Information Possibly Outdated
The information presented on this page was originally released on March 18, 2002. It may not be outdated, but please search our site for more current information. If you plan to quote or reference this information in a publication, please check with the Extension specialist or author before proceeding.
Numbers indicate fertilizer contents
MISSISSIPPI STATE -- The array of numbers on a fertilizer bag can intimidate gardeners into either applying the fertilizer indiscriminately or deciding to let the plants do without.
In order, the numbers represent the percentage of nitrogen, phosphate and potash contained in the bag. Varying amounts of these three macronutrients are essential to the survival of plants.
Larry Oldham, soil specialist with Mississippi State University's Extension Service, said the numbers make it possible for gardeners to supply the correct amount of these nutrients to the soil.
"Plants require 16 elements to complete their life cycle," Oldham said. "Plants obtain carbon, hydrogen and oxygen through their own processes. Most Mississippi soils supply 10 of the needed elements and gardeners must add nitrogen, phosphate and potash as fertilizer."
Plants use nitrogen to make protein, which is vital to various functions of the plant. This nutrient also gives plants a deep green color. Phosphate is used as the unit of energy exchange within the plant, transferring energy from place to place within living systems. Potash helps maintain the plant's chemical and internal water balances.
Oldham said gardeners should get a soil test done in early spring before beginning work on the garden. When doing a soil test, take samples from several locations within the garden area, and specify what will be planted.
Once the soil has been tested, the results will indicate the pH and nutrient status of the soil. Recommendations for soil additives are made based on what the gardener stated would be planted in the area.
"Follow the soil test recommendations when applying fertilizer," Oldham said.
The fertilizer grade is given as a percentage of 100 pounds, allowing gardeners to determine the quantity of each nutrient in the bag. For example, a 50 pound bag of 8-8-8 contains four pounds of nitrogen, four pounds of phosphate and four pounds of potash. The remaining 38 pounds are carrier material for the nutrients.
"Different grades of fertilizers can be combined for custom blending," Oldham said. "Several materials contain only one of the primary nutrients. This is indicated by a grade such as 34-0-0, which contains nitrogen but neither phosphate nor potash."
With a little math, gardeners can take the results of their soil sample and apply the correct amount of fertilizer to ensure the success of their gardens.