Before you purchase a fertilizer, read and understand the label to choose the right one. While commercially available fertilizers vary in the amount of nutrients contained in the bag and the content displayed on their products, state laws and agencies that control fertilizers require that the manufacturer guarantees the claims of nutrient content on the label. This is called the guaranteed analysis. South Carolina law requires that the manufacturer guarantees the claims on the label. Although not all fertilizer labels look alike, every label, such as the sample below, must contain the following information:
(1) All fertilizers are labeled with three numbers that indicate the guaranteed analysis or the fertilizer grade. The three numbers, 18-4-10, give the percentage by weight of nitrogen (N), phosphorus (P) expressed as phosphate (P2O5), and potassium (K) in the form of potash (K2O). Often, to simplify matters, these numbers are said to represent nitrogen (N), phosphorus (P) and potassium (K) or N-P-K, and are often calculated based on the amount that’s immediately water-soluble or available to plants.
For example, if the net weight of this 18-4-10 fertilizer was 100 lbs., then there are 18 pounds of nitrogen (N), 4 pounds of phosphate (P2O5), and 10 pounds of potash (K2O). The remaining weight (the total must add up to 100% or 100 lbs.) is comprised of an inert material or nutrient carrier that aids in the application of the nutrients.
This 18-4-10 fertilizer is a complete fertilizer because it contains each of the major plant nutrients: nitrogen, phosphorus and potassium. A balanced fertilizer contains equal amounts of nitrogen, phosphate (P2O5), and potash, such as a 10-10-10 fertilizer. Avoid the practice of applying a fertilizer that contains all three macronutrients when adequate levels of phosphorus and potassium are present and available to plants, particularly in clay soils and less so in sandy soils. When phosphorus and potassium are available in sufficient amounts, adding more may have no benefit or even be taken up by the plant, which wastes time and money. Besides, plants do not use these 3 macronutrients in equal quantities; nitrogen and potassium are required in higher amounts than phosphorus. Soilless mixes are an exception because these fast-draining container mixes with no mineral soil will benefit from the addition of all 3 nutrients, but not necessarily from a balanced fertilizer.
When a soil test report recommends only 1 or 2 macronutrients but not all three, select an incomplete fertilizer. For example, if your soil test results indicate sufficient levels of phosphorus, an incomplete fertilizer, such as 24-0-11, may be recommended.
These three numbers also represent a ratio that describes the relative proportions of N, P2O5, and K2O in a fertilizer. Calculate the fertilizer ratio by dividing the numbers in the fertilizer grade by the lowest number in the fertilizer grade. For example, the ratio of this 18-4-10 fertilizer is 4.5:1:2.5 or 4.5 parts nitrogen to 1 part phosphate to 4.5 parts potash. This ratio is important because soil test recommendations may include the ratios of suitable fertilizers ratios that satisfy the nutritional requirements of your plants.
(2) The guaranteed analysis is warranted by the manufacturer that the analysis by weight of each element is present in the product. The percentages by weight of the macronutrients listed in the fertilizer analysis.
(3) The total amount of nitrogen in the bag expressed in percent.
(4) The sources of nitrogen and their percentages help you determine the amounts of fast- and slow-release nitrogen in the bag. By law, a slow-release fertilizer must contain at least one-third or 33 percent of the total amount of nitrogen in a slow-release form. To maintain an extended supply of nutrients that can be absorbed by plants, consider a fertilizer that contains 50% or more of a slow-release nitrogen source.
To determine if this fertilizer can be considered fast- or slow-release, compare the percentage of slow-release, water insoluble nitrogen (WIN) [(sometimes listed as slowly available nitrogen (SAN)] with the total percentage of N in the fertilizer. In this example, the label states that 10% of the nitrogen is water-insoluble compared to the total amount of nitrogen—18%—in the bag. Since more than half of the nitrogen in the bag is slowly available and does not readily dissolve in water, it is a slow-release fertilizer. The remaining nitrogen in the bag is fast-release nitrogen.
(5) The percentages of secondary macronutrients by weight of the total amount of fertilizer contained in the package. Calcium, magnesium and sulfur are required in fairly large quantities; however, applying these nutrients may not be necessary because the calcium and magnesium contents of most SC soils are often sufficient for plant growth. Many soils along the coast are often very high in calcium because of the pulverized seashell material in the soil. Also, large quantities of calcium and magnesium are supplied when acidic soil is limed with dolomitic limestone. Sulfur is usually present in sufficient amounts from the slow decomposition of organic matter, which is an important reason for recycling grass clippings and leaves in the garden and landscape.
(6) The percentages of micronutrients by weight of the total amount of fertilizer contained in the package. Micronutrients present in some fertilizers may not be necessary, especially if the soil is not deficient in these elements. Therefore, plants will not respond to these added micronutrients. Sandy soils may have low levels of micronutrients, which can be determined by a soil test. Follow soil test results when applying micronutrients, because excessive amounts can damage or kill plants. A natural way to supply micronutrients to your plants is with organic matter that can be applied to the soil surface as mulch, such as fallen leaves, needles, or wood chips, or directly mixed into the soil. See HGIC 1604, Mulch, for more information.
(7) The elements in the fertilizer are derived from these various sources.
Originally published 05/22