Understanding the numbers on a bag of fertilizer helps you apply the right amount and ratio of fertilizer your yard and garden need, says LSU AgCenter horticulturist Dr. Tom Koske. Fertilizers come in different strengths and blends, with the three numbers on the bags showing the percentage by weight of the three major nutrients.
Koske explains the first number of the left-to-right sequence always is the percentage of nitrogen (N). The second is the percentage of phosphorus (P) as expressed in phosphate, which Koske notes is not pure phosphorus. The third number is the percentage of potassium (K) as expressed in the oxide called K20 equivalent.
"Recommended amounts of nitrogen, phosphorus and potassium are almost always given as these equivalents rather than the true elements," Koske says, adding, "The higher the number, the stronger that nutrient is in the fertilizer, so you can apply more of a weaker fertilizer to get the amount of nutrient needed or less of a stronger fertilizer." For example, about 7 pounds of cow manure can substitute for 1 pound of 8-8-8. Just look at the analysis on the bag.
Blended fertilizers have more than one nutrient, such as 0-20-20 or 8-24-24. A complete fertilizer is one that has some of all three nutrients, like 8-24-24, the horticulturist explains.
Muriate of potash is 0-0-60, and triple super phosphate is 0-46-0. Some nitrogen sources are ammonium nitrate 33-0-0, ammonium sulfate 20-0-0 or urea 46-0-0.
Other fertilizer materials are potassium sulfate (0-0-50), DAP (18-46-0), IBDU (31-0-0), SCU (32-0-0), UF (38-0-0), bone meal (2-20-0) or cottonseed meal (6-3-2). These materials are considered slow- or controlled-release fertilizers.
Dividing 100 by the percentage of the nutrient in the fertilizer gives the pounds of the fertilizer needed to supply 1 pound of that nutrient. For example, with a bag of 8-8-8 fertilizer Koske says that if you divide 100 by 8 you’ll get 12.5, which is the number of pounds that will supply a pound each of nitrogen, phosphate and potassium oxide.
The nutrient numbers on the bag also are ratios to one another, Koske adds, noting that 8-8-8 is a 1-1-1 ratio, while 5-10-15 is a 1-2-3 ratio. He stresses that different crops and soils may need different ratios.
A vegetable garden of average fertility may use a 1-1-1 ratio, a 1-2-1 ratio or a 1-3-3 ratio for sweet potatoes, peas and beans. Lawns generally like a 3-1-2 ratio, but centipede grass fertilizers are often a 1-0-1 ratio.
The best way to determine the nutrient needs of your grass, garden or crop, he advises, is to have the soil scientifically tested. Your county agent can tell you how to take a representative sample to send to the AgCenter’s Soil Testing Lab. Test results and fertilizer recommendations matched to the crop to be grown will be returned to the grower.
Related yard and garden topics are available by contacting an extension agent in your parish LSU AgCenter office. Also, look for Lawn & Garden and Get It Growing information on the LSU AgCenter Web site.