by Jeff Wasielewski 

Questions about fertilizing abound: when should I apply fertilizer; what do I apply fertilizer to; how much should I apply; and what type of fertilizer should I apply? The answers to these questions are simple, but they vary with each horticultural situation. Different soil types, plants and overall goals of the horticulturist or gardener can alter your fertilizer program. It is important to note that if you make wise plant choices and use large quantities of mulch, your garden will need very little fertilizer.

Choosing natives and plants adapted to grow in your soil and climatic conditions will substantially decrease the amount of fertilizer needed by your yard. Mulching also helps dramatically reduce the amount of fertilizer needed by adding an organic component to an otherwise barren soil.

The nutritional elements needed by plants can be divided into two categories: macro elements and micro elements. Macro elements are needed in larger quantities than micros; however, both types are needed in order for the plants to thrive. Macro element deficiencies generally manifest in all leaves of a plant while micro element deficiencies generally show up in new growth only.

The macro elements are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium and magnesium. Macro elements should be applied using a granular fertilizer that is, at least in some part, slow release. A slow release fertilizer is not able to be absorbed immediately by the plant, but is instead absorbed over time. Nitrogen (N), phosphorus (P) and potassium (K) are the three macro elements used in the greatest quantities by plants. When you read a fertilizer label, the numbers such as 8-3-9 are the percentages of N-P-K respectively found in that bag. Nitrogen helps the plants vigor and growth, phosphorus is for general health and potassium helps bloom and fruit development. A 6-6-6 fertilizer is an excellent general fertilizer, but palm enthusiasts may wish to purchase palm special, 8-2-12. This is more expensive than a 6-6-6 but is specially formulated for palms and can be used on most plants with good success. An 8-3-9 mix is formulated for most tropical fruit trees.

The first number on the fertilizer label, nitrogen, is broken down into four categories on the fertilizer label: nitrate nitrogen, ammoniacal nitrogen, water soluble nitrogen, and water insoluble nitrogen. The nitrate and ammociacal forms of nitrogen are quickly available to your plants after they are applied. Water soluble and insoluble nitrogen needs to be broken down by organisms found in the soil to the nitrate form in order to be absorbed by the plants. The time it takes to break down these two forms of nitrogen cause them to be absorbed slowly over time (slow release). This is a highly desirable characteristic of fertilizer. Slow release fertilizer can also be created by chemically coating the elements in plastics or sulfur. Osmocote and Nutricote (sold as Dynamite) are slow release, coated fertilizers. These two products are excellent for potted plants, but not cost effective for in-ground material. At least 2% of the nitrogen should come from water soluble or water insoluble nitrogen sources. If a fertilizer does not have a slow release quality, all the nutrients are available at once and will be quickly washed away by subsequent rains. Pictured: Frizzle Top caused by a lack of Manganese.

The micro elements are iron, zinc, manganese, copper, boron, molybdenum and chloride. In South Floirda, micro elements are not usually available to plants when applied in granular form due to a high soil pH. Micros should therefore be applied in a foliar spray that can be readily absorbed by the leaves. Foliar sprays should cover the leaves entirely and should be done on early morning or late afternoon. Iron, zinc and manganese can be applied directly to the soil if they are delivered in the form of chelates; which are specifically formulated, chemically altered forms of these elements that allow the plant to absorb the element from the soil. The chelated iron Sequestrene 138 is formulated specifically for South Florida’s alkaline soils and is mixed at 3 TBS per 5 gallons of water. Chelates should be applied as a drench to the soil when the soil temperature is warm and soil moisture is ample.

A proper fertilizer schedule is one that makes nutrition available to the plants when needed and is tailored to the seasons. South Florida has a marked wet (late May to early October) and dry season (late October to early May). More fertilizer is needed in the warm, wet season and less fertilizer is needed in the dry, cool months. A standard fertilizer program would apply fertilizer three times a year. Applications should be done in late May or early June, just as the summer rains begin. The second and third applications would be in mid-August and in mid-October. The June and August applications should be the slightly heavier as this is the height of the growing season. Granular (macros), foliar (micros) and drench (Iron) applications are done during these months as well. Some plants that are poorly adapted to growing in the alkaline soil of South Florida, such as hibiscus, ixoras and gardenias, will benefit from increased applications of all micro elements, especially iron. It is important to note that native plants and plants well adapted to South Florida need little to no fertilizer.

Granular fertilizer (macros) should be scattered slightly beyond the drip line of a tree or shrub. The drip line of a tree is an imaginary circular line drawn below the furthest point branches reach. The roots responsible for absorbing water and nutrients, feeder roots, are found just past the drip line. Do not place granular fertilizer near the trunk of the tree or in clumps anywhere around the tree. The feeder roots will be damaged by clumps of fertilizer and fertilizer placed near the trunk of a tree cannot be absorbed. Granular fertilizer should be applied at a rate of one large handful of most fertilizers per 1 inch of trunk diameter (measured 18″ above the soil line) for most plants. Do not over fertilize as it will do more damage than not fertilizing at all. Granular fertilizer can be applied on top of mulch. Foliar sprays (micros) should be mixed according to the directions on the product and applied to the leaves to the point of runoff. Mix soil drenches according to the directions on the product and pour just past the drip line of the plant.

These recommendations will help you figure out the many questions that homeowners have about fertilizers. The most important lesson to learn is that fertilizing can be kept to a minimum when the principle of putting the right plant in the right location is followed.