OFA Bulletin March/April 2013 : Page 3

Interior Plantscape nutrient disorders in tropical plants By Lynn P Griffith, Jr. My company has been providing soil and plant tissue analysis to growers for over 60 years. Being in Florida, we deal with tropical plants and their nutrient requirements on a daily basis. This article will briefly discuss nutrient function, but it will primarily deal with nutrient disorders that we commonly encounter in tropicals. I will also discuss some of the available corrective measures growers and interiorscapers may take. NITROGEN POTASSIUM Nitrogen has many functions in plants, but one of the primary ones is as a component of proteins and enzymes, which together are the primary “building blocks” and energy generators (via photosynthesis) within the plant that drives plant growth. When pothos are deficient in nitrogen, they develop progressively smaller leaves. Massangeana cane loses its variegated stripe when foliar nitrogen is low. African violets turn yellow and almost stop growing. Marginatas and most other plants will turn yellow and show either reduced growth, reduced leaf size, or no growth at all. To correct nitrogen deficiency, there are numerous types of nitrogen fertilizers. These can be applied either as drenches, topdresses, or foliar sprays. Urea is quite easily absorbed through the tissue of most plants. Many plants will absorb up to 50 percent of foliar applied urea within 8 hours of application. Nitrate nitrogen is the most readily available soil applied source. PHOSPHORY Potassium has over 200 functions in plants. One of the primary ones is in regulating the guard cells that surround the stomates. Where plants become deficient in potassium, the guard cells may not form correctly, and you get localized necrotic spots where the plant cannot close the stomates, usually in the older leaves. Potassium deficiency is often seen in many palm species, appearing as leaf tip burn or small, fairly regularly-spaced necrotic leaf spots. These spots may be confused with leaf spot diseases caused by fungi. Adonidias, Kentias, and Rhapis will often show small, round necrotic spots when deficient in potassium. Non-palms will usually show tipburn or necrotic flecking or marginal burn in older leaves. Potassium is fairly well absorbed through foliage of many plants, often from potassium nitrate as a fertilizer source. Multiple applications are often needed. SULFUR Phosphorus also has numerous functions in plants. It is very important in the formation of root systems in young plants. You may have read that phosphorus does not move much and does not tend to leach from field soils. Phosphorus, however, is quite mobile and leachable in most soilless media. Lack of phosphorus will often appear as a reddening or purple color in medium and older leaves. Among the Dracaenas, both Dracaena marginata and D. massangeana will develop purple coloring in the older leaves. Dracaena ‘Janet Craig’ sometimes will develop severely necrotic older leaves, where up to half of the leaf may die back. African violets and Gerbera daisies also will show a pronounced purple color when severely deficient in phosphorus. Many crop tropicals will not particularly show much in the way of visible foliar symptoms, though their root systems may be greatly diminished. Phosphorus can be applied as dry or liquid fertilizers, or as foliar sprays. Be aware that some phosphorus fertilizers are also sources of potentially damaging fluoride. Sulfur is required by plants in roughly the same amount as phosphorus. Sulfur is present in several of the amino acids needed for protein manufacture, especially structural proteins involved in growth. Sulfur deficiency is fairly rare when using granular fertilizers, as most granular products contain significant quantities of sulfur. Some of the soluble fertilizers applied as liquid feed may not contain any sulfur. When a plant becomes low in nitrogen, it becomes pale and quits growing. When plants become deficient in sulfur, they often become pale but keep growing, though sometimes you will see necrosis or chlorosis in older foliage. A plant commonly deficient in sulfur is the dwarf Schefflera or Arboricola. Sometimes this plant is extremely dark green, and other times it may be a fairly pale, dull green color. Lack of sulfur is usually the reason. Magnesium sulfate is the most common source of soluble sulfer. It can be applied as a drench or a foliar spray. Several sources are available for dry fertilizers, including potassium sulfate and K-mag or Sul-po-mag, among others. CALCIUM Calcium helps grow strong bones and teeth, and it has a similar function in plants. Calcium is very much involved in the formation of strong, sturdy cell walls in plants. Calcium is also often used by plants to neutralize toxins that may be inadvertently absorbed by the plant. Calcium often is supplied by the limestone used in media for pH adjustment. Many parts of the country also have fairly high amounts of calcium in irrigation CONTINUED ON PAGE 4 3

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