Fertilization and Fertilizers
Although the total amount of nutrients in the soil is important, the balance among them can be even more critical. Too much of a nutrient can be just as bad as too little. Excess magnesium may lead to calcium deficiencies, for example. Fertilization, or the addition of nutrients to the soil and plant, is the main method of adjusting the available nutrients. The degree of fertilization will depend upon the type of growth desired. Fertilization often is thought of in terms of greatest response, which might not always be the optimum response. Generally, in commercial crops, when the cost of fertilization is equal to or greater than the value of increased growth, there is little reason for continued fertilization.
When a fertilizer material contains nitrogen, phosphorus, and potassium, it is known as a complete fertilizer. The fertilizer analysis is the percent by weight of these three elements in the final preparation. For historical reasons, the nitrogen is expressed as elemental, phosphorus as P2O5 and potassium as K2O. A fertilizer analysis makes it easy to determine the exact amount of each element in a given quantity of complete fertilizer.
Since the needs of various crops differ, the plant requirements are expressed as a specific fertilizer ratio. If a plant needs twice the amount of phosphorus as it does nitrogen and potassium, using a material with a 5-10-5 analysis would be advisable since this fertilizer would have the needed 1-2-1 ratio. According to the analysis, 5 percent of the material by weight is nitrogen, 10 percent is phosphorus, and 5 percent is potassium. Therefore, when you apply 1 pound of the material to the soil you are applying 0.05 pounds of nitrogen, 0.10 pounds of phosphorus (as P2O5), and 0.05 pounds of potassium (as K2O).
Gardeners have the option of using two major groups of fertilizers: natural organic and synthetic chemical. Natural organics include dried blood, manure, fish scraps, and cottonseed meal. These compounds are derived from living organisms. The nutrients in most organic fertilizers generally undergo gradual chemical transformations into plant-available forms after they have been applied to the soil; thus nutrients from them are more slowly available to the plant than from chemical fertilizer sources. When applying most organic fertilizers to the soil, timing must be adjusted to account for the slower release of nutrients. For example, June-bearing strawberries have a high nutrient demand in the fall as they produce flower buds for the crop the following season. When using compost, it may need to be applied in the late summer so it will have sufficient time to decompose and release nutrients in time to meet plant needs in the fall. Chemical fertilizers such as ammonium sulfate or superphosphate are prepared from inorganic minerals. The nutrients in most of the natural organic fertilizers generally undergo gradual chemical transformations into available forms after they have been applied to the soil. Most chemical preparations, on the other hand, are available for the plant as soon as they are applied to soils containing adequate moisture levels.
For this publication, organic gardening is defined as gardening based on the production practices in the National Organic Standard (NOS) and pursued by noncommercial growers. If you plan on selling what you grow as organic, you must strictly adhere to the National Organic Standard, which can be viewed in English, Spanish, Japanese, and French. Information for commercial fruit production can be found in the Tree Fruit Production Guide or the Mid-Atlantic Berry Guide, which are guides produced for commercial growers. Nutrients from most chemical fertilizers are available to the plant as soon as they are applied to soils containing adequate moisture levels. Various combinations of organic and chemical fertilizers can be prepared depending upon your needs. With such a mixture, some nutrients are available to the plant immediately while the remainder is released slowly to meet the extended needs of the plant. When opting to use chemical or organic fertilizers, make sure that adequate nutrients are being applied. Table 1.1 (PDF) lists the nutrient analyses of some fertilizers.
Chemical and organic fertilizers can be purchased at local garden centers or through gardening catalogs. To locate organic fertilizers in garden centers, ask personnel which products are used in organic production. Gardening catalogs will typically identify a product as allowable in organic production.
Composts can be obtained through various sources. Many local municipalities have composting facilities where composts can be obtained for a nominal fee or, in some locations, for free. Composts can also be purchased from garden centers. Making your own compost is a great option because you control what the compost is made from in addition to reducing the amount of waste sent to landfills.
The nutrient content in compost varies depending on what materials make up the compost and on the composting protocols used. Therefore, it is recommended that composts are tested, particularly those that you make or obtain from local municipalities, to determine the amount of nutrients they contain (kits for doing this are available through local extension offices). Finished compost typically contains 0.5 to 2.5 percent total nitrogen. As a general rule, about 10 percent of the nitrogen will be available to the plant each year. Compost generally contains very little phosphorus for plant use, so phosphorus from alternate sources is typically needed to meet plant requirements. Potassium in composts is in a form that is readily available for plant use, but this form is also water soluble and, therefore, can leach out of compost piles. Placing a cover over a compost pile can help reduce the amount of potassium lost to leaching. In addition to determining the nutrient content of compost, the pH of the compost should be measured because it can be unsuitably high for fruit production, particularly for blueberries, which grow optimally in low-pH soils. When using compost to fertilize brambles, be aware that primocanes have difficulty emerging through large clumps; therefore, breaking up large clumps is necessary when applying compost.
The type and amount of fertilizer to use in a given situation has been the topic of study for many years. Present recommendations are based upon correlations between plant response and chemical tests on the soil or plant tissue itself. The nutrient availability is not a direct function of the total nutrient content of the soil. The available nutrients are related to the exchangeable cations, soil pH, and organic nature of the given soil. Over the years, quick tests have been attempted, but most are inaccurate.
See Table 1.2 (PDF) for the home fruit garden monthly maintenance schedule.