(Note - Because of the importance of this subject, I took the liberty of enlarging on Gil Whitton's program, using Florida Cooperative Extension Service publications: Bulletins SL-3, SL-8, 183-B and Circular 435. - Editor, 1982)

At least 94% of dry plant tissue is composed of the three elements, carbon (C), hydrogen (H), and oxygen (O), which are supplied by air (CO2) and water (H2O). The remaining 6% is made up of thirteen nutrient elements derived from the soil or from fertilizers. These thirteen are commonly divided into three groups. The Primary Elements (Macronutrients) are nitrogen (N), phosphorus (P), and potassium (K). The Secondary Elements are calcium (Ca), magnesium (Mg) and sulfur (S). The third group, the Trace Elements (Micronutrients), so-called because they are required in very small amounts by plants, are iron (Fe), manganese (Mn), boron (B), copper (Cu), zinc (Zn), molybdenum (Mo) and chlorine (Cl). A deficiency of any of these sixteen elements can have drastic effects on fruit crops.

The availability of soil nutrients to plants is dependent on various factors. One is soil pH, the measure of relative acidity or alkalinity expressed on a logarithmic scale of 0 to 14, 7.0 being neutral. The lower the number, the more acid the soil. The availability of each element to the plant varies with the pH level, the range of 6.0 to 7.0 being the optimum, usually.

The Florida Fertilizer Label
Florida law requires that each bag of fertilizer sold have a label showing specific information about the analysis and derivation of the contents. The "Guaranteed Analysis" section of the label is divided into the percentage of total nitrogen (N) - the sum of all forms of nitrogen present, available phosphoric acid (P), soluble potash (K), and each Secondary Nutrient present. A 'Derived from' section lists actual source materials for the Primary Nutrients.

A mixed fertilizer is ordinarily identified by its Primary Nutrient analysis, e.g., 6-6-6, 4-7-5 or 12-10-20, standing for N-P-K percentages.

Nitrogen (N)

1. Nitrate Nitrogen (NO3) or 'Chemical Fertilizer' is available immediately to the plant, but quickly leaches down through the soil and out of reach of the root system.

2. Ammoniacal Nitrogen (NH4) is converted to a nitrate form by organisms in warm moist soils with a pH above 5.0. About 50% becomes available to the plant within two weeks of application. The remainder may last 35 to 45 days in warm weather.

3. Water-Soluble Organic Nitrogen, usually Urea (CO(NH2)2), or 'Organic Fertilizer', will last 10 to 15 days. It is 'organic', but not a 'natural' material like cottonseed meal, bone meal, sludge, etc. Urea is made by synthetic chemical processes and changes to ammoniacal nitrogen within a few days after application. Therefore, it should be considered equivalent to Ammoniacal Nitrogen.

4. Water-Insoluble Nitrogen originally meant such natural materials as dried blood, seed meals, sewage sludge and tankage. Now, any form of water-insoluble nitrogen may be used, such as ureaform (urea-formaldehyde), magnesium ammonium phosphate, etc. All of these materials must be converted first to ammoniacal, then to nitrate nitrogen by soil organisms before the elemental nitrogen is available to the plant. Thus, water-insoluble nitrogen is a very slow-release fertilizer and is less subject to leaching loss. It is good for 60 to 90 days in warm weather.

When you are buying fertilizer, it is very important that you understand the nitrogen analysis, since the price of the bag is based on the percentage of nitrogen contained.

II. Phosphorus (P) is quoted as percent equivalent P2 O5 , phosphoric acid (properly 'phosphate'). Plants absorb phosphorus mainly as H2PO4 and HPO4 ions which are abundant in most Florida soils. Phosphate ion concentration may be as low as .001 parts per million in the soil solution for good plant health.

III. Potassium (K) is quoted as percent equivalent K2O, potash. Water-soluble potash comes mainly from muriate of potash, sulfate of potash magnesia, nitrate of soda potash, nitrate of potash and sulfate of potash. They are all equally usable by the plant.

