Successful vegetable gardening depends on soil health. Vegetables absorb nutrition from the soil in the form of 17 essential nutrients. Nitrogen, phosphorus and potassium are needed in the largest amounts but they cannot be absorbed unless soil has the correct balance between acidity and alkalinity. This balance is known as the pH level. Clay soil tends toward acidity and sandy soil to alkalinity. Both types are adjusted by the addition of soil amendments.
Acid and alkalinity in soil are measured in pH units which are expressed in a scale from 0 to 14. Acidity is associated with an increase of hydrogen ions and alkalinity with an increase in hydroxyl ions. A pH level between 6.0 and 6.8 is suitable for most vegetables. Fertilizer is material added to the uppermost layer of soil to enhance fertility. Soil amendments are incorporated into the soil.
Acid and alkalinity levels determine how plants absorb soil nutrients such as phosphorus. Phosphorus is an essential macronutrient that plants need to develop membranes, manufacture DNA and produce energy. Plants cannot absorb available phosphorus when soil is too acidic or too alkaline. A pH level of 6.0 to 7.0 is necessary for plants to effectively use phosphorus from the soil.
Standard chemical fertilizers list nitrogen, phosphorus and potassium as N-P-K on the label. If the pH level of soil is not in the necessary 6.0 to 7.0 range, chemical fertilizer will not be an effective method to improve plant growth. High amounts of phosphorus are not usable by vegetable plants. Phosphorus turns to an insoluble form that plants cannot absorb.
Overly acidic and overly alkaline soils benefit from the addition of compost, which can modify the pH level. Alkaline soil benefits from the addition of compost or peat moss. Clay soil is often overly alkaline. Acidic soil benefits from the high organic matter content of compost. Compost has been found to modify pH levels even when applied in a small amount.
Home soil-testing kits are sold at most garden centers. The kit provides a rudimentary analysis of the soil pH level. Professional soil-testing labs provide a more intricate and complete soil analysis, along with other important fertility determinants such as microbial population counts. The National Sustainable Agricultural Information Service provides a list of alternative soil testing labs (see Resources).