How to read Fertilizer Label
To purchase and apply turfgrass fertilizers intelligently, one must be able to read and understand a fertilizer label. Every fertilizer must be labeled stating guaranteed chemical analysis of the material and, in almost all cases, the following label information:
- the name or brand
- potential acidity
- manufacturer’s name and address
- net weight of the fertilizer in the bag
The guaranteed analysis is sometimes called the fertilizer grade, which is a listing of nutrients contained in the bag by weight. A complete fertilizer contains nitrogen (N), phosphate (P2O5) and potash (K2O), in contrast to an incomplete fertilizer that’s missing one of these elements.
The ratio of fertilizer is the relationship between N, P2O5 and K2O. A fertilizer with a 20-5-15 ratio contains 20% N. 55 P2O5 and 15% K2O by weight. A 50-lb bag of 20-5-15 contains:
0.5 x 20 = 10 lbs N
0.5 x 2.5 = 2.5 lbs P2O5
0.5 x 15 = 75 lbs K2O
Only the nitrogen is expressed as an element, while phosphorus and potassium are present as oxides, meaning the elements are contained in the phosphate and potash compounds, respectively. Phosphate contains 44% phosphorus, and potash contains 83% potassium and expressed in the formulas of their oxide forms, or P2O5 x 0.44 = P and K2O x 0.83 = K.
So, a 50 lb bag of 20-5-15 would have 10 lbs. of nitrogen but only 1.1 lbs. of phosphorus (0.5 x 5 x 0.44 = 1.1 lbs. P) and 6.2 lbs. of potassium (0.5 x 15 x 0.83 = 6.2 lbs. of K).
WIN and WSN, two other terms on a bag of fertilizer, stand for water insoluble nitrogen and water-soluble nitrogen. WSN is quickly available to the turf and provides fast green-up and growth. WIN is slow released by one of several mechanisms, less likely to burn the turf and provides a longer-lastin response than WSN.
Other secondary elements-such as calcium, magnesium and sulfur, and with minute amounts of boron, chlorine, copper, iron an manganese, molybdenum and zinc-are often found in fertilizers and are among the 17 essential nutrients for turfgrass.