Nutritional needs of fruit trees are defined as the amounts of all mineral nutrients that must be acquired during one growing season to sustain normal growth, high productivity, and optimal fruit quality. Nutritional needs may be determined by seasonally excavating well-producing trees to quantify the amounts of nutrients absorbed, but this is a costly and labor-intensive task because trees have large canopies and extensive root systems. Furthermore, one must account for contributions of nutrient reserves, which are stored in old roots, trunks, branches, and twigs, toward nutrient demands of new tissues. The use of stable isotopes as nutrient tracers, e.g., 15N, allows for distinguishing between the reserve and newly absorbed nutrients, thereby making such accounting possible.
A soil's natural store of nutrients may be sufficient to satisfy nutritional needs of fruit trees. Under most conditions, however, nitrogen (N), potassium (K), and calcium (Ca) and, less often, phosphorus (P), magnesium (Mg), boron (B), copper (Cu), iron (Fe), manganese (Mn), and/or zinc (Zn) must be supplied as fertilizers. Fertilizer needs of fruit trees are defined as the total amounts of all mineral nutrients that have to be applied as fertilizers to sustain normal growth, high productivity, and optimal fruit quality. Fertilizer and nutritional needs are numerically different because (1) the soil always satisfies at least a portion of plant needs for a given nutrient and (2) the efficiency of fertilizer recovery by plants is less than 100 percent due to losses caused by leaching, volatilization, or other chemical or biological interactions in the soil.
298 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT METHODS OF ESTIMATING FERTILIZER NEEDS
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