LSU AgCenter Horticulturist
As plants put on new growth this spring, they can benefit from essential nutrients that help them become healthy and strong. Plants can be very different and have their own unique requirements, but there is one universal gardening rule when it comes to fertilizing — and that is to make an annual application of fertilizer in the early spring when plants are in full growth mode.
Fertilizers encourage new growth and the production of flowers and fruits. This new growth is very tender and can be damaged by freezing temperatures. Early spring after the danger of the last frost has passed is the ideal time to fertilize.
Fertilizers are important because they contain the building blocks for plant growth, foliage and fruit production, root and bloom formation and overall plant health. There are three main macronutrients important to plant growth: nitrogen (N), phosphorus (P) and potassium (K).
When you see numbers across a fertilizer bag, they represent the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K). The numbers are printed in order of N-P-K.
For example, a bag of 10-10-10 fertilizer contains 10% nitrogen, 10% phosphorus and 10% potassium by weight. This means for every 100 pounds of fertilizer, there are 10 pounds of each nutrient.
The first number is nitrogen, which contributes to the growth and health of the foliage, stems and leaves. Phosphorus is an essential nutrient for root development and flower and fruit production. Potassium contributes to overall plant health, improves drought and stress tolerance, disease resistance and strengthens overall plant structure.
There are several types of fertilizers available for home gardeners, including water-soluble liquid drenches, stakes and slow-release granules. Each has different application rates and frequencies.
Consider the specific needs of the plants and their growth stage. A high-nitrogen fertilizer is best for leaf growth, while a higher phosphorus and potassium content is better for flower and fruit development.
Let’s take a minute to talk about responsible fertilizing. Plants can get many of the nutrients they need from the soil. If your soil has a balanced nutrient profile, you don’t need to add fertilizers. Often, however, our soils do not have this perfect balance of nutrients. It is recommended that soil tests be taken every three to five years; this way, you can understand exactly which nutrients are needed and apply only those.
Be sure to abide by the rate and application frequency listed on the label of the product you are using. Excessive applications in combination with heavy rains can cause nonpoint source pollution, as fertilizers can wash into storm drains. That stormwater eventually makes its way into bayous, rivers and lakes, causing pollution that can affect fish and other aquatic animals.
When fertilizing, don’t forget about trees and shrubs. Young trees, especially those with a trunk diameter of less than 6 inches, can benefit from regular applications of fertilizer. Large established trees and shrubs don’t typically require fertilizer; however, it can be a good practice to fertilize those that are surrounded by lawns or groundcovers. A general rule for fertilizing trees and shrubs is to use 1/4 to 1/2 pound of nitrogen per inch of diameter for trees 6 inches or more in diameter at chest height.
In general, plants grown in containers will require more fertilizer, as the growing media are designed to encourage good drainage. Plants such as vegetables benefit from heavier fertilizing to help increase growth and productivity. Stop fertilizing plants in winter when plants are dormant. Don’t fertilize new plants as they get established, and when in doubt, conduct a soil test.
The numbers on fertilizer labels represent the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K), and the numbers are printed in order of N-P-K. This all-purpose 20-20-20 formulation has 20% by weight of each nutrient. Photo by Heather Kirk-Ballard/LSU AgCenter
Fertilize responsibly by conducting soil tests to help determine the need for fertilizer. You can send samples to the LSU AgCenter for testing. Photo by Heather Kirk-Ballard/LSU AgCenter
Plant food spikes like these offer an easy option for fertilizing indoor container plants. Photo by Heather Kirk-Ballard/LSU AgCenter