- Soil pH indicates the acidity of your soil. Acid soils have a low pH, typically below 6.0, while alkaline soils have a pH above 7.0. Most vegetables and annual flowers prefer a neutral soil where the pH falls between 6.0 and 7.0. Some shrubs, including azaleas and roses, prefer soil with more acidity. A soil test indicates the pH of your soil and the amount of amending necessary to bring it into the correct range for the type of plants you are growing. Lime amendments lower the acidity and sulfur raises the acidity.
- The clay, sand and organic matter content in the garden determines the soil structure. Soil with a high clay content is more prone to compaction. It collects water and becomes soggy, which most plants can't tolerate. Sandy soils drain quickly and dry out. Desert and drought resistant plants, such as cacti and lavender, grow well in gardens with sandy soils. Loam soils have moderate amounts of clay and soil but most of the soil structure comes from decayed organic matter. These soils provide optimum drainage and most garden plants grow best in loam.
- Gardens that don't naturally contain loam soils benefit from the addition of organic matter before you plant. Organic amendments provide aeration to heavy soil so water drains at a suitable rate. The amendments also provide structure to sandy soil so enough moisture is retained for the plant roots to access. Compost provides a readily available option for an organic amendment. Peat, wood chips and composted animal manure also provide organic matter to soils that need improvement.
- Most soils contain trace amounts of nitrogen, phosphorus and potassium, the most vital nutrients for healthy plant growth. The amount of necessary soil nutrition depends on the specific needs of the plant variety, but many garden soils need a nitrogen amendment at the minimum and flowering plants usually benefit from additional phosphorus. Soil testing helps indicate how much fertilizer is necessary before you plant, but regular fertilization after planting replenishes the soil nutrients as the plants use them.