Clay soils are often formed of sticky pliable clods when moist,and rock hard clods when dry.
When squeezed in your hand it has a smearing quality.
It tends to drain slowly, hold water, and reacts slowly to temperature changes, staying cold for longer in spring.
There are basically two form of clay; Blue/Grey and reddish brown.
The blue/grey clays have poor aeration and must be loosened in order to support healthy growth.
The reddish clay usually has good aeration is quite loose, and drains well.
Most clay soils contain high nutrient levels and plants grow well if the area is well drained.
Improving clay soils:
Aerating clay soils is essential, pore spaces between clay particles are often filled with water, a situation that is worsened on compacted soils.
In autumn rough dig the area and then leave the soil in coarse lumps to over winter.
The action of the winter weather will break the clods into a friable material.
At the same time add a mulch of organic matter such as farmyard manure, straw or composted bark.*
* It is advisable to add some form of nitrogen if using bark, to supplement that used in the bark's decaying process.
Fungal growths decomposes the organic matter, and helps to bind the clay grains together, this in turn improves drainage and aeration.
Regular manuring or composting is essential to maintain this process.
Digging in sharp river sand or grit combined with organic matter will also improve the texture.
Before adding lime consider what is to be planted in the area.
Adding lime will produce fewer benefits than adding organic matter.
Lime will bind clay grains together and raise pH.
If it is deemed necessary to alter the pH, add Gypsum (calcium sulphate) instead of lime (calcium carbonate) this will preserve the acidity.
When planting anything out in clay, it is sometimes advantageous to excavate a planting hole that is much larger than the intended plants root ball and backfill with good soil / compost.
This particularly relevant when planting out perennials such as shrubs and trees.
Consider treating clay soil areas as features, e.g.
Form pools or ponds, or create a bog garden for moisture loving plants.
To avoid walking on wet areas, lay stepping stones or a system of duckboards.
Build a patio.
Form raised beds with walls of brick filled with imported soil.
Build a raised rockery.
Form a Kitchen garden many vegetables will tolerate clay soil.
To avoid walking on vegetable beds form 1.2m -1.5m (4ft -5ft) wide beds, and use the paths formed between them, for access.