Plant Nutrition: Is Your Plant Missing Something?

Is your plant missing something?

Yellowing leaves could be a sign of a nutritional deficiency, but it could also mean that the plant is either getting too much or not enough water or light. The pattern of yellowing on the leaves can be an indicator of the particular nutritional deficiency. Yellowing on the new leaves could be caused by a different deficiency than yellowing on older leaves. Failure to bloom could be a nutritional issue, but more than likely is caused by a water, light or temperature issue.

Basic Plant Nutrition

Plants absorb 13 mineral elements, mostly through their roots.

Primary macro nutrients are the ones that plants need in the largest quantities. Because the plants use so much of them, they are often not present in sufficient amounts in soils for optimum plant growth. These are Nitrogen (N), Phosphorous (P) and Potassium (K). Signs of deficiencies of these nutrients appear on older leaves.

Secondary macro nutrients are usually found in sufficient amounts in the soil so addition of these nutrients is not always needed. These are Calcium (Ca) Magnesium (Mg) and Sulfur(S).

Signs of deficiencies of these nutrients appear on older leaves.

Micro nutrients are those elements essential for plant growth which are needed in  very small (micro) quantities. These elements are sometimes called minor elements or trace elements. They include Boron (B), Copper (Cu), Iron (Fe), Chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Signs of deficiencies of these nutrients appear on newer leaves.

Soil pH The pH is a measure of soil acidity or alkalinity, and can also be a factor in a plants ability to take up necessary nutrients. Most plants do best at a slightly acid to neutral pH (6.5-7.0). Higher or lower soil pH may inhibit nutrient absorption illustrated in the chart below.

Nutrient availability in relation to soil pH table from

Soil testing maybe helpful in the diagnosis of plant nutritional issues. Kits are available for home owner use and usually test for primary macro nutrients and soil pH. Local county Extension Service offices often provide soil pH testing in house for a nominal fee, or provide materials for collection of soil samples and forms to send  it to a laboratory. The returned analysis can be a clue in determining what action to take to correct any nutritional issues.

It is also possible to submit a plant tissue sample  to a laboratory to have it analyzed  for deficiencies. Check with your local Extension Service for availability.    

Pictures and diagnostic key of nutritional issues of common Florida landscape plants can be found at.


Types 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. 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. 

Reading a fertilizer bag

The 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. A balanced fertilizer would contain equal percentages of N, P, and K. (6-6-6 ) A bloom booster would contain a higher amount of  Phosphorus (10-20-10).

Proper Fertilizer application

Keeping 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 Florida’s sandy soils. 

When otherwise healthy landscape plants show the signs of nutrient deficiencies, 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 How - When – Where -? questions of fertilizer.

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 5 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. Remember to always follow local ordinances regarding fertilizer application.

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.