The soil that you find in your local area may be quite suitable to use as a base for improvement but very few of us are fortunate enough to live in a place where it is suitable for the sustained growth of nutrient hungry vegetables. Whether you decide to improve your local soil or import new soil is an individual decision that should be made carefully as it will have great influence on the productivity of your raised garden bed, particularly when you first start off.
Soil or dirt is composed of particles of broken rock that have been altered by chemical and environmental influences such as weathering and erosion. Soil is grouped into different types according to particle size and falls into one of three groups, sand, silt or clay. Sand, silt and clay can be combined (either naturally or by design) to make a loam. Different sized soil particles have different properties that when combined can provide specific characteristics that will make it suitable for a given purpose, such as a vegetable garden.
Sand particles vary in size from 0.0625mm to 2mm in diameter and feels gritty when rubbed between the fingers. Sand has excellent drainage and aeration characteristics but does not retain plant nutrients well and by itself will only support growth from plants adapted to a dry low nutrient environment. When added to denser soils it makes an immediate and permanent improvement to the drainage character.
Silt particles are smaller at 0.0039mm to 0.0625mm in diameter and feels a lot like flour when rubbed between the fingers. Silt is generally transported and then set down by water and as a result has a very even fine structure that is usually very rich in minerals leached from soils at higher levels. Indeed, silt deposited by the annual floods of the Nile River created the rich fertile soil that sustained the Ancient Egyptians.
Clay has the finest particle size by far at 0.0039mm or less. The small particle size of clay allows it to become waterlogged very easily making it difficult to work with, the clay becoming heavy and very plastic. When dry most clays set harder than stone making them equally difficult to work. Despite clays high mineral content it is considered to be the hardest soil to start out with when creating your own garden soil.
Loam is soil composed of a mixture of sand, silt and clay. There also should be a reasonably high content of organic matter or humus as it is sometimes called. For good general purpose vegetable garden soil a loam comprised of 40% sand, 40% silt and 20% clay will provide an base that will be gritty, moist and retain water and nutrients well without become waterlogged and unmanageable. This mixture can then be built up by adding organic matter such as compost.
An advantage of raised garden beds is that dependence on the native soils is greatly reduced, you can fill your frame with any soil you wish and rely on the frame to prevent it from spreading to and mixing with the less desirable soil. Purchasing a soil mix from your local nursery or landscape supplier can be a short cut to a quick productive vegetable garden. Alternately you may prefer to improve your local soil however be aware sometimes this can be a slow process.
Examine your local soil and determine its type. Is it sand, silt or clay. If it is sand then adding silt and vegetable matter in suitable quantities will provide a fairly quick fix. If it is silt you may find that it does not drain well and this can be easily remedied by adding sand. Clay can be more challenging, this is the type of soil found in my local area and improving the natural soil has proven to be challenging. Adding gypsum slowly loosens the clay up and allows it to separate into loose particles. Drainage can be improved by adding river sand.
Regardless of the type of soil you use in your raised vegetable garden bed the nutrients contained in the soil will become depleted over a relatively short period of time and it is common for most gardeners to add additional organic material such as compost and manure every year or so at a minimum. Improving your soil and maintaining it is an ongoing task that can never be really completed.