils would support little plant growth.
Where really open texture is needed abundant coarse material can counter the clays' stickiness. Experiment on a small scale first, mixing a 7.5cm (3in) layer of grit into the top 15cm (6in) of the soil, adding more if, after a month, the soil still seems too stiff. Too little can be worse than none at all.
Wet clay swells preventing winter rain from escaping. In spring the water slowly evaporates, delaying planting, sowing and growth. Drainage or, on a small scale, raised beds counter this. Walking on wet clay damages its structure when it is compressed or smeared, so make paths or even use stepping-stones.
There are only brief periods in October and April/May when clay soil can be easily dug and seedbeds made. Freezing and thawing over winter of ground dug in October aids soil breakdown and helps to form spring seedbeds. If soil cannot be cultivated in these periods, grow perennial and woody plants or consider 'no-digging' beds where seeds and plants are set into undug soil. Deal with weeds by hoeing, pulling and spraying and adding a thick mulch of organic matter.
Nutrients are held in a readily available way within clays making these moist soils potentially very fertile. Although seedbeds are tricky, trees, shrubs and perennials usually grow very well once their roots penetrate the surrounding soil. Consider improving soil with cultivation and organic matter before planting and using smaller sizes of shrubs and trees. These are easier to plant and care for, and grow fast on fertile clay soils.