Gardening with Eileen: Understanding the chemistry of fertilizing plants

I thought a few more facts on fertilizers and plants' nutritional needs might help you with your fertilizing chores.

There are 16 known elements required for plant growth and development. Three of these ¡X carbon (C), hydrogen (H), and oxygen (O) ¡X are obtained directly from air and water. The other 13 elements ¡X nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), chlorine (Cl), copper (Cu), zinc (Zn), manganese (Mn), iron (Fe), boron (B), and molybdenum (Mo) ¡X are supplied by the soil.
The 13 elements plants obtain from soil must be in a slightly soluble form so they can be taken up by the plant's roots.

Each element has a specific function in plant growth and development. If one or more of these nutrients are present in excessive amounts, a toxicity or nutrient imbalance can occur.

Conversely, if one or more of these nutrients are in short sup ply, a deficiency can result. Either way, plant growth and/or quality may be affected.
Nitrogen, phosphorus and potassium are the main components, or macronutrients, of a suitable fertilizer.

Sometimes dolomite, a liming material providing both calcium and magnesium, may be included in growing media. Sulfur, the remaining macronutrient, may not be a component of the fertilizer and should not be overlooked. Look for fertilizer containing sulfur-coated urea nitrogen.

Nitrogen is available in two basic forms: inorganic and organic. Plants prefer the inorganic, or nitrate, form of nitrogen.

There are two primary organic forms of nitrogen. One is a manmade organic called urea.
The other is present in naturally occurring organic materials such as sewage, sludge and manure. This organic form of nitrogen is converted to the plant- preferred nitrate form by bacterial action in the soil.
The soluble nitrate form of nitrogen is quickly available to the plant, causing rapid growth rate that may cause thin cell walls to develop. Nitrate nitrogen doesn't last long in the environment. It can leach readily from the soil and may even be lost as a gas. Nutrients should be supplied at the same rate the plant can use them. This means soluble nitrates must be applied frequently in very small amounts.

The micronutrients to be used sparingly are boron, copper, iron, manganese, molybdenum and zinc. Chlorine ordinarily is not valued as a plant food in fertilizer. It can be injurious if high percentages are present, but small amounts may be beneficial under some conditions. If you have plants around your pool equipment or overflow that don't do well, chlorine toxicity may be the cause.

Micronutrients are required by plants in low quantities, so you should apply them cautiously. If a deficiency is suspected, it would be unwise to randomly apply all of the micronutrients. That might correct one deficiency while inducing a toxicity of another micronutrient.

The most accurate way to determine a micronutrient deficiency in need of correction is to use foliar analysis. The Collier County Extension Office can help you with this test. Individual micronutrients are available with suggested rates provided for application. However, it is essential that all micronutrients be provided in your fertilizer program at least once a year. Fertilizer formulations are available for shrubs, citrus, palms, etc., and contain a good balance of these necessary micronutrients. Nutritional sprays are liquid formulations that contain the micronutrients. These sprays allow the elements to enter the plant through the leaf surfaces. The micronutrients can be tied up in our alkaline soil, due to improper pH, when applied as a granular fertilizer and may not be available to the plant. Therefore, when a micronutrient deficiency is apparent, it is more effective to apply a nutritional spray.

Following are some general symptoms of nutrient deficiencies:

Nitrogen: Yellowing of entire plant with lower leaves worse and stunted.
Phosphorus: Main veins of old leaves become purple or reddish. On fruit trees, blossoms drop, fruit is small and matures slowly, and few flower buds are formed for next year's crop.

Potassium: Faint yellowing, then browning, of margins on old leaves. Then veins become yellow.

Manganese: Mottled chlorosis between midrib and primary veins. Entire leaf may turn yellow, but midrib and large veins stay green longest.
Frizzle top, yellowing, dwarfing and distortion.

Iron: Pronounced yellowing on younger leaves with veins appearing as fine green lines ¡X yellow to white if acute.
Dwarf leaves, leaf fall, dead wood, dead tips and reduced growth.

Magnesium: Yellowing begins on margin and near center of old leaf, progresses inward and downward; tip, upper margin and lower central veins may remain green; necrosis and leaf drop.

Molybdenum: Often mistaken for herbicide damage.
Dwarfed leaves with irregular, wrinkled margins and prominent midribs and main veins on leaves and shoots.

Boron: Plants grow slowly.
Terminal buds die and plant tends to be bushy. Later, lateral buds die, leaves thicken, and fruits, tubers and roots become cracked and discolored.

Copper: Usually confined to peat or muck soils. Slow growth or complete cessation of growth. Tips affected first and eventually die back.

Zinc: Leaves become long and narrow, turn yellow and become mottled with dead areas. Symptoms similar to iron deficiency.

I hope these facts make this necessary chore in your yard a little easier to understand.

Eileen Ward and her husband, Peter, have owned and operated Greensward of Marco Inc., a lawn maintenance and landscaping company, since 1982. Along with completing horticultural courses from the University of Florida, she has a commercial maintenance spray license and is a registered dealer in agricultural products in the state of Florida. Ward has been writing this column since 1996. To reach Ward, call 394-1413.

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