Fertilizer Application

Nitrogen
Nitrogen requirements are best determined by tree growth and performance. Soil tests for nitrogen have not proven useful in determining tree needs. Table 1-5 gives visual indices for judging nitrogen status of fruit trees. As mentioned previously, nitrogen levels in a foliar analyses should be higher for young nonbearing trees. The values should be towards the upper end listed in Table 1-2. Leaf nitrogen levels also tend to be higher in samples from trees carrying heavy crops. Biennial bearing trees in their off year or trees with a light crop generally have lower nitrogen levels. This is probably related to the inverse relationship between shoot growth and fruiting.

Nitrogen cycle and its availability is a complex situation. Nearly 80 percent of our atmosphere is composed of nitrogen. Some can be precipitated out during thunderstorms and rain events. Additionally, much nitrogen is recycled when organic matter decomposes. It has been estimated that for every 1 percent of organic matter in the soil as much as 20 to 45 pounds of nitrogen per acre per year can be made available for plants to absorb. In addition, much of the nitrogen in the leaves can be remobilized back into the tree to be stored in the structural portions of the tree. This stored nitrogen is what supplies the initial tree growth in the spring. The larger the tree, the more potential nitrogen the tree can store. Conversely, this means that the new smaller trees on a per-tree basis will store less nitrogen.

In apples, we have a better understanding of nitrogen levels based on cultivar. Cornell University researchers (Warren Stiles and W. Shaw Reid, Orchard Nutrition Management, Information Bulletin 219, Cornell Cooperative Extension) suggest the following nitrogen levels in mature trees based on cultivar (Table 1-3):

1.8 to 2.2 percent nitrogen: Soft cultivars such as Cortland, Gala, Golden Delicious, Jerseymac, Jonagold, Jonamac, Jonathan, Macoun, McIntosh, Mutsu, Paulared, Spartan, Tydeman’s Red, and other early ripening cultivars.

2.2 to 2.4 percent nitrogen: Delicious, Empire, Idared, Liberty, Melrose, Rhode Island Greening, Rome Beauty, Stayman, York Imperial, and other varieties.

Application rates: As a general rule, young trees may require about 0.01 to 0.04 pound of actual nitrogen per year of age up to 0.3 pound actual nitrogen per tree at maturity. These general levels should be adjusted up or down, depending on pruning, size of crop, cultural practices, indices presented above, and leaf analyses. Young nonbearing trees should grow up to 24 inches annually. Mature and heavily bearing trees should grow less, with apple and pear trees growing 12 to 18 inches; apricot, cherry, plum, and prune trees 15 to 18 inches; and peach and nectarine trees 18 to 24 inches.

Form of nitrogen: Fruit trees respond to any form of nitrogen fertilizer. Avoid using ammonium sulfate, urea, or ammonium nitrate if pH is below 6.0. Urea sprays may help increase fruit set but will not supply trees with all the nitrogen they require. Pear trees do not respond well to urea sprays. Applications of urea sprays to stone fruits are ineffective during the growing season. Calcium nitrate sprays should be avoided to discourage corking. (See Cork Spot and Bitter Pit Fruit Disorders.)

Time of application: Traditionally, we have said that nitrogen should be applied no later than 4 to 6 weeks before bloom. Recent research in the Pacific Northwest, however, indicates that application at this time results in the nitrogen chiefly accumulating in the foliage of the trees with little going into the flowers and developing fruitlets to help fruit set. Studies have shown that the majority of the nitrogen used to set fruit comes from the reserves within the tree and spring-applied nitrogen does not reach the developing fruit in time to be effective. In studies in apples in Oregon, late-summer to early fall nitrogen application was found to be preferentially translocated to the roots and the flower buds where it was mobilized from the roots and available in the flower buds next spring to increase flower strength and fruit set capabilities. The theory that nitrogen should not be applied after the middle of July may be incorrect; however, we have no sound research on this different application timing in Pennsylvania. Until we have adequate information, the safest method is to continue your past fertilization practices or to experiment only on a small scale.

Phosphorus
Phosphorus is important to plant growth because it is a catalyzing agent that induces metabolic reactions. It permits the plant to use nitrogen and to develop seeds. Failure of seeds to set often results in abortion of the young fruit and in misshapen fruit. Preplant soil test recommendations are made to attempt to raise soil levels to 100 pounds available phosphorus per acre.

Application rates: Rates should be based on either leaf analysis or soil analysis. Soil levels of less than 100 pounds phosphorus per acre indicate that an application of phosphorus is needed. Leaf analysis levels of less than 0.18 percent in apples and pears; 0.15 percent in peaches and nectarines; 0.23 percent in cherries; and 0.09 percent in plums indicate a need for phosphorus.

Form of phosphorus: The effectiveness of special forms of phosphorus under Pennsylvania conditions has not been documented.
Time of application: Application may be made anytime during the year in established orchards. Preferably, applications should be broadcast before the trees are planted and turned under with the previous crop. This aids in getting phosphorus down into the root zone.

Potassium
Potassium is believed important for maintaining water turgor in leaves and for functioning in the opening and closing of stomates. It is available in the soil as a cation; at excessive levels it competes with calcium and magnesium for uptake by the plant.

Application rates: If the potassium level in the foliage is over 1.5 percent, a response to potassium application is doubtful regardless of the soil test, unless leaf nitrogen or soil magnesium is too high. When soil test values exceed 4.5 percent base saturation of potassium, then no additional potassium is needed. Values lower than this call for an application.

