The first thing that you need to know when calculating the weight of Nitrogen, Phosphorus or Potassium in your fertilizer is to figure out the percentage rates of N-P-K.
The rates are expressed as three numbers separated by dashes. They will be on the packaging of your fertilizer and look like this: 16-4-8.
We will use our 1 Gallon Advanced 16-4-8 Lawn Food for this example.
The first number represents Nitrogen, the middle number Phosphorus, and the third number all the way to the right is Potassium. This means that 16% of the product is made up of Nitrogen, 4% of Phosphorus, and 8% Potassium.
First, you must figure out your product’s weight. This number is on the packaging represented in pounds (lbs) or kilograms (kgs). For this example, we will take the product weight per gallon and multiply it by the percentage of our nutrient (16% Nitrogen). A gallon of this product weighs approximately 10.32 pounds.
Total weight of product x percent of Nitrogen = Pounds of N in 1 gallon
10.32 x 0.16 = 1.65
Now you must determine what is the amount of N per ounce. Since there are 128 ounces in 1 gallon, we will divide the weight of N by 128 ounces. This will help you determine how much product to water ratio you need in your backpack sprayer.
Weight of N / Total Ounces of Product = Amount of N per 1 ounce
1.65 / 128 = 0.0129
So how much Nitrogen will be applied per 1,000 square feet? Since it is recommended to apply our Advanced 16-4-8 at 10 ounces per 1,000 square feet, we can now determine how much N will be applied per 1,000 square feet. You can even adjust your rates up and down based on the amount of N needed.
Ounces per 1,000 square feet x Nitrogen per 1 ounce = Total weight of N per 1,000 square feet
10 x 0.0129 = .129 pounds of Nitrogen per 1,000 square feet
That means at a rate of 10 ounces per 1,000 square feet we are applying 0.129 pounds of Nitrogen per 1,000 square feet.