There are pockets of heavy clay soil in Oxfordshire, and, having gardened on clay soil, I know about its disadvantages, but also its advantages, so I thought I’d write a short guide on gardening on clay soil.
You can tell what kind of soil you’ve got by taking a moist sample and rolling it into a sausage – if the sausage doesn’t hold together at all, you’ve got sandy soil, if it holds together moderately well you may have a good loam somewhere between sandy and clay soil, but if it sticks together well, it’s clay soil.
Clay soil has very much smaller particles than sandy soil.
These can stick together and form an impenetrable layer when compressed – our canals are made from clay that has been ‘puddled’ by horses or cattle so that they hold water. This lasts as long as the clay remains wet – once it dries it will crack.
Most of us don’t have this kind of pure clay soil in our gardens, but soil with a good proportion of clay in can be slow to warm up in spring, wet and muddy and hard to work. The heaviness of the soil makes it harder work for plants to get their roots down into, and if it’s heavy clay it might get waterlogged, so plants that like free draining soil will suffer. On the plus side, clay is full of nutrients and holds onto its nutrients far better than sandy soil, where they can quickly be washed out. As it holds on to water well, it is less likely to dry out in the warmer months and require less watering, and it is slower to cool down in autumn, so the growing season can extend beyond that on sandy soil. However, if it does dry out it will shrink and crack, which can damage plant roots.
It is important to improve clay soil by digging in plenty of organic matter (humus) in the form of garden compost or mushroom compost.
(Note mushroom compost is alkaline so don’t try growing acid loving plants in soil improved by mushroom compost.) This holds the particles apart and improves the soil structure, which makes it more free draining and easier to work and grow plants in. It is no good trying to improve clay soil by adding sand – it is far better to accept that you have clay soil, and work with it – adding compost is far more successful.
A lot of plants can be grown successfully in clay, as it has plenty of nutrients and retains moisture well. These include:
- Cornus kousa and other flowering dogwoods (pictured above)
- Fraxinus (Ash)
- Juglans nigra (Walnut – pictured right)
- Magnolia virginiana
- Pinus sylvestris Populus
- Prunus maackii
- Prunus serrula
- Quercus robur (oak)
- Salix (willow)
- Berberis thunberg
- Choisya ternata (pictured right)
- Cornus (dogwood)
- Cotoneaster (pictured right)
- Eleagnus umbellata
- Garrya elliptica
- Photinia x fraseri
- Ribes sanguineum
- Salix (willow
- Sambuccus (Elder)
- Spirea japonica
- Viburnus opulus (Guelder rose)
- Viburnum tinus
- Humulus lupulus (hop)
- Rosa filipes ‘Kiftsgate’
- Vitis coignetiae (ornamental vine)
- Acanthus mollis
- Anemone hupehensis
- Anemone x hybrida (Japanese anemones)
- Caltha palustris (marsh marigold)
- Doronicum orientale
- Eupatorium maculatum (Joe pye weed)
- Filipendula ulmaria (meadowsweet)
- Geranium ‘Orion’
- Geranium ‘Rozanne’
- Geum ‘Bell Bank’
- Gunnera manicatasinensis (grass)
- Helianthus ‘Lemon Queen’
- Houttuynia cordata ‘Chameleon’
- Iris laevigata
- Leucanthemum (pictured)
- Polystichum setiferum (fern)
- Primula japonica (candelabra primula)
You can also grow vegetables successfully in clay,
as long as you improve the soil with compost. Bear in mind that the ground is slow to warm up in spring, so your crops may be slow to take off, but the high nutrient level and good water retaining capacity of clay can be an advantage. Brassicas (cabbage family), particularly grow well on clay as they prefer firm ground.
So, as long as you remember the phrase ‘right plant, right place’, and you improve the soil with compost, it is possible to garden very successfully on clay.