We are not all blessed with perfect fertile soil, in fact this is impossible as plants have different requirements, for example some prefer alkalinity while others acidity. But, as Martyn Loach explains, a soil's composition can be improved and its character altered to accommodate what you wish to grow.
Before thinking about improving your soil, you have to get to know it. When a child goes to school he isn't automatically put into the top class and expected to produce the best results. The school get to know the student's capabilities, they then set about providing an education that is designed to allow his/her mind to flourish and eventually bear quality fruit.
In much the same way, analyze your soil and find out what its current capabilities are. On a basic level you can buy ph test kit to show acidity / alkalinity but you can probably measure this by looking at the type of plants your neighbours are growing. A more comprehensive report is best done professionally, in this way you will discover the soil's exact make-up and any deficiencies that need to be addressed.
In nature leaves and various bits of decaying vegetation plus animal manure give the soil the nourishment it requires. But we tend to clear that type of stuff away (hopefully you will turn the leaves into leaf mould and add the vegetation to the compost heap). This organic matter needs to be returned to the soil, dug in over the autumn and used as a mulch. As well as feeding the soil with nutrients, it will improve drainage in heavy soils and increase the water holding capacity of light soil.
Alternative soil conditioners can be used such as mushroom compost, wool shoddy, seaweed, composted pine bark, spent hops and peat. What you use and how much will depend on your soil's analysis and the plants you wish to grow. For example mushroom compost is slightly alkaline so will be good for acid soil, pine bark has no nutrients so is best used as a mulch, while seaweed is particularly rich in trace elements.
Here are five different types of soil and how they can be improved:
The main problem with clay is drainage, as a result it should be nutrient rich but hard to work. Dig over in the autumn, introducing organic matter, and let the weather do its work. Lime will encourage the tiny particles of clay to bind and improve drainage, but you must balance what you want to grow with the amount of lime you use. If its very heavy, work in course grit, you can also raise beds to help the soil both dry out and warm up.
Easy to work, but any nutrients are easily drained, so large amounts of organic matter can increase its water retentiveness and additional fertilizer can be used to replace nutrient deficiency. Try to mulch as much as you can and cover the soil with lots of vegetation all year round to minimize nutrient loss. If you are growing vegetables, sow green manure, when this is dug in it will add nutrients and improve the soil structure.
With a structure similar to clay, the biggest problem is drainage. It's unwise to to walk on this type of earth when it is wet as you will only compact it even more. Dig it over in the autumn so that frost can help to break it down, dig in coarse grit and organic matter and use raised beds to warm up and dry out the soil.
This like sand is a free draining soil from which nutrients can easily be leached. It is also very alkaline, so the range of plants that can be grown is limited. Digging in the autumn is not really necessary, leave that to the spring before you sow anything, and as there's probably not a lot of top soil, don't dig too deep. Add organic matter to increase acidity and aid drainage. Make sure that the ground has a good covering of vegetation throughout the year, including green manure in the winter and mulch with peat, grass cuttings and manure to increase acidity.
This type of soil has a tendency to be acid so use lime to balance the ph especially for vegetables and fruit. Peat is made up from decomposed vegetation so there is no need to dig or add any organic matter, but you may have to add an organic fertilizer to combat any nutrient deficiency.