A mulch is any layer of material covering the soil. This could be organic, biodegradable material like compost, leaf mould, straw, cardboard, woodchips or seaweed, or inorganic material, like gravel, black plastic or landscape fabric.
The main benefits of all mulches are to help soil retain moisture and to suppress weeds, but there can be other good outcomes, including deterring pests, protecting plant roots and keeping certain crops off bare soil (strawberries or pumpkins, for example). Mulches of organic material like compost or leaf mould have the important extra benefit of improving soil fertility and texture. This is achieved as earthworms pull the material down into the soil and consume it, producing rich casts (worm poo), which add nutrients to the soil.
A healthy soil is one teeming with life: good for biodiversity and good for your plants. Unless soil is waterlogged or has been treated with chemicals, it should contain large numbers of bacteria and fungi, and numerous invertebrates including worms, nematodes, mites, centipedes, springtails and many more. All these create a rich ecosystem, which breaks down organic matter and makes it available to plants via their roots with the help of a network of fungal mycelium (known as a mycorrhizal association). To routinely dig soil is to damage its structure and disrupt these ecosystems, so many organic growers practice the no-dig method (easier on the back too!) whereby the soil is disturbed as little as possible. Mulching is the perfect accompaniment to the no-dig method – let the worms do the work for you.
Mulches are best laid in spring and/or autumn over a warm, moist soil, ideally between 5 and 10cm deep. They can be laid around established plants (but careful to leave some space around the stems) or on to bare soil and then the plants planted through the mulch when the time comes. Pots can be mulched as well as beds, you may just need to scrape a little away now and again to see whether the soil is dry and the pot needs watering.
If you have a large weedy area you can mulch with a layer of cardboard or black plastic. The weeds will gradually die off and you can plant crops through small holes cut in the covering. If cardboard is used it will gradually rot down, especially if covered with a layer of compost or leaf mould mulch.