What causes green algae in rivers? The facts about fertilizers

Larry Figart
| For the Jacksonville Florida Times-Union USA TODAY NETWORK

Recently, the Duval County Health Department issued a warning about toxic blue-green algae in the St. Johns River. The cause of the algae bloom is said to be an excess of nutrients in the river, primarily nitrogen. Nitrogen is an important component of fertilizer applied to gardens and landscapes. The first number on a bag of fertilizer indicates the percentage of nitrogen, but do you know how much you really need to apply to keep your landscape healthy? Is there such a thing as too much? Read on to learn everything you need to know about nitrogen, its effects on plants and our environment.

What is nitrogen and how is it used in plants?

Nitrogen is an important nutrient (one of 17) that plants need. Our sandy Florida soils are often nitrogen-poor. Nitrogen is an important component of chlorophyll, which helps plants convert sunlight into energy. It is also a component of amino acids that make up plant proteins, which are essential for cell walls and important plant enzymes. Before the introduction of synthetic nitrogen, most nitrogen in soil came from decomposed plant residue and animal waste (manure). The nitrogen in these organic sources must be broken down by soil microbes and converted to ammonium or nitrate, which is available for plant roots to absorb. This process is called mineralization. Nitrogen in synthetic fertilizers is usually already in a form that plants can use immediately.

Why do we worry about too much nitrogen when it occurs naturally in the soil?

Nitrogen, in the form of nitrate or ammonium, is very mobile in the soil. That means it can easily be washed out of the soil by rain or irrigation. When it washes through the soil from plant roots into groundwater, or runs off the road or into a ditch, it ends up in a body of water such as a lake, stream, or the St. Johns River. Nitrogen flowing into these bodies of water can cause rapid growth of algae, or blue-green algae, resulting in an algal bloom. Blue-green algae blooms can produce toxins that are harmful to people, pets, and aquatic animals. In 2005, a toxic green algae bloom in the St. Johns River was so large it was dubbed “The Green Monster.”

How can we prevent the nitrogen we apply as fertilizer from causing algal blooms?

The short answer is: Don’t add more nitrogen than your lawn and plants can absorb through their roots. This can be difficult, but with a few common sense principles, you can add enough nitrogen to encourage your plants to grow and reduce the amount that washes away.

Use a slow-release or controlled-release fertilizer. These fertilizers are treated to release nitrogen slowly over a long period of time. They can be organic or inorganic fertilizers that have been treated to provide a slow release. Polymer or sulfur coatings are examples of slow-release fertilizers. The advantage of slow-release fertilizers is that the nutrients are available to plant roots for longer and are less likely to be washed away by the roots.

You can learn a lot about your fertilizer by reading a section on the packaging called “Guaranteed Analysis.” This box contains all the information about what nutrients and what amounts are in the fertilizer and whether they are fast or slow released. You can learn a lot about the quality of the fertilizer by looking at the guaranteed analysis.

You should not add nitrogen if the lawn is not growing. If you apply nitrogen to sparse, patchy, unhealthy turf, or before the lawn is actively growing to revitalize the area, much of the fertilizer applied to bare soil will not be absorbed by the roots and will be lost. The opposite is also true. If you apply too much nitrogen to an already healthy lawn, it will grow too lushly and be susceptible to insects and diseases such as large patch or gray leaf spot.

Map: Check out the recent algal blooms in Northeast Florida

More about algal blooms: Rain, temperature and nutrients are the main factors for algal blooms

How do I fertilize my lawn properly?

The lawn fertilizer bag will tell you how many square feet can be covered based on the nitrogen amount and composition. The fertilizer label will also give you a recommended spread setting based on your spreader. This amount was calculated with Florida Urban Turf Fertilizer rules in mind. Because “the label is law,” you can use less fertilizer than what is indicated on the bag. However, it is against the law to use more than what is indicated on the fertilizer bag label.

How often should I fertilize?

It is important to remember that there is no need to fertilize your lawn during the cooler months when it is not growing. Fertilizing then is a waste of money and results in more nutrient runoff. The most critical application times are mid-April (at the beginning of the growing season) and September (before winter).

Are there any other fertilization tips?

Do not apply fertilizer before heavy rain. Much of the fertilizer will be lost through runoff or leaching if heavy rain occurs within 8 to 12 hours of applying rapid-release urea.

By applying just the right amount of nitrogen at the right time and using it in a composition that resists its movement through the soil, we can avoid polluting our waterways while still maintaining a healthy, productive landscape.

Larry Figart is an Urban Forestry Extension Agent at the University of Florida/IFAS.