A fertilizer management program should start before an orchard is planted. At that time, fertilizers can be easily incorporated into the soil to the depth of 25 to 30 centimeters, i.e., the zone of high root activity. This is particularly important for P, K, Mg, and Ca fertilizers as well as Zn, Cu, Mn, and Fe, which move very slowly down the soil profile. When applied to the soil surface in mature orchards, these fertilizers will need a long time to reach the main part of the root system.
In the first two years after planting, fruit trees are usually fertilized individually by uniformly spreading fertilizers around each tree in a circle two to three times larger than the canopy. Depending upon local soil conditions, such applications may involve N and/or other fertilizers. In young, nonbearing orchards, N applications are often split into two or more applications that start in early spring prior to the beginning of current season growth and extend into midsummer. Later applications may unduly prolong vegetative growth, thus exposing trees to possible winter injuries.
In mature orchards, fertilizers are applied to weed-free strips within tree rows or are spread uniformly over the entire orchard floor. The recommended rates and timing of applications vary among fruitgrowing regions and are influenced by the results of leaf and soil analyses, soil N mineralization rates, tree size, and orchard floor management practices. Typical N rates in apple orchards vary from zero to 100 kilograms per hectare. An early spring N application, prior to budbreak, stimulates current year growth but has a small impact on the N status of current season flower buds and fruit set. Such application, however, will have a more profound effect on the N status of flower buds and fruit set in the next growing season. Nitrogen applied in summer or early autumn will build up tree N reserves that will be utilized for flowering and fruit set the following spring. These late applications, however, may have deleterious effects on winter hardiness and/or fruit quality in the season of application.
Foliar fertilization with sprays of N, Ca, and microelements is practiced by fruit growers the world over. These sprays can supply nutrients directly to the foliage and fruit at times when they are most needed from the standpoint of tree productivity and/or fruit quality. For example, high fruit quality and long storage potential of apples and pears require high fruit Ca levels. Since these levels are usually higher than what a tree can supply from normal root uptake, pre-harvest foliar sprays or postharvest fruit dipping in a solution of Ca salts are widely practiced to enhance apple and pear quality and storage potential. Similarly, autumn foliar sprays with urea are practiced by some growers to elevate tree N reserves and fruit set the following spring. Under alkaline and neutral soil conditions, sprays with microelements, except Fe, are preferred over soil applications because of their enhanced effectiveness and a rapid plant response.
Fertigation, or applying nutrients with irrigation water, is another way fertilizers may be applied to fruit trees. This method is considered very efficient because it places nutrients in optimally wetted soil zones where roots are most active.
Conventional orchard management systems rely on the use of synthetic fertilizers to maximize orchard productivity and fruit quality. Sustainable orchard production systems, however, also emphasize orchard profitability, environmental protection, and soil conservation. Commonly recognized sustainable systems include integrated pest management (IPM), integrated fruit production (IFP), and organic production (OP). Under the IPM and IFP systems, soil conservation is emphasized and external inputs of synthetic fertilizers are minimized to achieve ecological stability and high profitability. Soil and/or plant chemical analyses are used to determine the need for fertilizer applications. Also, the methods, rates, and times of applications must be such as to minimize the risk of groundwater and surface water pollution. Mulching the soil strips beneath tree rows with organic composts is often practiced to supply plant nutrients, improve soil structure, and control weeds in order to minimize competition for water and nutrients. Under the OP system, no synthetic fertilizers are permitted, and soil fertility can be regulated only by applying natural composts or composted manures, or through the use of legume cover crops to enrich the soil in nitrogen and organic matter. Additionally, mineral nutrient sources that are mined or otherwise naturally occurring may be used, e.g., limestone to correct soil pH, calcium chloride for foliar sprays or postharvest dips to control physiological disorders on apples and pears, potassium sulfate, rock phosphate, copper hydroxide, etc. Leaf and soil analyses are pursued to verify the need for any particular element addition.
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