Fertilizer Happens

When Squanto showed the Plymouth Colony pilgrims how to use fish to enhance the growth of their corn crops, he was sharing with them a form of organic fertilization.  Throughout human agrarian history methods of using manures, seaweed, and mulches have been used to improve crop yields.  Synthesized forms of mineral fertilizers are a relatively recent innovation, first developed in England and Ireland in the early 1800’s.  As the worldwide human population ballooned from 1.6 billion in 1900 to 6 billion in 2000, the pressure to feed these mouths drove the development of new agricultural techniques and products.  Homeowners and horticulturists today benefit from the convenience and effectiveness of modern synthetic fertilizers, but can there be dire side effects of this technology?

A debate on this very topic is currently underway as environmentalists, landscapers, fertilizer manufacturers, and local and state government deliberate over proposed fertilizer ordinances.  At the root of this issue is declining water quality in both fresh-water and marine environments.  Recent concerns over outbreaks of algal red tide and “dead zones” found in the Gulf of Mexico are fueling the debate.  Is fertilizer leaching the cause of these phenomena?  While scientific data can’t definitively answer that question yet, fertilizer leaching is certainly a suspect contributor.  Are there effective steps homeowners and property managers can take to minimize fertilizer leaching while ordinances are being argued?  Absolutely.

Let’s start by gaining a basic understanding of fertilizers.  Plants absorb 13 mineral elements, mostly through their roots.  Nitrogen (N), phosphorous (P), and potassium (K), are consumed in the largest quantities and are the main components of synthetic fertilizers.  These primary nutrients are expressed as a percentage of overall weight; thus a 50 pound bag of fertilizer with an “analysis” of 16-2-14 would contain 8 pounds of nitrogen, 1 pound of phosphorous, and 7 pounds of potassium.  Organic fertilizers such as composted cow manure often have a much lower analysis, something like .05-.05-.05.  

Keeping these nutrients where they belong depends on more than just the type of fertilizer you use.  The plants you choose for your landscape are also a determining factor in the amount of fertilizer you will need to apply.  Native plants are generally happy without any fertilizer once established, and slow growing woody plants typically require less nutrients to stay healthy than fast growing herbaceous plants.  Lawns and palms also require more fertilizer, particularly in our sandy soils. 

When otherwise healthy landscape plants show the signs of nutrient deficiencies (see http://hort.ifas.ufl.edu/nutdef/index.html), then some type of fertilizer will need to be applied.  Keeping this fertilizer in the root zone and out of waterways is a goal that can be achieved by correctly answering the What, How, When, Where questions of fertilizer.

What type of fertilizer?

Organic fertilizers such as composted yard waste, manures, bone meal, and fish emulsion are the best choice for a healthy environment.  Their nutrients are released slowly, which minimizes leaching and makes nutrient burning of plants nearly impossible.  Synthetic fertilizers are popular because of their convenience and potency, but are more likely to escape the root zone.  Nitrogen and phosphorous are the nutrients most often implicated for causing algae blooms, as they readily move through sandy soils.  Because many Florida soils are naturally high in phosphorous, fertilizers with a phosphorous content no higher than 2% is now recommended. 

A synthetic fertilizer’s form is also a key to its propensity for leaching into the water table.  Liquid fertilizers are already mixed in water, so move rapidly through the soil.  Granular forms are better, but many release their nutrients quickly once they get wet.  Modern controlled-release prilled fertilizers slowly and consistently discharge their nutrients over a period of time that allows plants to absorb them as they are released. 

How much to apply?

When fertilizing plants, more is not better, and can actually be quite damaging to both plants and the environment.  Many homeowners over-fertilize unwittingly, and need to remember that plants absorb nutrients rather slowly.  Current best horticultural practices recommend applying less than 4 pounds of nitrogen per 1000 square feet every year in the landscape.  Because their nutrient content is lower, organic fertilizers can generally be applied more liberally.

When to apply it?

Once you know how much fertilizer to apply annually, you will need to determine how to portion it out.  As a rule of thumb, fertilizer is less likely to leach if low doses are applied more frequently.  In the case of controlled-release fertilizers, this estimation has been done for you; apply at the manufacturers recommended interval.  Fertilizers are best utilized during periods of active plant growth; in Florida this period coincides with our rainy season.  Heavy rains can quickly move soluble nutrients into waterways, which creates a conundrum.  To minimize this risk, use organic or controlled-release fertilizers, and don’t apply when the weather looks stormy.

Where to apply it?

Plant roots absorb fertilizer, so any fertilizer applied out of their reach is a waste.  Distribute fertilizer evenly from the stem to the drip-line (farthest reaches of the branches); clumps or rings of fertilizer are too concentrated to be useful to the plant.  Make sure to keep your fertilizer off hard surfaces such as driveways, sidewalks, and streets, as this errant material is often washed into storm drains by rains and irrigation.  Also remember to keep your fertilizer at least ten feet away from bodies of water such as ponds and ditches.

Beautiful landscapes make our part of Florida a wonderful, lush place to live.  But so do healthy lakes and bays teeming with aquatic life.  Armed with some knowledge, you can ensure that both of these conditions can exist in harmony.   

Plant Nutrients

Plants require 13 mineral nutrients for proper growth, but they are not all needed in equal amounts.


Primary Macronutrients – used in the largest amounts

            - nitrogen (N)

            - phosphorous (P)

            - potassium (K)

Secondary Macronutrients

            - calcium (Ca)

            - magnesium (Mg)

            - sulfur (S)

Micronutrients – used in very small amounts, organic fertilizers are a good source

            - boron (B)

            - copper (Cu)

            - iron (Fe)

            - chloride (Cl)

            - manganese (Mn)

            - molybdenum (Mo)

            - zinc (Zn)