Several separate but related factors control vegetable garden soil conditions. These include drainage, organic matter content, pH reading and nutrient content. All can be manipulated and managed to some extent by a gardener who understands what qualities are needed by leafy and root vegetables.
Sandy soil drains well, while clay soil drains poorly. The ideal lies between these extremes. A sandy silt loam soil will allow water to drain slowly but easily. Percentages may vary, but a soil of this type will have about 45 percent sand, 40 percent silt and 15 percent clay. A classic loam has percentages closer to 40-40-20.
A silt loam will have a fairly high organic matter content--perhaps as much as 5 or 6 percent. This organic matter is sometimes called humus. It consists of vegetative matter (compost or cover crop residues) that has decomposed to a point at which it is stable and can break down no further. In the garden, it helps retain moisture and plant nutrients.
Soil pH Reading
The soil pH scale is a measure of acidity and alkalinity that runs from 0 (acid) to 14 (alkaline), with pH 7 being neutral. Most vegetables like soil that is slightly on the acid side of neutral, measuring about pH 6.5 to pH 6.8. Achieving the correct pH balance is essential, because if the soil is too acid or too alkaline, plants will be unable to access nutrients, even though they might be present in great abundance. A relatively small shift in soil pH makes large quantities of nutrients unavailable to plants.
Organic gardeners feed the soil so it provides natural nutrients to feed their plants. Synthetic chemical substances that feed plants directly often pollute the environment and make the plants totally dependent on artificial additives. A soil test by a qualified laboratory will show the range and quantity of nutrients present in soils, allowing any deficiencies to be corrected.
Sandy soils are so porous that they fail to retain many nutrients. Clay soils can retain nutrients well but drain so poorly that they flood, allowing nutrients to be washed away. This is why a sandy silt loam soil with a high humus content---which retains nutrients in plant-available form---is the ideal most vegetable growers seek to achieve.
A laboratory soil test will show the quantity of basic plant macro- and micro-nutrients, the organic matter content and the soil pH. It will also recommend which soil amendments to use, along with the quantities needed, to bring the soil into balance for successful vegetable growing. Advice on selecting a soil testing laboratory is available from your local county Cooperative Extension Office, which is listed in the phone book and can be found online.