How to Calculate How Much Fertilizer You Need
The amount of fertilizer needed depends on the size of the area being treated and the application rate of the fertilizer product. For example, a fertilizer label may give an application rate in pounds per 1,000 square feet. Once you know the total lawn or garden area, that rate can be used to calculate the total amount of fertilizer required.
For irregular lawns or gardens, it can be easier to divide the property into several smaller sections, calculate the area of each section, and add them together. This can provide a more useful estimate than treating the entire property as one large rectangle.
What Do Fertilizer Numbers Mean?
The three numbers commonly printed on fertilizer packages describe the fertilizer's guaranteed analysis. They represent the percentages by weight of nitrogen (N), available phosphate (P₂O₅), and soluble potash (K₂O), in that order.
For example, a 10-10-10 fertilizer contains 10% of each of those three components by weight. A 20-10-5 fertilizer contains 20% nitrogen, 10% available phosphate, and 5% soluble potash. These percentages can be used to calculate how much fertilizer product is required when you already have a nutrient application target from a soil test, product instructions, extension recommendation, or another reliable source.
Buying the Right Number of Fertilizer Bags
Fertilizer is normally sold in fixed bag or package sizes, so the exact calculated quantity may not match the amount you can actually purchase. If a calculation shows that you need more fertilizer than one package contains, you will need enough whole packages to cover the required amount.
Knowing the package size before buying can also help estimate how much fertilizer will remain after the job. Leftover fertilizer should be stored and handled according to the manufacturer's label instructions. Always follow the fertilizer label for application rates, restrictions, safety precautions, and proper use.