Guaranteed analysis is also known as the N-P-K label on bagged fertilizer products. Nitrogen is the nutrient most needed by plants and is the most important chemical to boost plant growth. Guaranteed analysis chemicals are absorbed by plant roots but do not enhance soil productivity. Nitrogen overload caused by excessive use of nitrogen fertilizers has been identified as the primary cause of global toxic pollution problems, according to the World Resources Institute.
Fertilizer product regulation falls under state jurisdiction but "guaranteed analysis" labeling is industry-standard. According to the Wisconsin Department of Agriculture, Trade and Consumer Protection, "The guaranteed analysis tells the user the guaranteed percentage of the nitrogen, phosphate and potash within the product. The product cannot contain more or less of a listed guarantee."
Nitrogen, phosphorus and potassium are macronutrients needed in the highest levels by plants. Chemical fertilizers identify the percentage of each nutrient on the N-P-K label. A label that reads "5-10-10" has 5 percent nitrogen, 10 percent phosphorus and 10 percent potassium. Many fertilizers contain micronutrients as well, but these are not listed as part of the guaranteed analysis.
Boron, manganese, chlorine, zinc, copper, calcium, sulfur, cobalt, copper, iron, sodium and molybdenum are known as trace elements or micronutrients. Their percentage amounts are most often listed after the guaranteed analysis label. Labeling requirements may differ by state. Florida fertilizer regulations state that "The guaranteed analysis shall specify that secondary plant nutrients are present in elemental form."
Organic Fertilizer Labels
Some manufacturers of organic fertilizers also use the guaranteed analysis label to identify the percentage content of nitrogen, phosphorus and potassium in their products. Nitrogen in organic fertilizer is derived from natural sources such as bat guano and blood meal; phosphorus from fish waste products and bone meal; potassium from seaweed. Backyard compost is rich in the full range of nutrients needed for optimum plant growth.
Organic Materials Review
The USDA National Organic Program has stringent requirements for organic labeling. The fertilizer industry is not regulated by this agency and does not have standards for their organic label. Many organic fertilizer manufacturers rely on the standards set by the Organic Materials Review Institute, which provides lists of products that meet the USDA organic labeling standards.