The Importance of Plant Nutrition

Healthy plants spring from rich soil. Naturally fertile soils arefound in other parts of the United States but most of our growing ground is made of sand, seasoned with lime rock, and peppered with shells. If it looks like it used to be the ocean floor, your are right. Here is one of the greatest challenges to plant growth and success.

The amount of "data" and advice on plant nutrition is dizzying and often contradictory. This is a summary of various articles by experts on Florida plantnutrition that makes sense and will give you the good results that you desire in Southwest Florida.

The amount of salt that you put on your steak and how you like it done is strictly a matter of taste. But plants don't have opinions, they have needs. The following describes plant's nutritional needs and how gardeners can provide them.

PALM NUTRITION

Palms are among the most important ornamental plants in Florida landscapes. Palms suffer quickly and conspicuously from improper mineral nutrition, whether due to insufficient or incorrect fertilization. They also may exhibit certain nutritional disorders in unique ways compared to other ornamental plants. You can slowly kill large, valuable palms by putting-on the wrong fertilizer.

Seventeen elements are considered necessary for the growth of green plants: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Cl), Magnesium (Mg), Sulfur (S), Iron (Fe), Zinc (Zn), Manganese(Mn), Boron (B), Copper (Cu), Molybdenum (Mo), Chlorine (Cl), and Nickel (Ni).

Plant nutrients are classified as macronutrients and micronutrients. The term “macronutrients” refers to those elements that plants require in large amounts (N, P, K, Ca, Mg, S). “Micronutrients” applies to plant nutrients that are essential to plants but are needed only in small amount (Fe, Zn, Mn, B, Cu, Mo, Ni, Cl).

All it takes is just one of these 17 elements to be lacking and a palm will decline, deform, and die.

Nutrient availability directly affects how well plants grow and produce. A sufficient supply of all nutrients is critical to nutrient management and sustainability.

SOIL TYPES

The dominant soil type in most yards in South Florida comes from limestone rock. Because our native limestone does not readily hold nutrients or make them available to plants, most plants need to have fertilizer supplied to them. Choosing Fl Natives and plants adapted to grow in our soil and climatic conditions can decrease the amount and frequency of fertilizer needed. Mulching can also help to reduce the amount of fertilizer needed by adding an organic component to an otherwise barren soil.

Soil pH is a measure of the acidity or alkalinity of the soil. Homeowners and gardeners alike should be interested in soil pH because it affects the growth and quality of landscape plants.

Florida soils can vary widely in pH levels. Soils in pine woods can be quite acidic. Soils that were formed from calcium-rich materials, such as limestone or sea shells, tend to be alkaline. This is particularly true of soils in coastal areas. Building materials in the home landscape including concrete and stucco, may create alkaline conditions as well.

Alkaline soil can adversely affect many plants that require acidic soil conditions. Some familiar acid-loving plants include Ixora, Magnolia, Azalia, Gardenia, Hibiscus, Rose, and Blueberries. On the pH scale of 14, a value of 7 is considered neutral, pH values less than 7 are acidic, and pH values greater than 7 are alkaline.

Lowering the pH of strongly alkaline soils is difficult and there is no way to permanently lower the pH. Soil amendments such as peat moss, manure and elemental sulfur can temporarily lower the pH of the amended area, but the effects are localized to the amended area and the effect is temporary.

Always consider the pH of your soil when selecting new plant material for your home landscape or garden. Have your soil tested for pH and lime requirement (if soil is acidic). If you are interested in growing specific plants that are not suited for your soil pH, consider growing them in containers, where you will be able to amend small volumes of soil to reach the desired pH.

Are you confused yet? I am including information about palm trees from Dr. Tim Broschat of the U of Florida. He is a world renowned expert on the nutritional needs of Palm trees. This information also applies to all of our green plants.

FERTILIZER FORMULATIONS and CONTROLLED RELEASE

Nutrient deficiencies are much more easily prevented than corrected once they occur. The correction of nutrient deficiencies can take as long as two to three years for some elements. Research at the U of FL has shown that regular use of a fertilizer having an analysis of 8N-2P-12K+4Mg (Magnesium) with micronutrients can correct mild to moderate deficiencies and prevent their recurrence in all soil types in south Florida. However, not all fertilizers having that analysis with micronutrients are effective.