Plant Nutrition and Deficiencies
1. Nitrogen (N) is a constituent of all living cells and is a necessary part of all proteins, enzymes and metabolic processes involved in the synthesis and the transfer of energy. Nitrogen is a structural part of chlorophyll, the green pigment in plants, which is responsible for photosynthesis. Nitrogen stimulates plants into rapid, vigorous growth, increasing seed and fruit yield and improving the quality of leaf and forage crops. Most Florida soils do not supply nitrogen in sufficient quantity to meet the desired growth requirements of fruiting plants. A nitrogen lack will cause slow growth and perhaps a pale yellow (chlorotic) coloration, especially on the lower (oldest) leaves.

2. Phosphorus (P) is also an essential part of the photosynthesis process. It is most abundant in young, growing tissue. It transfers readily within the plant from older to younger tissue. As a plant matures, most of its phosphorus moves into the seeds and/or fruits. Phosphorus influences flowering and fruiting habits of plants, hastens maturity (useful in the fall for hardening tender plants), encourages root development, increases disease resistance, improves drought and cold resistance and increases vigor and yield. Phosphorus deficiencies are rare in Central Florida. The deficiency is exhibited in the older leaves, which may have a dark bluish-green color with some tints of bronze or purple. Other abnormalities include thin stalks, small leaves, coarse-textured or spongy fruit. Root crops may be severely affected.

3. Potassium (K) plays an essential role in the metabolic processes of plants and perhaps in protein synthesis and photosynthesis. Although potassium is not found in organic combination with plant tissues, it is absorbed by plants in greater amounts than any other mineral element except nitrogen and, in some cases, calcium. Potassium reserves in Florida soils are very low, and only small quantities are retained in surface soils. During heavy rainfalls, potassium leaches through the soil severely. Potassium deficiencies also first appear on the oldest leaves, usually as reddish discoloration or 'scorch' at leaf edges, gradually working to the center, leaving a torn and ragged appearance. Other symptoms are slow growth, long, slender, weak stems, and low yields of misshapen fruits.

4. Calcium (Ca) is an essential element in plant cell walls and provides for normal transport and retention of other elements within the plant. Calcium exists in delicate balance with magnesium and potassium in the plant. Most Central Florida soils are acidic and deficient in calcium. They require lime or dolomitic limestone to raise the pH to optimum levels for plant growth. Fertilizing with potassium (K) and magnesium (Mg) in the presence of sodium (Na) may further limit the calcium available to the plant roots. Calcium is relatively immobile within a plant, not easily redistributing when deficient. Plant health requires a continuous supply of calcium. Black heart in celery, tipburn in lettuce and cabbage, blossom end rot in peppers and tomatoes, carrot cavity and fading of chlorophyll along the edges of citrus leaves, followed by early leaf drop, are all examples of calcium deficiency.

5. Magnesium (Mg) is part of the chlorophyll in all green plants and essential for photosynthesis and the activation of many growth enzymes. It is a relatively mobile element within the plant, relocating from older to younger plant tissue as the need develops. Magnesium is slightly more abundant than calcium in Florida soils, but still short of required levels. Dolomitic limestone is an excellent source of magnesium. Fertilizers and organic materials are other sources. Because of its mobility, plants exhibit magnesium deficiency symptoms in the older leaves at first. Leaf chlorosis advances from the tips and edges of leaves towards the center, progressing between the veins, which remain green. Eventually the leaves become brown and very brittle, then gradually shed.

6. Sulfur (S) is a constituent of the amino acids and hence, certain proteins. Vitamins, enzymes and co-enzymes contain sulfur. Characteristic odors and flavors of mustard, onion and garlic are sulfur compounds. Sulfur is most readily available in warm weather favoring organic matter decomposition. Sulfur deficiency is most likely in cool weather or in soils with low organic content. Deficiency symptoms are leaves with uniform chlorosis, i.e., no green veins, and plants with an overall light-green color resembling the early stages of nitrogen deficiency. Symptoms, however, are more severe in young leaves. Orange, red or purple pigments may appear and leaves and plants may become stunted.

7. Manganese (Mn) is believed to activate many enzymes affecting metabolic processes. The sandy soils of central Florida are deficient to begin with, and raising the pH (sweetening the soil) will further reduce the availability of manganese. Therefore, liming to cure other problems may induce manganese deficiency. On the other hand, high levels of manganese may induce iron deficiency in plants. Low manganese levels produce symptoms similar to iron deficiency, and in some plants, closely simulates magnesium deficiency, except that lack of manganese affects the younger leaves, not older. Leaves may be mottled with wide, dark-green veins which narrow as the deficiency progresses. The usual cure is the soil application of manganese sulfate.

