Any gardener will tell you that successful gardening always starts with the soil. Look after your soil and you’ll be able to grow a far greater range of plants. Ideally what you need is a fertile, well-drained, moisture retentive loam. But chances are your soil will be anything but that. In my neck of the woods here in Dulwich it’s a heavy London clay. If uncared for it becomes compacted and waterlogged in winter and cracks and dries as hard as concrete in summer.
But a bit of time and effort can change all that and now is the time of year to do it. The best way to improve any soil, clay or otherwise, is by the addition of organic matter. This could be homemade garden compost or leafmould, rotted farm manure or composted bark. When added to soil organic matter improves the aeration, water-retention and drainage of the soil and, as it is broken down by soil organisms, adds nutrients as well. My preference is a well-rotted farm manure as it tends to have higher nutrient levels and when mixed with the soil is great for making clay more workable.
If your soil is in particularly poor condition and hasn’t been blessed with organic matter for many a year, forking it into the soil will really help, especially if new plantings are to follow. Otherwise, you can just spread it over the surface as a mulch and let the worms and other soil critters do the rest. When spread thickly about 5cm (2 in) deep, mulches also help to conserve soil moisture and suppress weed seeds from germinating. I also think the dark rich texture really sets off garden plants as they burst into life in spring. And burst into life they will, if only to thank you for looking after their world.