As it sounds, this method involves a deeply dug garden bed. The preparation is more labor intensive but the yields much greater…up to 4 times that of a conventional bed. Deep bed gardening aka the Chinese method or the French intensive method has been around for hundreds of years used in various countries by farmers that lived just outside of the city on small expensive land and needed high yielding crops.
A deep bed is essentially a double dug bed which is exactly what it sounds like. The bed is dug in trenches about the depth of a spade and the soil at the bottom of the trench is then loosened to the depth of a pitch fork- each trench being “dug” twice. The soil from the first trench is set aside and each trench dug is filled with the soil of the previous trench. The last trench is filled with the soil of the first. Compost, manure, or peet would be added along the way. Suggestions are either to layer it on top of the soil before you begin digging, or layer it into the trenches as you go.
The size of the bed is determined by what is comfortable for the gardener. The most important rule of deep bed gardening is to never compact your soil which means never ever walk on it. Take this into consideration when planning your bed size. Make paths if needed or keep your bed small enough that you can comfortably garden from the perimeter.
Deep bed gardening has been very successful for many years because it allows the roots of the plant to grow down rather than sideways. This frees up space in the garden and your plants can have much closer neighbors. It is recommended that generally plants can be spaced 4 times closer and in triangular or diamond patterns rather than the traditional row. The goal is to space the plants out so when mature the leaves just barely touch their neighbors. This will create an environment that conserves moisture in hot dry climates. There is no consensus on whether to double dig each year or if forking through the soil is sufficient.
Studies have shown a consistant 4x higher yield than a conventional garden. The US department of agricultural stats claims the average adult eats roughly 320 lbs of vegetables each year. A 100 square foot double dug garden bed can produce between 200 and 400 lbs yearly.