In most years it is only necessary to dig the soil, or fork it, to the depth of a spade blade. However, every three to four years it is a good idea to dig more deeply. This stops a compressed layer forming below the cultivated section which will impede drainage and it allows compost or manure to be included at a deeper level. The deeper cultivated soil encourages plant roots to go downwards, drawing nutrients from a greater depth, and this means that they are also more able to withstand drought. Plants can also he grown closer together.
There are two methods of digging based on how deep the soil is dug. Single digging is digging trench of one spade level deep and replenishing the soil back. Double digging is done in a similar way to single digging except that the soil at the bottom of the trench is loosened to a further blade’s depth with a fork. A good layer of manure or compost is put into the bottom of the trench, then the soil from the second trench is spread over the manure in the first and soil and compost are forked to mix them. The last trench is filled with the soil from the first.
Heavy soils are usually dug in autumn. The surface is left to rot so that frost will help to break up the heavy clods. The surface is forked lightly in the spring, before sowing or planting. The soil in a narrow bed can be formed into a long ridge and covered in manure. This helps drainage and still allows the weather to break up the soil. With light soils it is better to cover the surface with compost or manure through the winter but leave any digging until the spring.
Once the soil has been dug it is important to keep off it. The structure of the soil is easily destroyed so it is wise to keep beds to a size that allows you to attend to growing plants without going on to the soil. Lay a plank on the surface if you have to walk on it.