The Leaflet Volume 4, Issue 1

The Leaflet banner image

May 12, 2021

Numbers on the fertilizer bag

Have you ever wondered what the numbers on a bag of fertilizer mean? I often talk about “triple 13” or “8-24-24” without stopping to explain what those numbers mean. Bagged fertilizer generally has three numbers located somewhere on the bag. Fertilizer numbers are consistent in the United States and represent the percentage of a specific nutrient inside the bag. The first number represents nitrogen (N), the second phosphorus (P), and the third potassium (K)- in that order. We call it “NPK” for short. For instance, a bag of 13-13-13 contains 13% nitrogen, 13% phosphorus, and 13% potassium. Similarly, a bag of 8-24-24 contains 8% nitrogen, 24% phosphorus, and 24% potassium.

Occasionally you may encounter a product with 4 numbers on the bag. The fourth number represents sulfur. For example, 21-0-0-24, or ammonium sulfate, contains 21% nitrogen, 0% phosphorus, 0% potassium, and 24% sulfur.

Understanding the numbers on fertilizer bags will help you decode soil test results and decide which fertilizer is best for your situation. If I see a soil test come back with very high P & K values, I may recommend a product with N only (33-0-0 or similar). Tests with low sulfur require elemental sulfur or a fertilizer like ammonium sulfate which contains sulfur.

Sometimes fertilizer calculations require basic math. The LSU AgCenter Azalea publication recommends applying 1 pound of nitrogen per 1000 square feet around azaleas. Let’s pretend you have a bag of 13-13-13 laying around at home. How much 13-13-13 would you need per 1000 square feet to meet the 1 pound of nitrogen recommendation?(Remember, 13-13-13 contains only 13% nitrogen).

Use this equation to figure it out:


I want 1 pound of nitrogen and I have 13% nitrogen. Therefore:

Equation 2png

You will need to apply 7.69 pounds of 13-13-13 per 1000 square feet to apply 1 pound of nitrogen per 1000 square feet around your azaleas.

Serena series Angelonia

Each year I like to test Louisiana Super Plants in my flower beds to gain personal experience with what works well in our area. Louisiana Super Plants are tested by LSU AgCenter researchers and boast superior performance in Louisiana’s tough growing conditions. By planting them myself, I can speak about them from personal experience, not just what the specialists tell me. This spring I chose to plant the Serena series Angelonia and I have been very happy with them. The deer have not eaten them (yet), and they took off growing and blooming almost immediately. I was worried that our extremely wet spring may hinder the plants, but so far, they are thriving.

The Serena series comes in multiple colors including pink, white, and purple. They are often referred to as the summer snap dragon because their flowers resemble an open-mouthed dragon. I prefer the wispy, delicate, cottage garden look that Angelonia provides over snapdragons.

Serena series Angelonia have a 10-14 inch spread and reach 10-14 inches tall. They are great planted around borders or planted in mass plantings. For best results, plant in full sun and space plants 12 inches apart. Apply a slow-release fertilizer at planting and an occasional dose of liquid fertilizer. Once established, the plants are fairly drought tolerant, but they may require occasional supplemental water.

Serena series Angelonia also perform well in containers. Give them a try, I think you will be pleased! Learn more about Serena series Angelonia here.

Watering plants

Louisiana receives an average of 60 inches of rainfall per year making it one of the rainiest states in the United States. With this much rainfall, watering plants is not as critical in Louisiana as it is in other states, but Louisiana gardeners should monitor their plants and learn to recognize when they should water.

Follow these tips to ensure your plants stay healthy and hydrated:

  1. No plant should be watered every day. Most plants, including fruits and vegetables, need about 1 inch of water per week. If you receive at least one inch of rain per week, your plants should be fine. If not, water once per week. You may need to increase watering to twice per week when the weather turns dry. Sandy bedding mixes will dry out more quickly than our native soil, so monitor them closely.
  2. Monitor your plants and use your finger to test the soil for moisture. Stick your finger in the soil 1 inch deep and if the soil is dry, it is time to water.
  3. Water deeply and infrequently. Watering plants once or twice per week deeply is more beneficial than sprinkling them with a little water every day.
  4. Water at the base of plants if possible. Utilize soaker hoses or drip irrigation systems to reduce diseases and ensure the plant is getting water where it needs it- the roots.
  5. Utilize mulch in beds and gardens to conserve moisture in the soil.

3/4/2021 2:56:40 PM
Rate This Article:

Have a question or comment about the information on this page?

Innovate . Educate . Improve Lives

The LSU AgCenter and the LSU College of Agriculture