Edible Gardening Series: Question of the Week – fertilizer and plant nutrients

By Carol Wyatt-Evens and Sarah Bostick

Gardening in Florida can be incredibly rewarding and incredibly frustrating, at the same time. If you are new to the region, you soon learn that gardening in the Sunshine State can quickly become a full-time job. While our subtropical climate is perfect for growing an abundance of different vegetables, fruits, and herbs, it also can present some overwhelming challenges.

We can help! UF/IFAS Extension Sarasota County agents and staff have created an online edible gardening resource center. The website features short videos from our 25-episode “Edible Gardening Series” webinars, along with blog posts and resources lists for episodes. Get help on an array of topics that befuddle many gardeners.


This week’s Question of the Week:

How do I know how much fertilizer to give my plants?

Have you ever walked into a garden center and felt overwhelmed by the number of fertilizer options on the shelves? If so, you are not alone. Feel confused about how often you should be fertilizing your garden? You aren’t alone there either.

The topic of fertilizer and plant nutrition is a complicated one, but there are a few key pieces that every gardener should understand.

How plants get the energy and nutrients they need to thrive

  • Leaves: plants rely heavily on the energy they get from sunlight.
    Plants get much of the energy they need directly from the sun.

    Access to sunlight is what drives a crucial process called photosynthesis. Photosynthesis is the process in which plants use sunlight, water, and carbon dioxide to create oxygen and sugars. Those sugars provide the plant with the energy it needs to grow and function. To learn more about how access to sunlight impacts a plant’s ability to thrive, check out this blog post in our Edible Gardening Series.

  • Roots: Most plants derive some of the nutrients and energy they need directly from the soil. To learn more about the nutrients plants need and how the plant accesses those nutrients, check out this article from Texas A&M University.


The nutrients plants need

Remember the Periodic Table of the Elements from chemistry class? Plants need more than a dozen elements to grow, mature, reproduce, and stay healthy.

Most gardeners are familiar with the letters N-P-K on the front of a fertilizer bag or bottle. N-P-K stands for nitrogen, phosphorous, and potassium. These are known as macronutrients because plants need more of these three nutrients than other nutrients. Just as important to plants but needed in lesser quantities are secondary nutrients: calcium, magnesium and sulfur. And last but not least are micronutrients, which are needed in the smallest quantities: boron, chlorine, copper, iron, manganese, molybdenum, and zinc. Plants need all of these nutrients for healthy growth and reproduction.


Knowing what nutrients your garden needs

Each time you harvest a carrot or a leaf of kale, you are removing nutrients from your garden and putting those nutrients into your own body. Your body digests the nutrient-rich food, utilizing most of the nutrients to keep your body healthy and functioning. Your body expels what it does not need in the form of poop. A flush of the toilet finishes the process of removing the nutrients from the system.

By comparison, in a natural system like a forest, nutrients are simply recycled over and over again in the same place. A tree grows big and tall and drops its leaves each year. Those leaves decompose, releasing their nutrients back to the soil where the tree can reabsorb them.

In a garden, we have to periodically replace the nutrients that harvesting removes. Macronutrients need to be replaced most often, secondary nutrients occasionally, and micronutrients very infrequently. There are many things that determine how often different nutrients need to be replaced: climate, temperature, rainfall, soil pH, soil type, what plants you are growing, and more.

It is tempting to guess what nutrients your plants need and also tempting to assume that more is better. This is not advisable! Too much of any nutrient can be just as damaging to a plant as not enough of a nutrient.

Too much nitrogen, for example, can lead to major outbreaks of aphids (which spread plant diseases), plants that produce huge amounts of leaves and stems but very little fruit, and root vegetables with big, beautiful leaves but very small roots. Boron toxicity, for example, is a serious condition caused by too much boron. The symptoms are yellowing along the edges of leaves that progresses towards the veins. Severe boron toxicity causes leaves to dry out at the tips or turn black and eventually fall off the plant. Chronic boron toxicity can lead to plant death.


Resources for knowing what your plants do and do not need

  1. Soil tests
There can be many causes of plant stress. This picture shows corn leaves. From left to right: herbicide damage, fungal disease, drought damage, magnesium deficiency, nitrogen deficiency, potassium deficiency, phosphorous deficiency, and a healthy leaf.

If you are a home gardener, it is a great idea to have your soil tested at least once every few years. If you live in Florida, you can get your soil tested by the University of Florida’s Extension Soil Testing Laboratory in Gainesville or at a private laboratory. The UF/IFAS Extension Sarasota County’s website provides all the information you need to decide where to get your soil tested.

Your test results will let you know if your soil has too much, too little, or the right amount of each nutrient. If your soil does not have enough of a particular nutrient, your test results will tell you how much you need to add to your soil.

  1. Plant tissue tests

If you are experiencing particularly unusual issues with a plant, an expert might recommend that you send a plant tissue sample to a laboratory for analysis. In the laboratory, the leaves of your plant will be ground up and analyzed to determine if they are deficient in a certain nutrient or if they are experiencing a toxicity effect from too much of a particular nutrient. Plant tissue tests can also determine whether or not the issue your plant is experiencing is nutrient or disease related.

  1. Learn to “read” your plants

There are dozens of very neat guides to “reading” the strange colors and patterns on your leaves. Most guides are set up like a choose-your-own-adventure story. By answering yes or no to each question, you can arrive at a possible reason for the issues your plant is having. It is important to note that not all plant mysteries can be solved in this way. Many plant diseases and nutrient issues can look quite similar, but it can be helpful to start the process of troubleshooting plant issues by using a guide to eliminate possibilities.

Here are a few guides to plant nutrient deficiencies to get you started:


The Edible Gardening Series and blog series is a partnership between the following UF/IFAS agents and Sarasota County staff:

  • Sarah Bostick, Sustainable Agriculture Agent
  • Carol Wyatt-Evens, Chemicals in the Environment Agent
  • Mindy Hanak, Community & School Gardens Educator
  • Kevin O’Horan, Communications Associate

Posted: May 6, 2021

Category: Agriculture, Fruits & Vegetables, Home Landscapes
Tags: Ag, Agriculture, Edible Gardening QOTW, EdibleGardeningSeries, Fertilizer, Garden, Pgm_Ag, Pgm_Chemicals, Plant Nutrients

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