Orchard Nutrition: Micronutrients
Tree deficiencies of zinc, iron, copper, manganese, calcium, magnesium, and boron are common in high pH soils (pH>7.7) of the Intermountain West. These nutrients, with the exception of Zn, can be successfully corrected with soil amendments and temporarily with foliar applications. Zinc is often tightly bound in the soil and is best corrected through foliar sprays using either zinc sulfate or chelated zinc. Fruit trees showing severe deficiency symptoms may respond temporarily to some of these other nutrients applied as sprays. However, these sprays should only be used in conjunction with soil applications of the same nutrient. The sprays will provide temporary relief until the soil-applied nutrient can be translocated throughout the tree.
Chronic micro-nutrient deficiencies, typical on peaches on alkaline soils, can most times be corrected by reducing the soil pH through acidification of irrigation water and/or application of elemental sulfur to the soil. Adjusting soil pH, however, can be costly and difficult especially in soils with a high buffer capacity, indicated by high levels of calcium. Soil application of chelated micro-nutrients can correct these deficiencies from 1 to 3 years depending on soil pH. Several areas of Intermountain West soils are typically deficient in boron regardless of soil pH.
Boron is critical to pollination, fruit formation and shoot growth. Deficiency often presents as blossom blast, shoot die back, dwarfed leaves and deformed fruit. It can also mask symptoms of Zn deficiency. Leaf analysis results show some boron deficiencies in peaches and apples. However, pears are the fruit crop that most often shows boron deficiency including "blossom blast" or wilting of the flower buds in early spring. Boron deficiency is common in Intermountain West soils and trees.
Boron deficiency can be corrected through soil and foliar applications until deficiencies are corrected. Boron should be soil applied (2-5lbs/acre) in fall on heavier soils or in spring on coarser soils as it is very mobile in the soil and can easily leach. A single maintenance spray, applied each year at a low rate (0.5-1lb/100 gal), should supply enough boron to prevent the development of a deficiency when soil levels are sufficient. While the spray may be applied at any time, fall applications when leaves are still green or spring pre-bloom applications are recommended. Proper calibration of equipment is important when applying boron as the range between deficiency and toxicity is very small, risking over application. Monitor boron levels with foliar and soil testing to prevent over application and adjust rates accordingly.
Calcium is vital to cell wall development and hence fruit quality and storability and can cause bitter pit and corking especially in certain cultivars of apple like Honeycrisp. Despite often high levels of calcium in soil, uptake and allocation of calcium within the plant and fruit may still be inadequate. For example, in a survey of Montana orchards calcium levels in the soil were high in 61% of orchards, but 73% of orchards still demonstrated low foliar calcium. Several factors affect Ca uptake and partitioning in the plant including root health, spring soil temperatures, interactions with other nutrients, crop load, tree vigor and irrigation.
Foliar sampling and physical evaluation of fruit should be used to determine if Ca fertilizers should be applied in conjunction with addressing other potential barriers to uptake. Under average conditions, three sprays are suggested, and up to twelve when deficiency symptoms are severe. Calcium chloride is the most effective and inexpensive option for both conventional and organic growers. Rates of 2-4lbs/acre calcium chloride are recommended per acre for a total application of 15-50lbs/acre depending on orchard needs. The first spray should be applied about one week after petal fall (mid-June). Subsequent sprays should be applied through mid-August and spaced according to total number of sprays needed to meet desired application rate. Young and very vigorous trees or trees with large fruit which have a history of serious bitter pit, may require more sprays than older trees or cultivars less prone to storage issues. The more severe the history of bitter-pit, the more frequently calcium should be applied, however, growers should keep in mind other factors influencing Ca uptake and address those as well.
Magnesium deficiency is observed as interveinal chlorosis in leaves. Most soils contain sufficient amounts of Mg and issues with uptake are either due to competition with K and Ca limiting uptake or saturated soils. If foliar samples indicate Mg is deficient, but soil tests show adequate levels, address irrigation or soil nutrient issues and apply foliar Mg in the form of Epsom salts to temporarily address Mg needs. If soil supply is low, use soil tests to inform application rates.
Yellow leaves, with green veins and mid-ribs are symptoms of manganese deficiency. Manganese deficiencies are especially common in arid climates with high pH soils. Peach orchards located on highly alkaline soils are especially susceptible. This deficiency is often masked by zinc and iron deficiencies, the latter of which has similar symptoms. While it may not be visually detectable, a tissue analysis will identify the deficiency. The deficiency also can be induced by applying excessive amounts of iron chelate. Usually one foliar application of manganese sulfate at the labeled rate, applied when the first leaves are fully expanded, is sufficient to maintain an adequate level of manganese in the leaves.
Iron deficiency is usually indicated by pale, yellow or nearly white leaves and green veins. This symptom is referred to as chlorosis and often appears early in the season on new growth. Chlorosis can be caused by a variety of factors including poor root growth, cold wet spring soils, high soil pH, water high in bicarbonates and poorly drained or compacted soils.
Iron sulfate is not readily take up by trees, however, iron chelates usually give temporary correction of chlorosis. Peach trees tend to be less responsive than other fruits to iron amendments. Soil and foliar sprays of chelated iron can provide temporary relief but quickly become unavailable to plants. Iron chlorosis will persist if the cause is related to irrigation or soil physical properties. These issues should be addressed alongside applying fertilizers. While soil pH is difficult to change in calcareous soils common to the Intermountain West, using acid fertilizers containing sulfur, allowing soil to dry out between irrigation events and reducing compaction will improve iron uptake.
Zinc deficiency symptoms are common in the Intermountain West and present as blind wood, stunted growth or little leaf syndrome. Soil applications of zinc have not proven effective as it often binds with phosphorus or on organic matter becoming inaccessible to plants. Where zinc levels are known to be low, annual foliar spray applications should be made to avoid deficiency symptoms. Once symptoms are detected, they should be treated as soon as possible to avoid further injury. Several precautions should be taken to avoid phytotoxicity when applying zinc.
- Verify need by tissue analysis or visual deficiency symptoms. Zinc sprays can cause severe injury to shoots, buds, fruit, and leaves. Adjust the rate, formulation, and time of application according to the kind of fruit, the season of the year, and the amount of zinc required.
- Applications made within 3 days before or after an application of oil can cause injury. Longer periods may be required during cool weather. Application of zinc sulfate spray within five days of any oil-containing spray may damage apples and should be avoided during that time.
- Because of the problem of multiple applications of oil to pears in the spring, it may be necessary to apply zinc in the fall instead.
- Do not use fall applications on apricot because of potential injury.
- When using zinc sulfate crystals, be sure all crystals are dissolved before spraying because of potential injury.
- Zinc sulfate is highly corrosive. The spray tank, pump, lines, and nozzles should be thoroughly rinsed and flushed after using.
- Foliar application during or followed by damp weather may result in spray injury on some varieties of stone fruits.
Higher rates of zinc can be applied in the spring before the buds are open than during the growing season. Sprays are more effective and appear to cause less injury when delayed as late in the spring as possible, but before buds scales open.
Zinc can be applied after the trees have begun to go dormant (usually after mid October), but while the leaves still remain green and active. Fall applications are usually less effective than spring dormant applications, but the former may be needed in cases of severe deficiency. With sweet cherry, both a fall and a dormant application may be necessary.