Good soil is a precious and wonderful thing but needs to be managed correctly to keep it in good shape. Just as poor soils can be improved by good husbandry, good soils can be be ruined by bad practice. Organic matter is the key to soil fertility and it comes in different forms. Garden compost, leaf mould, well rotted farmyard manure, seaweed, crop debris and bagged compost from the Garden Centre are all good examples and should be added to your soil at every available opportunity. Organic matter will improve the water retention of thin sandy or chalky soils and help breakdown heavy clay soils into something more manageable. It also encourages earthworms which digest it and in the process create fissures and channels in the soil improving aeration and drainage.
Soil contains a network of channels called capillaries through which water flows and the action of frost, freezing and thawing, expands these capillaries, forcing the soil particles apart, and breaking up the soil. This is why farmers plough their fields and gardeners dig their vegetable patches over in Autumn, exposing as much soil to the action of frost as possible. The benefits of frost can easily be lost however by attempting to cultivate soil too early in the year when it is still cold and wet. Far better to wait and lose a week or two in cropping time than to damage your soil. Mechanical rotivation is a great help on a big plot but, again, only attempt it when the soil is not sticky otherwise damage will occur. Frequent rotivation, particularly with shallow soils, can lead to the formation of a “pan”, or hard impenetrable layer 8″ or so below the surface. This can impede drainage and restrict root growth. Farmers use a “subsoiler” to break up the pan, and serious (and fit!) gardeners use a technique known as double digging, in which not only the top spade full of soil is turned over, but also another spade depth below, taking care not to mix the subsoil with the topsoil.
Finally, mulching soil with bark or organic matter will help retain moisture, keep weeds down and improve worm activity.
can you suggest any solution for extra damp soil covered under trees
It’s quite unusual for soil under trees to be damp, its usually very dry. There are plenty of moisture loving perennials which will grow in a damp shady position, e.g. Astilbes, Irises, Hostas, Arum lilies, marsh marigold, and if you have plenty of room you could try Gunnera manicata which is like a giant rhubarb plant with leaves reaching 6ft in a single year. Addition of sharp sand or coarse grit may inprove the physical structure of the soil but is unlikely to help the excess water to drain away.