Although conditioning the soil with well-rotted organic matter improves its structure and helps drainage, there arc extra methods to help with waterlogged soils, caused by tightly packed particles of silt and clay, or a high water table.
Where wet soil is a problem, analyze the symptoms and causes carefully and decide on a plan of action. Winter wetness resulting from badly cultivated, heavy clay soils can sometimes be alleviated with good cultivation and the addition of coarse sand or grit (not fine sand, which can further clog the soil). Try digging a hole. If this fills with water, then the water table is high and artificial drainage will be needed. Sometimes there is a hard layer or pan lying 12-18 inches under the surface. Pans arc often found where soils are rich in iron and aluminum and this thick, impenetrable layer stops water from draining away from the top layers of soil. Deep digging to break up the pan (a mattock or crowbar might be needed in the process) is the long-term solution.
Providing artificial drainage
Where a whole garden is badly drained, investing in a proper drainage system will carry water away and make gardening possible and enjoyable. The most common system is land drains, generally laid in a herringbone pattern to collect the water and discharge it either to a ditch or, if there is no accessible ditch, to a infiltration device. Lay the main drains to run parallel with the slope of the ground, making trenches 2-5 feet deep and 12 inches wide. In most heavy soils, a mini-digger could be used to excavate the ditches. Do this when the ground is dry and avoid criss-crossing the site to minimize soil compaction. You would general space the side-drains coming off at angles from the main drain, 15 feet apart on clay and 25 feet apart on loam.
The central drain pipe (usually perforated plastic these days but they used to be clay lengths butted up to each other) should be 4 inches wide and the side-drains 3 inches wide. Lay the pipes on a good 2-inch layer of shingle and cover with more shingle before the soil is replaced. When excavating, make every effort to separate subsoil and topsoil, so the topsoil can go back in at the top. Sometimes a French drain or rubble drain is sufficient to drain a small area. Dig a 2-5 toot trench at the same spacing as land drains, but instead of laving a pipe, simply fill with broken bricks, flints or similar rubble, top with shingle, and cover with soil. On tint land, the trench and pipes must slope down into the ditch so the water can run off.