Camellias do very well in a wide range of soils, whether sandy, loamy or clay. They do not demand the same soil as rhododendrons, as most people seem to think, though they grow well alongside them.
The only soil conditions that really are too hard for them are limestone and chalk, where their leaves go yellow, making them look very sick (Yellow leaves may also be virus).
They can't cope with really boggy soil either, because the roots drown and rot. If the soil is wrong, camellias are still easy to grow, in pots, tubs and raised beds - see below.
Camellias are never happier than when they get their roots down into clay subsoil and this is one of the reasons why they do so well in London gardens.
Once established they can get everything they need from the soil, but it is well worth preparing the soil, and mulching and feeding, especially when a young plant is establishing itself.
Any well-rotted humus-rich material will help, dug into both the surface layer and deeper. This feeds and helps to retain water and is especially helpful on light, sandy soil.
We all yearn for the perfect well-rotted manure, but if not available, the bags of tree and shrub planting compost are ideal. Composted bark, mushroom compost, leaf mould, peat and garden compost are all ideal and make a very useful mulch as well. Beware chipped bark - it needs to be composted unless you want to mulch the surface.