[UNDER REIEW! vCalc MANAGEMENT DOES NOT RECOMMEND THE USE OF THIS CALCULATOR AT THIS TIME.]
The NPK Fertilizer Calculator uses the area of the lawn or garden, NPK (nitrogen, phosphorous, and potassium) percentages, desired application rates, and application instructions to calculate the amount of fertilizer needed.
INSTRUCTIONS: Choose units and enter the following:
The NPK numbers listed for a fertilizer are the percentages of nitrogen (N), phosphate (P2O5), and potash (K2O) of the fertilizer's weight. Most often if the fertilizer does not contain phosphate or potash, the label will indicate the hypothetical weight of phosphate or potash that would provide the same amount of P or K.
Nitrogen (N) is required in the highest amounts by plants. Nitrogen deficiency results in the yellowing of leaves. Phosphorus (P) is important for photosynthesis, energy metabolism, and structure. Phosphorus deficiency may result in slower growth and abnormally dark leaves. Potassium (K) is necessary for photosynthesis, respiration, and maintaining osmotic potential in plant tissues. Potassium deficiency results in yellowing of leaf tips and weaker stems.
To view more information about soil nutrients and how they affect plants, view the Atomic Weight of Soil Nutrients.
1. To find the rate of fertilizer application, use the formula:
fertilizer application rate = Nr / ( N / 100 )
where:
`Nr` = Rate of nitrogen application
`N` = Percent of nitrogen in the fertilizer
Example: One bag of fertilizer has a NPK ratio of 14-0-10. How much fertilizer do you need to reach an application rate of 40 lbs of nitrogen per acre?
fertilizer application rate = 40 pounds per acre / (14 percent / 100) = 285.71 pounds of fertilizer per acre
2. To find the amount of fertilizer for one lawn or garden, use the formula:
amount of fertilizer = Fr ⋅ L
where:
`Fr` = Fertilizer application rate (from previous equation)
`L` = Lawn area
Example: If one lawn is two acres and the fertilization application rate (previous example) is 285.71 pounds per acre, how many pounds of fertilizer is needed?
amount of fertilizer = 2 acres ⋅ 285.71 pounds per acre = 571.43 pounds of fertilizer
3. To find the number of bags of fertilizer for one lawn or garden, use the formula:
number of bags of fertilizer = Fw / Bw
where:
`Fw` = Fertilizer weight (from previous equation)
`Bw` = Bag weight
Example: If 571.43 pounds of fertilizer is needed for one lawn, how many 50 pound bags of fertilizer are needed?
number of bags = 571.43 pounds of fertilizer / 50 pounds in one bag = 11.43 bags of fertilizer
4. To find the amount of phosphate and potash in the total fertilizer amount, use the formula:
phosphate or potash amount = (P or K) ⋅ Fw
where:
`P` or `K` = percent of P or K as indicated by the fertilizer
`Fw` = Fertilizer weight (from equation 2)
Example: If the percent of P equals 5 and the percent of K equals 6, how much of the fertilizer is phosphate or potash?
phosphate amount = (5 / 100) ⋅ 571.43 pounds = 28.57 pounds of phosphate
potash amount = (6 / 100) ⋅ 571.43 pounds = 34.29 pounds of potash