It is essential that 100% of the N,K, and Mg in such a fertilizer be in slow or control-release form. Since Florida’s soils have very low capacities to retain these elements in the root zone during periods of heavy rainfall or irrigation, the only effective way to keep these elements readily available to plants during a three month interval between fertilizer applications is to use slow release sources. A water-soluble source applied one day could be completely leached out of the root zone the next day by a heavy rainfall or irrigation. Since control release fertilizers deliver nutrients more slowly than water-soluble fertilizers, they are also less likely to burn plant roots during periods of drought. In addition, control-release fertilizers dramatically reduce fertilizer runoff which can leach into our groundwater.

The use of the 8-2-12 fertilizer is recommended for the entire landscape. This simplifies fertilization by using a single product for all of our plants. Palm tree roots on large specimens typically extend 50 feet or more from the trunk in all directions frequently under turfgrass, and will take up all fertilizers that have been applied to the turf. The typical turf fertilizers can cause serious problems for the affected palms and the 8-2-12 should be applied on turfgrass around these palms.

We recommend Nurseryman’s Sure-gro 8-2-12 which was developed in conjunction with Dr. Broschat for ALL plants. It also acts to acidify our alkaline soils. The control release features will ensure that it will provide nutrients for 90 to 120 days. These fertilizers can be applied on top of mulch and don't need to be watered in.

Sure-gro’s 12-4-12 Palm fertilizer provides even more acidity for our acid-loving plants. It is also a great palm fertilizer, and again can be used on all plants. Their All Purpose fertilizer is highly recommended for citrus and edibles as well as a general purpose fertilizer effective for up to 120 days. They have also developed a control-release fertilizer specifically for Bougainvillea and other blooming plants.

Fertilizing Details and Recap

Palms:

• Use only 8-2-12+4Mg and micronutrients palm maintenance fertilizer.
• Broadcast using a calibrated rotary spreader under the canopy area, entire bed area, or entire landscape at a maximum rate of 15 pounds of fertilizer (not Nitrogen) per 1000 sf. every 3 months.
• Never use turf fertilizers within 50 feet of a palm. The recommended palm fertilizer can be used on turf and other ornamentals within range of the palm(s).
• Fertilize in February, April, June and early October (once established, Sabal palmetto do not require additional fertilizers).

Established Trees and Shrubs

Shrub beds:

• Apply a complete fertilizer at the rate of 1 pound of actual N per 1,000 sf.
• Apply in February, April, June and no later than late September.
• Broadcast throughout the shrub bed directly onto the soil or mulch. Do not fertilize next to the trunk(s).
• Water any wash fertilizer off of foliage.

Flowering and Shade Trees:

• Apply a complete fertilizer such as 8-2-12+4Mg, 12-4-12. or an 18-6-12.
• Apply fertilizer to the soil or mulch surface no more than 1 ½ - 2 times the canopy diameter. Fertilize the mulch area only and base fertilizer calculations upon the mulch square footage).
• Trees planted more than 5 years (especially those surrounded by turf) do not need supplemental fertilizer unless they exhibit a specific nutrient deficiency.

Plants in Containers:

• Use a general purpose fertilizer such as Sure-gro’s 18-6-12 on plants growing in containers. It is a 100% control-release product and will not burn. It will also last 3 months and is effective for palms growing in a container.

Bedding Plants:

• Recommendation is to use an purpose 14-12-14 for annual beds. This is a 3 month control-release fertilizer, which will not burn and lasts 3 months. A 6 month product is available. This simplifies the fertilization process and homeowners can avoid mixing and measuring soluble fertilizers.

Measuring Calculations for Control-release Fertilzer:

• 1 ½ pounds per 100 sf. or 15 pounds per 1000 sf.
• There are 16 tablespoons for each pound; 16 tablespoons equal 1 cup; and 2 cups for each pound.
• A newly planted 3g container plant will need approximately 3 tablespoons of fertilizer. Remember to spread the fertilizer evenly around the root-ball area.

Travelers & Snowbirds:

• Sure-gro has formulated a 6 month 8-2-12 palm fertilizer which will need to be applied only twice a year. This simplifies the fertilization process and allows “snowbirds” to be able to provide much-needed nutrients during the fast-growing summer months when they are away.

References:

Soil pH and Plants: http://edis.ifas.ufl.edu/MG092

The Florida Fertilizer Label: http://edis.ifas.ufl.edu/LH014

Fertilization of Field-grown and Landscape Palms in Florida: http://edis.ifas.ufl.edu/ed261

Palm Nutrition Guids: http://www.floridaplants.com/horticulture/palm.htm

South Florida Horticulture: http://southfloridahorticulture.com/v2/fertilization

Palm Nutrition Guide: http://edis.ifas.ufl.edu.ss-orh-02

Soil pH and the Home Landscape or Garden: http://edis.ifas.ufl.edu/ss480

Bob Cook, FNGLA Instructor