Fertilizer Buying Guide
Nutrients in Fertilizer
Plants need several types of nutrients. Macronutrients are necessary in large quantities. Those that may not be easily available in soil in the right amount are the primary nutrients you find in fertilizer:
Nitrogen for plant growth, leaf development and the production of vivid, green color
Phosphorous for root growth and the creation of fruit, seeds and flowers
Potassium — sometimes called potash — for root development and resistance to drought and disease
Secondary nutrients — oxygen, carbon, hydrogen, calcium, magnesium and sulfur — are also necessary macronutrients, but are often available in soil or air. Micronutrients — including boron, chlorine, cobalt, copper, iron, manganese, molybdenum, nickel and zinc — are needed only in small amounts.
Before you shop for fertilizer, perform a soil test. You can purchase a home test kit or send a soil sample to your local cooperative extension office for testing. The results will tell you what to add to the soil to make it ideal for the plants you plan to grow. For more information on soil tests, read Test and Improve Your Soil.
How to Read Fertilizer Numbers
Three prominent numbers on a fertilizer package — known as the NPK value, guaranteed analysis, or fertilizer grade —tell you the percentage of available primary macronutrients by weight in the package:
Nitrogen (N) content is the first number.
Phosphorous (P) content is the second number.
Potassium (K) content is the third number.
A bag marked 16-4-8 contains 16% nitrogen, 4% phosphorous and 8% potassium. To determine how much of each is in the bag, multiply the percentage by the weight of the bag.
Example: For a 50-pound bag:
.16 x 50 = 8
.04 x 50 = 2
.08 x 50 = 4
The bag contains 8 pounds of nitrogen, 2 pounds of phosphorus and 4 pounds of potassium.
The remainder is typically inert material which helps distribute the fertilizer evenly and prevent chemical burn. There may also be secondary nutrients or micronutrients in the formula.
Types of Fertilizer
Liquid fertilizers are fast-acting. Plants absorb them quickly through the leaves or roots, so you need to apply them every two to three weeks. Most are concentrates you mix with water. Some are available as hose-end bottles that create the mixture as you apply them, others you mix yourself and apply with a watering can. Liquid fertilizers work well for container plants, but you can also find liquid lawn fertilizers.
You apply granular fertilizers dry — with a mechanical spreader or from a shaker container —and water them in. Fertilizer for lawns and gardens are often in granular form. They're easier to control because you can see how much you're using and where you're dispersing them. There are two formulations of granular fertilizers:
Quick-release — known as water-soluble nitrogen (WSN) — fertilizers provide nitrogen to plants immediately. They generally last for 3 to 4 weeks, depending upon temperature and rainfall.
Slow-release or water-insoluble nitrogen (WIN) fertilizers are available in sulfur-coated varieties, which last for about 8 weeks, and polymer-coated varieties, which can last for about 12 weeks. The time estimates may vary depending upon the amount of rainfall. You don't need to apply these fertilizers as often, and they produce more even growth. In addition, burning caused by nitrogen is less of a concern with slow-release fertilizers.
Plant food spikes are a solid form of fertilizer you drive into the soil to dispense nutrients over time. They provide a simple means to feed houseplants, trees and shrubs.
Lawns have specific fertilizer requirements, depending on the season and the type of turfgrass. Read the instructions on the package carefully to make sure it's formulated for your lawn. The packaging also gives you the square footage the product will cover and spreader settings to apply it at the correct rate. See Fertilize Your Lawn for instructions on feeding turfgrass.
Weed and feed is a lawn fertilizer that contains weed killer for broadleaf weeds such as dandelions or grassy weeds such as crabgrass. Look on the label for a list of weeds that the product is effective against to be sure it fits your needs. Applying these products at the proper time is necessary for success. Pre-emergents, such as those commonly used to prevent crabgrass, are weed killers that you apply early in the season, before weeds germinate. They're ineffective if the weeds are already growing. Post-emergents kill actively growing weeds on contact, but don't kill weeds that haven't germinated. If you're also sowing grass seed, check the weed and feed packaging for the proper interval between applying weed and feed and sowing seed — weed control products can prevent germination or kill immature grass seedlings.
There are two other special formulations you may see. Winterizers are fertilizers with high levels of potassium to help cool-season lawns deal with the stress of winter. Starter fertilizers have high levels of phosphorous to help new lawns develop strong roots.