DALY: What do the numbers on the fertilizer bag mean?

Fertilizers are frequently applied to plant material as a supplement because the soil does not supply enough of the needed nutrients. Many types of fertilizers exist and not all are created equal. By knowing some basics about fertilizers and the various types, you can choose one that will be the most beneficial.

Prior to applying fertilizer, consider having your soil tested through Gwinnett County Extension. The results will show the nutrient status and the relative acidity of the soil (pH) through soil testing. Applying fertilizer without a soil test can result in the application of too much or too little lime and fertilizer required for optimum growth. Refer to the article that I wrote on February 5, 2014 titled “Soil testing essential to the quality of your soil” for more information on soil testing.

Plants require several nutrients for sustenance. Nitrogen, phosphorus and potassium are the ones they need in the highest quantity whereas sulfur, magnesium and calcium are required in lesser amounts. Some nutrients, such as zinc, iron and manganese, are only required in small quantities.

The numbers on the fertilizer bags are labeled with three numbers that indicate the guaranteed analysis, or the fertilizer grade. These three numbers give the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K), commonly referred to as the N-P-K ratio. For example, if a 100-pound bag of fertilizer is 20-10-15, it has 20 pounds of nitrogen, 10 pounds of phosphorus and 15 pounds of potassium for a total of 45 pounds of nutrients and 55 pounds of filler material. Other nutrients are sometimes listed on the label. The law requires the fertilizer manufacturer to guarantee that the N-P-K ratio of fertilizer on the label is correct meaning the nutrients listed on the label are contained in the fertilizer. If you have your soil tested, the report will recommend the type of fertilizers and amounts that should be applied.

Various formulations of fertilizers are used. Complete fertilizers contain all three of the major nutrients. Incomplete fertilizers, such as 0-20-20, do not contain all three of these nutrients. Slow release fertilizers include a coating of materials that allow the nutrients to be slowly made available to the plants over a period of time. Water, heat and microbes break the material down. Some fertilizers are combined with pesticides. However, timing for a fertilizer application and treatment for the pest may not coincide. For example, some fertilizers are formulated with a pre-emergent herbicide. In the spring, the best time for use of a pre-emergent is in March. The fertilizer should not put down on warm season grasses at that time since it could cause them to come out of dormancy prematurely and increase the risk of cold damage from a late season freeze.

Many gardeners question whether the use of organic fertilizers is advantageous over synthetic fertilizers. Bone meal, cottonseed meal, blood meal, manure, compost and other sources are considered organic fertilizers. The advantage to using them is increasing the organic matter content of the soil and improving its physical structure. They also are less likely to burn the plant material. However, the nutrients in organic fertilizer are not readily plant available. They have to be broken down by soil microbes to be released into the soil. With synthetic fertilizer, the nutrients are immediately released. Also, the nutrient levels from organic sources are relatively low. Even if you use a synthetic fertilizer, consider incorporating organic matter into the soil to improve it.

Remember, not all fertilizers are alike. Fertilizer choice is dependent on the type of plant material that you are growing and the type of soil where it will be grown. Knowing your soil fertility needs from soil testing and understanding the different types of fertilizers will help in finding the right one for your particular situation.

Timothy Daly is the Agricultural and Natural Resources Extension Agent with Gwinnett County Extension. He can be contacted at 678-377-4010 or tdaly@uga.edu.