Mineral Nutrition

F or normal growth and optimum production, fruit the quantities that they are in certain annual crops. Generally, nitrogen is the only macronutrient that must be consistently supplied to fruit trees in California. Under certain conditions, other nutrient deficiencies can develop. Toxicities can also become a problem. These will be discussed in the sections for individual nutrients. trees require 13 essential nutrients in varying amounts (table 13.1). Those needed in relatively large amounts are termed macronutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Those needed in smaller concentrations are called micronutrients -chlorine (Cl), iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu), and molybdenum (Mo). Nutrient problems (deficiencies and excesses) can often be identified visually, but it is advisable to chemically analyze leaf samples to confirm the problem and quantify its severity. Even without obvious problems, leaf samples should be taken to check the effectiveness of one's nitrogen fertilization program and to watch for gradual changes in nutrient levels. Leaf samples should be taken in June or July when nutrient levels are relatively stable. For peach and nectarine, basal to midshoot leaves on moderately vigorous fruiting shoots (10 to 20 inches long) are sampled. For plum, the standard is nonfruiting spur leaves. Each sample should include 60 to 100 leaves taken randomly from within the orchard. The leaves are washed, dried at 150 Growers must learn to manage these nutrients for optimum growth and production and to minimize adverse environmental effects. Most important is balancing the nutrients -when one element is deficient, its absence can negatively affect plant processes and thereby inhibit optimum uptake, utilization, or distribut ion of o ther e lements . On the other hand, an excess of any element may be toxic to trees, thus affecting the availability of other nutrients in the soil. Besides the great variation in amounts of different nutrients needed by plants, concentrations of nutrients in individual organs and at different times during the season also differ substantially. For instance, the calcium content of leaves can be 10 to 30 times higher than that in the fruit (table 13.1). Also, the percentage of leaf calcium steadily increases over the season, while leaf nitrogen content declines. Therefore, efficient nutrient management calls for supplying the right nutrient to the right organ at the right time. Most of the elements essential to trees need not be supplied in a regular fertilizer program because (1) many elements are abundant (and available) enough in most soils to provide an adequate supply; (2) perennial trees recycle a substantial portion of some nutrients from the leaves back into the tree structure before leaf fall, and (3) for some nutrients (i.e., phosphorus), very little is removed in the crop, leaves, and prunings. Because of the last two reasons, nutrients like phosphorus are not needed in nearly o to 160 F (66 to 72C), and sent to a chemical laboratory for analysis. Optimum leaf concentrations for each element are shown in table 13.1.