The three numbers you find on all commercial fertilizers make up what is known as the N-P-K ratio. The name comes from the chemical symbols for the elements that the numbers represent. The three numbers, such as 13-13-13, on a fertilizer bag reflect the percentage of nitrogen (N), phosphorus (P) and potassium (K) in the mixture -- always in that order. The proper N-P-K ratio promotes healthy plant growth.
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Plants need nitrogen, phosphorus and potassium in greater quantities than any of the other soil-based plant nutrients. Nitrogen, the first number in the N-P-K ratio, helps plants gain energy and supports strong, leafy green growth. Phosphorus, the second nutrient, helps plants take up nutrients and promotes blooming and fruit development. Potassium, the third nutrient, helps plants absorb other plant nutrients and encourages overall plant health. When all three numbers on a fertilizer are the same, such as 13-13-13, the product is called a balanced fertilizer.
Examining the Numbers
A balanced fertilizer, such as 13-13-13, is also considered a complete fertilizer because it contains all three of the essential primary nutrients. A 100-pound bag of 13-13-13 fertilizer contains 13 pounds of nitrogen, 13 pounds of phosphorus and 13 pounds of potassium in the form of potash. Fertilizer recommendations often call for actual pounds of nitrogen. With a 13-13-13 fertilizer, it would take 7 1/2 pounds of the 13-percent nitrogen fertilizer to deliver 1 pound of actual nitrogen.
Testing Soil First
Before you add a 13-13-13 fertilizer to your garden, test to determine the level of available nutrients in soil. Soil tests determine if soil is lacking any nutrients, which determines the proper fertilizer N-P-K ratio to apply. Although plants require nitrogen, phosphorus and potassium for healthy growth, too much nitrogen can cause problems. It reduces flowers and fruit and may increase susceptibility to disease and insect damage. However, phosphorus and potassium are harder for plants to absorb from soil and rarely cause problems from overfertilization. Your soil may not need all three nutrients your 13-13-13 fertilizer holds.