Essential Plant Nutrient Elements:

Vegetable plants, like all other green plants, require several nutrient elements for growth, development and productivity. Among the necessary plant nutrients are carbon, hydrogen and oxygen, available from the atmosphere and from water, and nitrogen, phosphorus, potassium, calcium, magnesium and sulfur supplied from the mineral portion of the soil. These elements are used by plants in relatively large amounts and are referred to as macronutrients. Micronutrients, or trace elements, used in small quantities, include boron, chlorine, copper, iron, manganese, molybdenum and zinc. These 16 plant nutrients, whether used in large or small quantities, are absolutely essential for plant vigor and productivity. A deficiency of any of these nutrient elements can limit plant growth and development and, ultimately, yield. Most soils contain sufficient amounts of the micronutrients needed to support plant growth. However, soils may be lacking in some of the macronutrients, particularly nitrogen, phosphorus and potassium. It is the job of the gardener, therefore, to ensure the presence of all the essential elements supplied by the soil in the right quantities and the right chemical forms for plant use. This is done by supplying organic matter and by the judicious use of fertilizers to maintain or increase soil fertility.

pH and Soil Fertility:

Soil pH is the level of acidity or alkalinity of the soil. Most vegetable crops prefer a slightly acid pH of 6.0 to 6.8, although this varies with each vegetable crop. Nutrient elements in the soil may be chemically tied up or bound to soil particles and unavailable to plants if the pH is outside of this range. Soil pH can be raised by applying ground agricultural limestone containing calcium and some magnesium. Lowering soil pH is achieved by adding sulfur, either in its elemental form or as a component of some fertilizers.

Regular Fertilizer Applications:

The gardener cannot assume fertile soils will not require a periodic fertilizer application. Plant nutrients are lost from the soil over time through use by plants and by leaching with water. It is important that these lost nutrients be replaced. Low-fertility soils will require not only maintenance fertilizer applications to replace lost nutrients, but also applications that will improve the overall soil fertility and support plants at a productive level. It is impossible to determine the fertility level and pH of garden soil by looking at it--fertilizer and lime applications are best made based on the results of a soil test. Contact the URI Cooperative Extension for information on soil testing, or see the GreenShare Factsheet on soil testing for an order form in PDF Format.

Commercial Fertilizers:

Nitrogen is the plant nutrient most often in short supply in the soil. It is usually necessary to supply additional nitrogen each season. Phosphorus and potassium are also required in large amounts. Nitrogen is essential for vigorous vegetative growth and development. Phosphorus is necessary for good root development and for fruit and seed production. The role of potassium is not as well understood, but is important for overall plant development. These are the three nutrients that are almost always present in commercial fertilizers.

Commercial fertilizers, whether organic or inorganic, always have three numbers printed on the bag or container. These numbers or analysis represent the percentage of nitrogen (N), phosphorus (P2O5) and potassium (K2O) in the product. For example an 8-16-16 analysis fertilizer contains 8 percent nitrogen, 16 percent phosphorus and 16 percent potassium. The rest of the product is an inert carrier material. A complete fertilizer will have a number for all three nutrients. Some fertilizers are incomplete, meaning that they contain only one or two of the nutrients. Urea, for example, has an analysis of 45-0-0, and contains 45 percent nitrogen with no phosphorus or potassium. Inorganic or chemical fertilizers usually have a higher analysis than most organic fertilizers and are less expensive. The nutrients in chemical fertilizers are in a form that is readily available to plants. Organic fertilizers contain nutrients in forms that must be chemically changed in the soil before the plants can use them. These nutrients, although not immediately available, are usable to the plants over a longer period of time. Some chemical fertilizers will contain nutrients in both readily available and slow-release forms. Organic fertilizers are less likely to burn plants and often contain several micronutrients in addition to nitrogen, phosphorus and potassium.

Application Methods:

Fertilizers are often applied over the garden area before planting and then are tilled or spaded into the soil. Calibrated spreaders used for lawn fertilizer application can be effectively used for applying garden fertilizer. Do not use fertilizers containing herbicides in the vegetable garden. Row or band applications make the most efficient use of small amounts of fertilizer. This method allows for fertilizer placement near the plants where it will be of the most use. Make small furrows about three inches to each side of the row and two to three inches deep. Use caution, however, so that seeds or roots do not come into direct contact with the fertilizer. In lieu of a soil test recommendation, apply two to three pounds of fertilizer per 100 feet of row. Select the fertilizer analysis according to the type of vegetable grown. For leafy vegetables, high nitrogen fertilizers such as 12-12-12 or 15-15-15 analyses are appropriate. For vegetables grown for their fruits, seeds, roots or bulbs, fertilizers such as 6-24-24, 6-12-18 and 8-16-16 or equivalent are satisfactory.

Growing healthy, productive plants in the vegetable garden involves attention to many cultural details. Since fertilization is just one of these details, fertilizers should not be considered a cure-all for all gardening problems. When properly used, fertilizers are extremely valuable in obtaining good gardening results. Fertilizers cannot correct or improve poor soil structure that requires additions of organic matter, nor can it compensate for an incorrect pH or unfavorable weather conditions. Choosing inappropriate vegetable cultivars and not practicing weed or pest control cannot be corrected by applying fertilizers. The use of fertilizers is just one of a number of integrated practices that lead to gardening success.

Adapted from Marianne Riofrio, Ohio State University Extension, 2000