As soon as you walk into your favorite garden center, you can’t help but notice the stacks of fertilizer bags waiting for you. There are many different types, formulations, and brands to choose from. In this article we'll give you some answers to questions you may have about fertilizing your lawns and landscape.
Plants need nutrients to grow, and usually get them from the soil. We typically add fertilizer to promote plant growth and provide needed nutrients. It’s not always necessary to fertilize your plants or lawn.
However, if you need to supply needed nutrients through fertilization, it’s important to do so properly. Over fertilization can result in nutrients leaching through the soil and into our local water bodies and rivers. The recent spike in manatee deaths in Florida has been attributed to nutrients leaching into the Indian River Lagoon, causing algae blooms that in turn shade out the eelgrass that manatees feed on.
The key in fertilizing our lawns properly is to apply the right formulation, at the right amount, at the right time.
When should you fertilize?
The best time to fertilize is when your turf is actively growing. The turf will not absorb the fertilizer unless it is growing. Applying fertilizer to turf too early means more nutrient leaching. A good way to think of it is to wait until you are having to routinely mow the turf before you apply fertilizer. Typically, mid-April is a good target date.
What fertilizer should I use? The 2007 Urban Turf Fertilizing Rule makes this easier. To follow this rule, fertilizer companies selling fertilizer in Florida must meet certain formulation requirements. To fertilize lawns and turf, select a fertilizer that says, “for use on home lawns, or urban turf.” Most Florida soils have plenty of phosphorus, so the middle number on the fertilizer bag should be 0 or 2. Get your soil tested if you suspect a phosphorus deficiency.
Next, look at the nitrogen source. Find a fertilizer that has 30% or more of the nitrogen in a form that allows it to be released into the soil slowly making it available to the turf for a longer period of time. You may find it listed on the fertilizer bag as “controlled release,” “slow release,” “water insoluble nitrogen” or “polymer coated.”
To calculate the percentage of slow release nitrogen in a fertilizer, take the number listed for slow release, divide by the first number on the fertilizer bag (total nitrogen) and multiply by 100. For example, if a 15-0-15 has 7% water insoluble nitrogen, divide 7 by 15 and multiply by 100 to get 47%, which is very good.
Then look at the label on the bag for secondary plant nutrients and micronutrients. For lawns, iron and manganese are the most important, especially in soils with a pH over 6.5.
“Weed and Feed” type fertilizers are not the best product to apply at this time. It’s a timing issue. The herbicide is traditionally a pre-emergence herbicide, which should have been applied earlier (mid-February to March 1), when weeds were germinating, to be effective. That was not the right time to fertilize because lawns are dormant.
How much fertilizer to purchase and apply
Only buy what you need for one application or at least one growing season because fertilizers do not store well. First, determine the overall square feet of lawn area by dividing your lawn into smaller rectangles. Multiply the length by the width (60 feet x 30 feet = 1,800 square feet) of each rectangle. Add these together to get the total square feet of fertilized area.
Another option is to look at the total area of your property. Then subtract the area for the house, deck, driveway, sidewalks and garden area (all non-fertilized areas) to get the total square footage.
The fertilizer bag will state how many square feet can be covered based on the analysis. The fertilizer label will also give you a recommended spreader setting based on the spreader you have. This rate has been calculated with the Florida Urban Turf Fertilizer rule in mind.
Since “The Label is the Law,” you can use less fertilizer than the bag says, but it is against the law to use more than is stated on the label on the fertilizer bag. Applying too much fertilizer can encourage turf insects and disease, as well as causing nutrient rich runoff to pollute our waterbodies.
Another option is to use the University of Florida publication "Homeowner Best Management Practices for the Home Lawn" at https://edis.ifas.ufl.edu/pdffiles/EP/EP23600.pdf. This includes a table that gives the pounds of fertilizer to apply based on nitrogen content and square feet of lawn area.
How often to fertilize
One important thing to remember is your turf does not need to be fertilized in the cooler months when it is not growing. Applying fertilizer then only wastes money and creates nutrient runoff. How often you fertilize during the growing season depends on the turf you are growing and how much maintenance your turf needs.
Here are some guidelines: Apply 1 to 3 pounds of nitrogen per 1,000 square feet per year to bahia grass. Apply .4 to 2 pounds of nitrogen per 1,000 square feet per year to centipede grass. Apply 2 to 4 pounds of nitrogen per 1,000 square feet per year to St. Augustine grass. Apply 2 to 3 pounds of nitrogen per 1,000 square feet per year to zoysia grass.
Notice there is a range. Lower amounts of nitrogen are for lower maintenance lawns versus larger amounts are for a higher maintenance lawn. For lawns that require two pounds of nitrogen per 1,000 square feet per year, the most critical application times are mid-April (at the beginning of the growing season) and September (going into the winter). To figure out how many pounds of nitrogen are in a bag of fertilizer multiply the percentage of nitrogen as a decimal and the pound weight of your bag. For instance, a 20-pound bag of 16-0-12 has 3.2 pounds of nitrogen. (20 pounds x .16 (percent nitrogen from the label) = 3.2 pounds).
Other fertilizer tips
Do not apply fertilizer prior to a heavy rain. Much of the fertilizer will be lost due to runoff or leaching if a heavy rain occurs within 8 to 12 hours of a quick release fertilizer application.
Lightly water-in fertilizer with one-quarter inch of water to prevent nitrogen loss and burning the grass.
For more Florida-Friendly tips on managing lawns, go to https://ffl.ifas.ufl.edu/about-ffl/9-principles/principle-3-fertilize-appropriately/.
Larry Figart is an urban forestry extension agent with the University of Florida/IFAS.
This article originally appeared on Florida Times-Union: Garden Help: It's spring, time to fertilize your lawn