Question from our Help Desk: I had a soil analysis done and it recommended that I apply fertilizer. I’m not sure how to read the fertilizer labels to choose the right one. Can you help?
Answer: Bravo! Having a soil analysis done is the best way to determine what nutrients may be missing in your soil and get recommendations as to how much of each nutrient should be applied to healthy plants in your garden or landscape. Here’s a summary of how to read fertilizer labels to help you choose the right one.
Essential Elements for Plant Growth
Plants require 16 elements for plant growth. Of those, nitrogen (N), phosphorous (P), and potassium (K) are macronutrients that are required for maximum growth. These three elements are considered fertilizer macronutrients and the amounts of each are always found on fertilizer labels.
Calcium, magnesium, and sulfur are secondary macronutrients needed for plant growth. These elements are often available in sufficient quantities in the soil, however, they can be found in fertilizers in the form of other materials such as limestone.
There are 10 other micronutrients plants need in small amounts: Chlorine (Cl), iron (Fe), manganese (Mn), boron (B), zinc (Zn), copper (Cu), molybdenum (Mo), sodium (Na), cobalt (Co), silicon (Si), selenium (Se), and nickel (Ni).
It is important to note that there can be too much of a good thing, in other words, over-application of nutrients through fertilizers can be toxic to your plants.
There are several key things to look for on fertilizer labels:
- Fertilizer analysis – refers to the amount of each element based on the percentage of weight.
- Characteristics – refers to different types of fertilizers such as special-purpose fertilizers for certain types of plants (e.g., roses, lawns, vegetables), organic fertilizers, slow-release delivery fertilizers, etc.
- Directions for Use – the instructions for application methods, amounts, etc.
- Cautions – safety cautions, toxicity information, disposal, and cleanup, etc.
- Manufacturer – name and contact information including mailing address and phone number.
All fertilizers are labeled with the percentage by weight of nitrogen (N), phosphorous in the form of phosphate (P2O5), and potassium in the form of potash (K2O) in that order. These are represented on the label as three numbers such as 16 – 4 – 8 which is 16% nitrogen, 4% phosphorous (in the form of phosphate), and 8% potassium (in the form of potash). This is often referred to as the N-P-K ratio or fertilizer grade.
For example, a 40-pound bag of fertilizer labeled 16-4-8 contains 6.4 pounds of nitrogen (N), 1.6 pounds of phosphorous (P) as phosphate (P2O5) and 3.2 pounds of potassium (K) as potash (K2O). The remaining 28.8 pounds is filler to help the fertilizer spread evenly, coatings for slow release, or carriers.
Texas and federal laws require manufacturers to guarantee what is claimed on the label. The guaranteed amounts will appear under “Guaranteed Analysis” on the label. There may also be secondary macronutrients and micronutrients in the fertilizer that may not be listed on the label or are listed elsewhere on the label.
You’ll also hear fertilizers referred to as complete, incomplete, or balanced.
- Complete fertilizers contain all three major macronutrients nitrogen, phosphorous, and potassium.
- Incomplete fertilizers are missing one of the major macronutrients. For example, an N-P-K of 34-0-0 contains 34% nitrogen, and no phosphorous or potassium.
- Balanced fertilizers contain an equal ratio of nitrogen, phosphorous and, potassium. An example is an N-P-K of 20-20-20 contains 20% nitrogen, 20% phosphorous, and, 20% potassium.
- Special-purpose fertilizers are marketed by the manufacturer for use on certain types of plants and will include the plant types on the label.
- The nutrients in organic fertilizers come from once-living organisms or their by-products. Look for organic materials on the label such as meals (blood, bone, cottonseed, cocoa shell), manure and guano, oyster shell, peat, sewage sludge, and wood ashes. Note that organic materials release their nutrients slowly over a long period of time as they break down in moist, warm soil.
- Slow-release fertilizers are made of materials that release nutrients over a period of time. These may include materials that dissolve slowly, resin or sulfur coated granules that dissolve at different rates, or micro-organisms that release nitrogen over time. These fertilizers are typically more expensive, however, they don’t have to be applied as often, minimize the risk of fertilizer burn, and allow the plants to take in the nutrients throughout their growth.
Directions for Use: Fertilizer labels by law must include directions for use such as:
- Timing of application – examples include application before or after rain or irrigation, frequency or number of applications over a period of time, and time of year to apply
- Amount to apply – for example in terms of pounds per 1000 square feet, or ounces per plant, or ounces per tree trunk or tree canopy diameter
- Application method – including broadcasting, banding, using starter solutions, side dressing, drenching, and foliar feeding.
- Broadcasting: Spreading fertilizer at the recommended rate over the growing area
- Banding: Applying narrow bands of fertilizer in furrows next to vegetable seeds or plants
- Starter Solutions: Liquid fertilizers applied to newly planted plants
- Drenching: Liquid fertilizers applied in the plant’s root zone
- Foliar Feeding: Liquid fertilizers applied to the leaves of the plant
Cautions: Fertilizer labels must also include instructions for first aid, storage, use around children and pets, disposal, and clean-up.
Manufacturer Contact Information: Texas and federal laws require fertilizer labels to include the manufacturer’s name and contact information for questions or information about the product. Please carefully read and follow all label instructions including directions for use and cautions before using any chemical products including fertilizers and pesticides.
Sources & Resources
Griffin, Becky, and Waltz, Clint. “Turf Grass Fertility, Understanding Fertilizer Labels, Macronutrients, and Micronutrients.” caes.uga.edu, University of Georgia Extension, 1 Nov. 2015, secure.caes.uga.edu/extension/publications/files/pdf/C%201058-2_1.PDF
Herrman, Tim. “Commercial Fertilizer Rules.” tamu.edu, Texas A&M University System, 2 Nov. 2017, otscweb.tamu.edu/Laws/PDF/CommercialFertilizerRules.pdf
Masabni, Joseph. “Fertilizing a Garden.” tamu.edu, Texas A&M AgriLife Extension, 2 Feb. 2017, agrilifeextension.tamu.edu/library/gardening/fertilizing/