Like All Plants, Conifers Require Adequate Nutrition to Stay Healthy and Grow Properly
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by Dr. Bert Cregg LIKE ALL PLANTS, CONIFERS REQUIRE ADEQUATE NUTRITION TO STAY HEALTHY AND GROW PROPERLY. HOWEVER, SOME ASPECTS OF CONIFER NUTRITION DIFFER FROM THOSE OF BROAD-LEAVED, DECIDUOUS TREES. THIS EDITION OF CONIFER CORNER DISCUSSES THE BASICS OF PLANT AND SOIL NUTRITION AND HIGHLIGHTS SOME WAYS IN WHICH CONIFERS DIFFER FROM BROAD-LEAVED TREES. Bare Essentials ver the years, scientists have identified the chemical elements that are needed for plant Ogrowth. These 16 elements are referred to as essential elements. In addition to the essential elements, nickel (Ni) and silicon (Si) have been identified as beneficial to plant growth. Three of the essential elements (carbon, hydrogen and oxygen) are derived from air and water. The remaining elements are supplied by the soil (see Table 1). 42 The Michigan Landscape™ (Photo credit: Ronald Hallett, Canadian Forest Service) (Photo credit: Ronald Hallett, Canadian Forest Service.) (Far left): Test, don’t guess. Soil and tissue sampling are critical managing nutrition and diagnosing nutritional problems. Be sure to request pH along with any soil analysis. (Above left to right): Needle twisting induced by copper deficiency; Yellow tipping of conifer needles associated with magnesium deficiency; Phosphorus deficiency symptoms may include a purple cast to needles, often referred to as “purple heart.” Typically, essential elements are nutrient relations between conifers and 6. The overriding effect of soil pH on divided into those needed in the largest broad-leaved trees. conifer nutrition is difficult to overstate. amounts (macronutrients) and those Foliar nutrient levels and nutrient Anytime a nutrient problem is suspected needed in relatively small amounts requirements. In general, foliar nutrient in conifers, soil pH heads the list of (micronutrients). In analyzing plant tissues levels are lower in conifers than in factors to investigate. If soil pH is for nutrients, analytical labs usually report broad-leaved trees. For example, higher than 5.5–6, consider applying macronutrients in percent (%) dry weight, adequate foliar N levels in conifers are elemental sulfur or fertilizing with whereas micronutrients are usually usually around 1.5–2.2 percent dry ammonium sulfate to reduce soil pH. expressed in parts per million (ppm). weight, whereas foliar levels in broad- Note that the effect of sulfur on pH is The likelihood of a nutrient element leaved trees usually range between 2–4 transitory. To avoid over-correcting pH, becoming limiting depends on plant percent. Compared to broad-leaved limit sulfur applications to 400 lbs per uptake relative to soil supply. In most trees, conifers have relatively low acre per year (see Table 2). situations, nitrogen is the element most nutrient requirements. This make sense Approximately 2.6 lbs of ammonium likely to become limiting. This is due to when we consider that broad-leaved sulfate is needed to provide the same two factors: (1) nitrogen is the element trees need to ‘rebuild’ their entire acidifying effect as 1 lb of sulfur. required by plants in the largest photosynthetic factory each spring, Remember that ammonium sulfate also amounts and (2) soil nitrogen is highly while most evergreen conifers retain contains 21 percent N that needs to be dynamic and subject to losses through three to six years’ worth of needles. In included in your nitrogen management microbial activity. The growing fact, ecologists suggest that their low program. environment (field versus container) nutrient requirement is one of the key Symptoms. Diagnosing nutrient also impacts conifer nutrition. Aside traits that enable evergreen conifers to deficiencies in conifers can present from nitrogen, most field soils are able dominate vast portions of cold, boreal unique challenges. For instance, many to supply adequate nutrients for plant regions of the world where nitrogen standard symptoms used to diagnose growth. Key exceptions are sandy soils, availability is limited. broad-leaved plants, such as inter-venal which have low ability to supply cations Soil pH. With a few exceptions, chlorosis or marginal necrosis, do not such as potassium and magnesium, and conifers grow best on soils that are apply. Nevertheless, certain nutrient high pH soils that limit uptake of iron, more acidic than optimum soil pH for deficiencies often result in symptoms in manganese and magnesium. Growing deciduous trees. For example, Fraser fir conifers that may aid in diagnosing trees in containers provides an additional is native to sites in North Carolina with nutrient problems: challenge since most container media has soil pH as low as 3.5. The optimum n Nitrogen deficiency—general low inherent fertility, and all nutrients pH for most conifers is around 5.5; chlorosis (yellowing) of the entire must be supplied either through a most broad-leaved trees grow best a pH plant. Since N is mobile within the constant-feed program or through 6.5. This phenomenon is due largely to plant, deficiency may