The single most important ingredient to successful planting is soil preparation. Planting in inappropriate soil is a waste of time. Most garden plants require soils with good drainage yet good moisture retention, a slightly acid pH (5.5 to 6.5) and moderate fertility. Whether your thumb is green or brown, plants started in such soil will usually perform well.
If you live in farm country where topsoil’s are uniformly 6 to 7 feet deep, plants usually thrive with a minimum of soil preparation. But many urban and suburban gardeners live where soil was scraped level or filled and compacted just prior to construction. Most plants grow poorly, if at all, under these conditions, so the soil will need to be improved.
The easiest way to improve soil quickly is to add topsoil. Another long-term method is to add organic matter such as compost, sawdust, wood chips, peat moss or other organic byproducts. Organic matter benefits soils in several ways. It supports the diverse and essential population of soil microbes, without which the whole soil system begins to break down.
Beyond adding organic matter and adjusting pH, soils are typically deficient in nitrogen. Other necessary elements may also be in short supply. A soil test is the best way to find out the make up of the soil in your area. As a general guide, spread 5 pounds of 10-10-10 fertilize per 100 square feet of soil before planting.
Fast growing annual flowers and vegetables, such as petunias, lettuce and cabbage, benefit most from the addition of fertilizer before planting. They are significantly stunted by a slight shortage of essential nutrients. Perennial plants, trees and shrubs also benefit from the addition of fertilizers at planting time, but are rarely irreversibly stunted. You can wait until you see deficiency symptoms, such as yellow leaves or slow growth, before fertilizing.
Gypsum, a natural mineral containing calcium and sulfur, is often recommended for heavy clay soils. It is especially useful if a soil test shows excess sodium in relation to calcium. It is also useful if the soil test shows that the pH is good but calcium content is low. In these specific situations, gypsum loosens and improves clay soils.