The label on all fertilizer bags is required to show the percentage by weight of nitrogen, available phosphate and soluble potash. This is called the guaranteed anaylsis of the fertilizer.
- The first number is Nitrogen (N), which promotes overall grass shoot growth.
- The second number is available Phosphate (P₂O₅), which promotes strong root growth.
- The third number is soluble Postash (K₂O), which helps grass withstand stress, drought or disease.
For example, a 24-2-8 fertilizer has 24% nitrogen, 2% available phosphate and 8% soluble potash. Nitrogen, phosphate and potash are also sometimes referred to as N-P-K.
To understand how much of each nutrient is being applied to your lawn, you must multiply the weight of the fertilizer bag by the percentage of each nutrient. For example, a 30 lb. bag of fertilizer rated 24-2-8 contains:
N: 24% x 30 lbs = 7.2 lbs. Nitrogen
P: 2% x 30 lbs = 0.6 lbs. Available Phosphate
K: 8% x 30 lbs = 2.4 lbs. Soluble Potash