show up in controlled release fertilizers. reduced ability of conifers to take up older foliage first. nutrients as pH increases. Nutrients: Conifers In some of our recent research on n Phosphorus deficiency—may show exotic firs, we found that needle variable symptoms, but one of the vs. Broad-Leaved most diagnostic is a purplish cast to In designing a nutrition program for chlorophyll content and photosynthetic foliage sometimes referred to as conifers, it is important to remember efficiency of container-grown firs “purple heart.” that there are several key differences in dropped rapidly as pH increased above September/October 2005 43 n Magnesium deficiency—conifers Table 1. Sufficiency concentrations for essential macronutrient and micronutrient in conifers deficient in magnesium may show a pronounced chlorosis on the tips of Macronutrient Chemical symbol % Nitrogen N 1.4–2.2 the needles. Phosphorus P 0.2–0.4 Micronutrients can sometimes be Potassium K 0.4–1.5 deficient to the point of causing visible Calcium Ca 0.2–0.4 symptoms. Among the most common Magnesium Mg 0.1–0.3 micronutrient problems in conifers are Sulfur S 0.2–0.3 iron, manganese, copper and boron. Micronutrient Chemical symbol Parts per million (ppm) Like N deficiencies, iron deficiencies Iron Fe 60–200 cause needle chlorosis; however, iron is Manganese Mn 100–250 less mobile and symptoms may appear Zinc Zn 30–150 on new foliage first. Manganese is Copper Cu 5–20 sometimes deficient in conifers in Boron B 20–100 Michigan, especially on high pH sites. Chlorine Cl 2–20 Molybdenum Mo 0.25–5 Copper deficiency can lead to a distinctive symptom of twist needles Source: US Forest Service Container Nursery Manual near the shoot apex, Boron deficiencies may be indicated by soil or foliar analysis. Conifer growers and landscapers should remember that boron has a narrow window between example, needle scorch caused by grades can remove surface feeder sufficiency and toxicity. If boron is drought or high temperature may roots and lead to drought injury. recommended for your site, fertilize result in needle browning that may Have any herbicides been applied carefully, double check equipment suggest a nutrient problem. recently? Some herbicides that are calibration and consider split application Eliminating environmental causes safe to use over conifers when they to allow time to observe response to the will require knowledge of the are dormant may injure shoots initial application. history of the site. Has the site during active growth. been flooded recently? Has rainfall (3) Check visible symptoms. As Diagnosis of Conifer Nutrient or irrigation been adequate or has Problems indicated earlier, many nutrient the planting been subjected to deficiencies have characteristic As noted previously, managing drought stress? Is there evidence of nutrition in conifers is quite different symptoms that can aid in diagnosis. insects or disease? Remember, Also, certain trees are prone to from the nutrition of broad-leaved trees many insect pests are extremely and shrubs. One characteristic that is specific nutrient disorders (e.g., small and may require a hand-lens pH-related nutrient problems in similar in both cases, however, is the to see. Also, the pest organism may process for identifying and diagnosing firs). However, many nutrient have moved on and left only its disorders and abiotic or biotic suspected nutritional problems. damage behind. Identifying nutrient problems in stresses have similar symptoms. nurseries or in landscape plants can (2) Consider cultural history. What Therefore, visible symptoms alone often be a difficult chore and can has been done on the site recently? may not be sufficient to diagnose a involve significant detective work. Has fertilizer been applied? nutrient problem. Although many nutrition guides and Remember, some fertilizers such as (4) Test, don’t guess. Soil and foliar extension publications imply that ammonium nitrate have a relatively analysis are usually required to nutrient deficiencies can be diagnosed high salt index and can cause salt make a complete diagnosis of a from visible symptoms, in reality the injury. Some elements like boron nutrient problem, and even these situation is rarely that simple. If you or chloride can cause a direct techniques have limitations. suspect a nutritional problem, the toxicity injury, while others can interfere with uptake of other (5) Sample the soil. Numerous following steps should be useful in private labs and the MSU Soil and sleuthing the problem. elements (e.g., potassium-induced magnesium deficiency). Have there Plant Nutrient lab conduct soil (1) Eliminate other causes. Some been any grade changes on the nutrient analyses. To collect a soil nutrient deficiencies can look like for analysis, use a soil probe, shovel other biotic or abiotic injuries. For site? Raising a grade can deprive roots of oxygen whereas reducing or trowel to collect 10 to 15 samples 44 The Michigan Landscape™ Table 2. Approximate pounds of sulfur per acre nutrient status of plants in a extension office or the MSU Soil and to lower soil pH by 1 pH unit nursery or landscape.