Frequently Asked Questions About Fertilizers

By Cindy Haynes
Iowa State University Extension

As the growing season gears up, it is also a great time to start fertilizing plants for a healthy garden. But figuring out how and when to use fertilizers can be confusing and improper fertilization can be harmful to plants. Here are a few commonly asked questions about fertilizers.

What do the numbers on a fertilizer bag represent?  

The numbers indicate the amounts or percentages of nitrogen, phosphate and potash in the fertilizer. These three nutrients are needed in relatively large quantities by most plants. The first number refers to the amount of nitrogen, the second refers to the amount of phosphate, and the third refers to the amount of potash. For example, a 10-6-4 contains 10 percent nitrogen, 6 percent phosphate and 4 percent potash.  

There are so many fertilizers out there, which one is best for my plants?

Deciding what type of fertilizer to use can be a bit confusing. Let the numbers and the labels on the bag be your guide. Fertilizer labels often tell you the type of plant it is best for.  For example, lawn fertilizers generally contain high levels of nitrogen (first number) to promote vegetative or leafy growth. Lawn fertilizers are great for your grass, but lousy for annual flowers (too much nitrogen promotes excessive vegetative growth and inhibits flowering). General purpose fertilizers are often complete (contain nitrogen, phosphate and potash) and are good for a wide range of plants in the garden, landscape or containers. 

What form of fertilizer should I use?

Home gardeners commonly use granular or liquid forms of fertilizer. Granular fertilizers can be water soluble (fast) or slow-release materials. Slow release fertilizers are formulated to release nutrients over several months so one application in spring may be all that is necessary. Liquid fertilizers are fast-acting and can be applied when you water. 

How much and how often should I fertilize my plants?

The rate and frequency of application depends on the nutrient analysis of the fertilizer, plant species, soil type, and other factors. For outdoor or garden plants always start with a soil test to determine the amounts of fertilizer needed. There are some general guidelines on how often to fertilize plants. 

A granular fertilizer is often applied to vegetable gardens at the beginning of the growing season, usually at planting or prior to planting. Annual flowers may require frequent fertilization throughout the growing season, especially if they are growing in containers. Houseplants need regular fertilization in spring, summer, and fall. Many houseplants do not need fertilizer in the winter. Established perennials may need fertilizer once in the spring every other year. Established trees and shrubs rarely need fertilizer. But these are general recommendations and could vary, so watch your plants. Poor or slow growth and overall yellowing are signs that a plant may be lacking essential plant nutrients and would benefit from an application of fertilizer.

Can you fertilize too much?

Yes, over-fertilization can burn plant leaves or stunt growth.  More is rarely better when using fertilizers. Always read and follow label recommendations for fertilizer rates, dilutions, and application guidelines. If you are ever in doubt, always err on the conservative side or use less fertilizer.

My plant looks sick. Should I fertilize it?

First you must figure out why the plant is not doing well. Ask yourself the following questions before pulling out the fertilizer bag: “Has it been over-watered or under-watered? Is it receiving enough light or too little light?  Are there signs of disease or other pests?” Fertilizer doesn’t fix all problems, and in some situations it can make the problem worse.

Why are some fertilizers blue or pink?

Dyes (usually blue) are added to some fertilizers to identify them as a fertilizer and so that you can loosely determine how much you are applying. As you would expect, a dark blue solution contains higher amounts of nutrients than a light blue solution.

Are fertilizers harmful to pets or kids?

Fertilizers, like many other household chemicals, should be kept out of the reach of children and pets.  

Hopefully these questions and answers will help you make good decisions on using fertilizers in your garden or landscape this year.  

Sidebar - Some general tips on fertilizers.

Always read and follow label directions before applying any fertilizer.

Avoid applying fertilizers when the soil is dry. This increases the chances for burning the foliage.

Newly planted trees, shrubs and perennials rarely need fertilizer during the first growing season.  Regular applications of water, to aid in root establishment, are more important.

For container plants in soilless media:

  • Regular fertilizer is important when the plant is growing.

  • If you use water soluble fertilizers, avoid fertilizing every time you water.            

  • Occasionally use water without the addition of fertilizers to help prevent salt buildup.

Contacts :
Cynthia Haynes, Horticulture, (515) 294-4006,
Jean McGuire, Communications and Marketing, (515) 294-7033,