Form of potassium: Any added value of one type of potassium fertilizer over another has not been determined.

Time of application: Same timing as for phosphorus.

Calcium
Calcium plays a vital role in reducing the incidence of corking and bitter pit. Low soil-test values are often found in very sandy or shaley soils, but such values usually do not indicate a need to apply calcium or lime. Calcium in the form of limestone is important for maintaining soil pH. Improper soil pH can lead to deficiencies or toxicities of other nutrients. The need for lime is best determined by a soil test. Calcium, like potassium and magnesium, is available to the plant as a cation, and an excess of potassium or magnesium can reduce calcium uptake.

Application rates: Soil application depends on soil pH, buffer pH, and depth of pH change desired. Calcium chloride applied as a foliar spray is recommended to prevent corking and bitter pit in the fruit. Current recommendations and additional information are listed in Cork Spot and Bitter Pit Fruit Disorders.

Form of calcium: Growers should base their limestone-purchase decisions on the price per ton of calcium carbonate equivalent, including spreading costs. Most limestone contains some impurities, the most common being magnesium. Continual use of dolomitic or high-magnesium lime can cause problems in a tree’s uptake of calcium. Dolomitic lime should not be used unless the soil test or the leaf analysis indicates a need for magnesium.

Time of application: In preplanting situations lime is most effective when broadcast and incorporated at least 6 to 12 months prior to planting. In established orchards any time is suitable, although postharvest or early spring applications allow rains to move lime into the soil. Regardless of how much rain accumulates, the process of raising the soil pH is slow, with the effects of lime moving downward at about 1 inch per year.

Magnesium
Leaf analysis values below 0.2 percent in apples and plums, 0.3 percent in peaches and pears, and 0.49 percent in cherries suggest that trees may respond to applications of magnesium. In soil tests, when the ratio of the percentage base saturation of magnesium to potassium is less than 2.0, magnesium applications may be needed.

Application rates: Foliar applications of magnesium sulfate at 10 pounds per acre should be viewed as quick but temporary measures. A more permanent solution is to apply a magnesium containing limestone at rates recommended in the soil analysis.

Form of magnesium: Magnesium is normally found in all but the purest limestone. The Penn State soil test results give a separate recommendation for magnesium. Growers should examine the purity of the limestone to determine whether the percentage of magnesium contained in it will also satisfy the magnesium requirement.

Time of application: Magnesium sprays are effective only when applied during the growing season. Applications may be single or split (i.e., applied at petal fall or in the first two cover sprays). Foliar sprays appear to be most effective when applied separately from pesticide applications. Ground applications can be made when lime is applied.

Boron
Boron is a nutrient often found to be low or deficient in orchards. Deficiency symptoms are observed more often in apple, plum, and pear trees than in peach and cherry trees. In apple trees, a deficiency may be expressed as internal cork in the fruit, as a dieback of shoots, and as bark necrosis. In pear trees, a deficiency may be expressed as internal cork, withering of blossoms, or poor fruit set. In peach trees, toxicity symptoms appear as necrotic lesions on leaves, crinkling of margins and tips of leaves, reduced flower bud formation and set, and pit splitting.

Tissue test values less than 35 ppm in apples indicate a shortage of boron. Values between 35 and 60 ppm indicate sufficiency. In the soil, values less than 0.5 ppm are low, while values between 0.5 and 1.0 ppm indicate a sufficient level. Caution: Excessive boron can be extremely toxic. Leaf values of over 80 ppm or soil values of over 1.0 ppm are excessive.

Application rates: In orchards where boron is low, apply 0.8 to 1.6 pounds per acre of actual boron (4–8 lb/A of Solubor 20.5% B) in two separate sprays at bloom, petal fall, or first cover; or apply a single postharvest foliar spray of 1.6 pounds per acre of actual boron. For trees more than 3 years old, apply boron annually to the soil. Apply 0.12 pound of actual boron per acre for 4-year-old trees. For each additional year of tree age up to 16 years, increase the rate by 0.02 pound per acre of actual boron. Use the lower rate as a maintenance program when no leaf analysis has been made. Eight pounds of Solubor per acre is recommended for proven cases of low boron.

Form of boron: Boron is most effective when applied as a foliar spray. Special formulations such as Boro-spray or Solubor should be used. Agricultural borax is adequate for ground applications.

Time of application: Soil applications can be made anytime. Foliar applications can be made during bloom or postharvest while leaves are still green and active on the tree.

Copper
Copper is a catalyzing element in plant metabolic reactions. Foliar analysis is best for determining copper deficiency. Values below 5 ppm indicate the need to apply copper.

Application rates: When leaf analysis indicates a deficiency, 4 to 6 pounds per acre of copper sulfate is recommended.

Form of copper: Copper sulfate (22% Cu) is the most readily available and cheapest material to use.

Time of application: Foliar applications should be made during the dormant season or after the fruit is off and while the leaves are still active and green. Soil applications can be made anytime.

Zinc
Deficiencies of zinc are becoming more common in Pennsylvania. Small leaves clustered on the end of shoots indicate zinc deficiency but may be confused with winter injury. Zinc applications are recommended when leaf levels are below 20 ppm. Foliar applications of zinc sulfate should only be made during dormancy or postharvest. Do not apply zinc sulfate with oil or apply within 30 days of the application of oil.