8. Iron (Fe) is essential to many organic compounds in plants and for the synthesis of chlorophyll. Florida soils are low in iron, but generally have adequate amounts for plant growth except where excessive liming has occurred (as near building foundations). Also, excessive application of copper or manganese will inhibit iron uptake by plants. The first deficiency symptoms to appear are light yellowing of terminal leaves, similar to manganese deficiency. This progresses to overall yellowing with narrow, dark-green veins. To cure, spray with chelated iron (pronounced, 'key-lated') followed by an application of iron sulfate on the soil surrounding the plant. Take care with iron sulfate, as it will badly stain concrete, or any other surface.

Any restriction of root development, as by nematodes, disease, excess water, adverse temperatures or excessive nitrogen fertilization may cause iron deficiency. Also, high pH and high phosphate levels will inhibit iron availability. Adding acid peat, manure or other organic materials to the soil will increase the availability of iron to plants. Potting media or seedbed media mixes are commonly low in iron and must be supplemented.

9. Copper (Cu) is essential for growth and also activates many enzymes. A deficiency interferes with protein synthesis and causes a buildup of soluble nitrogen compounds. Excessive copper may induce iron deficiency. Toxic quantities of copper can be counteracted by applying dolomitic limestone (CaMgCO3) or gypsum (calcium sulfate). The calcium displaces the copper and the copper is then tied up as copper carbonate or copper sulfate.

Although Central Florida sandy soils are usually lacking in copper, deficiency symptoms are relatively rare, as copper is often applied as an impurity in phosphate fertilizers. Deficiency symptoms are unusually dark-green leaves, sometimes abnormally small. Occasionally, white veins are present, or wilting or 'scorched' leaf tips.

10. Zinc (Zn) is essential because it controls the synthesis of an important plant growth regulator. Also, it is an enzyme activator. Most Florida soils are low in zinc, and a high pH will further reduce availability. Zinc deficiency often occurs together with manganese deficiency because of the pH effect. Symptoms may closely resemble those of manganese, iron or magnesium. Lack of zinc causes stunted plants and thickened leaves. Stem internodes are much shorter than usual and chlorotic zones appear between leaf veins. On fruit trees, small leaves and internodes so short that all leaves seem to originate from one point on the stem ('rosette'). Leaf margins may be distorted into a wavy, twisted or corrugated shape. Pecan trees may develop a deep bronze cast in July or August during the nut filling period. In severe cases, twig or large branch dieback may occur the following winter.

11. Boron (B) primarily regulates the metabolism of carbohydrates in plants. Different crops have widely-varying needs. The requirement for one crop can be toxic to another. Boron is relatively lacking in Florida soils. Further, it easily leaches out of sandy soils, requiring frequent applications. Mild boron deficiency may cause tip growth or terminal bud dieback and excessive sprouting of secondary buds. Progressive symptoms include thickened, wilted or curled leaves, white veins, and cracking or rotting of fruit, tubers or roots. In citrus, gum spots in the rind and excessive fruit drop are seen.

12. Molybdenum (Mo) is essential to enzyme activity in nitrogen fixation and nitrate reduction. Adjusting the pH to the range 5.5 to 6.5 will usually make adequate quantities of molybdenum available in Florida soils. Some peat and muck soils have potentially toxic levels and the pH must be kept below 5.0. Symptoms approximate nitrogen deficiency due to the interference with nitrogen transformation within the plant. Only trace amounts of molybdenum are required to cure deficiencies.

13. Oxygen (O) starvation usually occurs in flooding or in very heavy soils.

14. Chlorine (Cl) was added to the list of essential elements in 1954. It is rarely a problem in Florida, occurring naturally from salt air and rainfall plus the general use of muriate of potash (KCl) in fertilizers.

Since different plants have different nutritional requirements, the symptoms described here may not always appear. Soil tests and plant tissue analyses are the best aids to determining nutrient balance in the soil. Caution is advised in that repeated applications of copper, zinc, manganese or molybdenum may result in levels in the soil toxic to both plants and animals.

Gil Whitton

DATE: September 1982

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