How to Calculate Pounds of NPK in Fertilizer

Calculating the amount of fertilizer in your turfgrass or gardening program can be a bit confusing. Just like the human body requires vitamins, minerals, and nutrients, so do plants. Keeping your lawn and garden supplied with the correct amount of nutrients will help it grow stronger and greener, and increase yields. A healthier lawn and garden will also fare better in its disease resistance.

The 3 main macronutrients that are most important for plants, are:

• Nitrogen
• Phosphorus
• Potassium

Nitrogen Function in Plants

Nitrogen is the nutrient that is needed in the largest quantities by plants. Nitrogen is involved in the structure of all amino acids, proteins, and enzymes. Nitrogen is also a component of chlorophyll, where plants get their green coloring. A Nitrogen deficiency in plants will cause yellowing in older leaves, stunted and slower growth, reduced root growth, and yield of fruit is significantly reduced.

Phosphorus Function in Plants

Phosphorus is responsible for many essential plant functions. Phosphorus buffers cell pH, regulates the activity of enzymes that manage respiration in ripening fruit, and the conversion of starch to glucose. Phosphorus also acts as an energy carrier and is involved with the transfer of energy within the cells and plant. Phosphorus deficiency results in stunted growth, and leaves may turn dark green/purple. There is also a limited/reduced production of seeds, flowers, and fruit. In turn, phosphorus toxicity can also reduce growth.

Potassium Function in Plants

Potassium is involved in the formation of proteins, is involved in the activation of enzymes involved with plant metabolism, maintaining plant water movement within the plant, maintaining cell turgor pressure, and controlling the opening and closing of the stomata. Potassium also plays an important role in the upward transport of nutrients including Nitrate, Phosphate, Calcium, and Magnesium. Potassium is also important for the utilization of carbohydrates in the synthesis of cellulose for providing strength and rigidity to plant structures. A Potassium deficiency presents as yellowing older leaves with severe deficiencies showing leaves that look burned across the edges.

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.

How much fertilizer should you apply to your lawn? Our product coverage will vary depending on the product and whether you are applying to a lawn or garden. For lawns, generally our products will have the following coverage rates:

Note: Granular fertilizers may contain higher pounds of N per square foot. This is because these products are slowly released over a period of time. Our liquid fertilizers provide immediately available nutrients and should be applied no more than the recommended amount at one time.

To calculate the size of your lawn, you can measure the property yourself or you can head to this website which is an easy way to calculate the size of your lawn.

To measure your lawn with a tape measure, follow these steps:

1. The first piece of information you need to know is the shape and dimensions of your lawn, particularly the width and length. Measure the width and length of your lawn using a tape measure.
2. Multiply the length and width. (Length x Width = Square Footage). For example, if your yard has a length of 10 feet and width of 10 feet, you would multiply 10 by 10 to get 100 square feet.
3. If your lawn is not a perfect rectangle, you can break up the lawn into sections to measure and then add the measurements together. If you have areas in your yard like flower beds, you can measure the square footage of those areas, and subtract that from your yard’s square footage.

If you have a 20,000 square foot property and a garden bed that is 4 ft by 4 ft, you would multiply 4 x 4 to get 16 square feet for the garden bed, and then subtract that from the total square footage of the lawn.

20,000 square feet - 16 square feet = 19,984 square feet

Now that you understand how to measure the square footage of your lawn, you can effectively calculate the right amount of fertilizer for your lawn. Understanding how to calculate the weight of NPK in lawn fertilizer is important so you can adjust your applications if needed.