Clay Soil

Gypsum if your pH is already where you want it. Lime only if a test shows the soil is genuinely acid, because lime raises pH as well as adding calcium. Most British clays are neutral to slightly alkaline, which is why gypsum is usually the right answer.

Liquid gypsum starts working within weeks; a measurable change in structure takes a season, and the rock dust and humic acid are multi-year inputs. Organic matter, mulched on every year, is what makes it permanent. How long liquid gypsum takes to work has the detail.

Yes; the liquid gypsum and the wetting agent are the two most useful things you can put on a compacted, puddling lawn. See lawn care and is liquid gypsum good for lawns.

The rest of the amendments, including the calcium and magnesium sources, are in soil conditioners; the whole liquid gypsum question set is in the liquid gypsum FAQ.

You can, but mix the first few centimetres into the clay rather than laying a clean layer on top. Where good soil sits directly on unbroken clay, water drains through the topsoil and then stops at the clay, leaving a wet, airless band where roots rot. Loosen the clay surface, work some topsoil and compost into it, then add the rest.

Clay usually holds nutrients well, and many clays are naturally rich in potassium, so the limit is structure rather than fertility. Feed modestly with a balanced, slow-release feed, and put most of your effort into organic matter and drainage. Heavy feeding on cold, wet clay in early spring is largely wasted, because roots cannot use it until the soil warms and drains.

Plants from dry, free-draining places struggle: lavender, rosemary, thyme and most silver-leaved Mediterranean plants, many alpines, and root crops such as carrots and parsnips, which fork in heavy ground. Plenty thrive on clay once drainage is reasonable, including roses, hostas, astilbes, asters, most fruit trees and many flowering shrubs.

A sturdy garden fork does more good than a spade, because it loosens clay without smearing a smooth, sealed face. A mattock helps on very hard ground. Timing matters more than the tool: work clay only when it is moist but crumbly, never when it is wet enough to stick to your boots, or you will compact it further.

Usually not. To change the texture of clay you would need sand in very large quantities, roughly equal to the volume of soil you are treating. Small amounts mix with the clay to make something closer to concrete. Organic matter, gypsum where the clay is dispersive, and keeping off wet ground are far more effective.