Clay soil

Last revised: 20 September 2018

Clay soil found on Jubilee Road allotment has very good fertility and needs less fertiliser than other soils. Manuring and mulching is often sufficient to keep up fertility. In addition, this will enhance soil structure and is preferable to adding sand or grit. Sand or grit must be added in large quantities to make a difference. Jubilee soil is not in the heavy clay category as there is plenty of good loam top soil.

Across majority of cultivated ground at Jubilee Road allotments there is good spade depth of soil. Below this layer the ground tends to clay, with the main clay bed appearing at about half metre (20 inches). Plots vary a little depending on volume of mulch, compost and manure dug in during previous years.

The clay bed prevents water draining away quickly. An advantage in dry conditions, a disadvantage after prolonged periods of rain. Where the ground on some plots has become compacted and not replenished with organic matter, the plot will remain sodden for a long time and will likely become compacted during heat of summer.

Cultivate the soil

During autumn, when most crops have been harvested, digging in leaves, straw, rotted manure and compost improves the soil structure, drainage and replenishes the soil with nutrients. Digging during March and April will have difficult wet conditions, and during winter the ground may be too hard. Digging the ground in autumn opens it up to winter frosts which will help break down clods and kill off many pests.

For most people it is not possible to dig a entire plot every year. Dividing a plot into 4 or 5 areas for crop rotation provides a good reason to dig one area each year, adding lots of organic matter. The other areas will benefit from a deep rotted mulch simply laid onto top of the ground, perhaps around fruit bushes and trees, shrubs and other permanent plants.

Potassium deficiency

While Jubilee allotments are not heavy clay, potassium deficiency is worth considering when checking fruit trees and bushes.

A heavy clay soil will likely suffer from potassium deficiency. On fruit trees and shrubs this is usually indicated by the edges of the leaves curling up and going brown as if scorched, with brown patches on the undersides, and yellowing of leaf veins. Purple spots may also appear on the leaf undersides.

This can be remedied by applying bonfire ash, seaweed meal, composted bracken, comfrey liquid or other organic potassium-rich fertilisers. An excess of potash, however, can upset the balance so that fruit is produced at the expense of young growth. In the longer term the soil structure should be improved by adding plenty of well-rotted compost or manure. Wood ash has a high potassium content but should be composted first, as it is in a highly soluble form.

Soakaway drainage

Some plots have been dug leaving drainage gully around the edges. While this pushes extra soil into the middle and slightly raises the centre ground, it does encourage water to drain away from the centre, wastes potential growing space and leaves a dangerous gap for passers by to trip into. However, it does discourage grass from the paths creeping into the cultivated ground.

To achieve a raised area on a plot it is better to dig two trenches across the plot turning the soil into the space between. Fill the trenches with organic matter like manure, straw and mulch which will allow drainage and rot down. The raised area will usually become less wet and allow earlier planting.

Many crops like clay soil

Some crops like clay because it retains water, and for these crops you may find watering need is less frequent once the plants are established. Clay, however, is rich in nutrients and just needs help, and the right plants, to release them.

Brassica like cabbage, kale, cauliflower, and Brussels sprouts are good examples of long standing crops that thrive in clay soils. The plants grow tall on large shallow roots, using the heavy soil as an anchor. Plant well established seedlings, not seed which will rot quickly or be eaten by pests.

On Jubilee plots we see sweetcorn, pumpkin, squash and beans do well. No doubt these benefit by extra feeding during flower and fruit periods for bonus crops, but usually need little extra other than regular watering.

Most fruit trees prefer a solid ground to anchor themselves. Across the allotment site there are good examples of apple, pear, plum, gage and cherry. Apply deep mulches around trees, shrubs and other permanent plants to reduce summer dry and help conserve moisture.

Grape vines seem to do well, despite being waterlogged in spring. Plot holders will be able to provide cuttings rather than buying an unknown variety.

Although not great clay lovers, crops like parsnip, peas, carrot, beetroot and lettuce grow reasonably well provided they get a good start and the underlying soil has been loosened by a light dig. These crops usually get started by sowing seed in a shallow drill of general compost.

Green manures

Field beans, forage rye and white clover are recommended for clay soil. However, experiments on Jubilee ground were not very encouraging. We saw good results with phacelia, vetches, ryegrass, mustard and alfalfa which may reflect that the surface soil is quit good. More information about green manure.