Posted: 09/10/2015 at 11:54
As I understand it, fertile soil is full of minerals which feed the plants so nitrogen, phosphate, potassium, iron, magnesium and so on. The soil structure needs to be good to release these goodies to the plants so heavy clay soils need opening up with houmous and grit to help roots grow and penetrate. Lighter soils need houmous to retain the minerals and moisture and stop it all draining away so fast the plants can't absorb what they need.
Humous rich means there's plenty of organic matter which opens up the soil and keeps it aerated as well as retaining moisture but allowing drainage. Adding well rotted manure will do this for you and increase fertility.
Garden compost is less nutrient rich but is a good soil conditioner and adds micro organisms which help release nutrients to plant roots. If you have lots of nettles in the compost, they will add nitrogen which is good for leafy plants and comfrey will add nutrients good for flowering and fruiting plants. You can also make liquid "teas" with these two plants for feeding specific plant groups according to their needs.
Spent compost and leaf mould are also good soil conditioners and mulches which don't add many nutrients but do help improve soil structure and retain moisture in the soil when used as a mulch.
An annual autumn mulch just spread over the soil and around the plants will be worked in over winter by the worms so is the easiest option once your beds are planted and the choice of material used - manure, garden compost or spent compost and leaf mould will determine how many nutrients are added.
Lavenders come from hot, dry places with alkaline soil so they need to have good drainage to stop their roots rotting and full sun to flower well. The British varieties such as Hidcote and Munstead Dwarf are the hardiest in UK winters but still need good drainage to do well. I grow mine at the top edge of a bank next to the retaining wall which gives them dry feet in cold, wet winters. The soil is neutral to alkaline clay improved with plenty of garden compost and they are in full sun.