Fertilizer Calculator, Formula, Calculation & Example

Fertilizer Calculator

Is it greener on the opposite side? This calculator will help you calculate how much fertilizer to apply per acre (or 1000 square feet). This tool can be used to keep your crops and lawn green.

Answers to questions such as:

• Why fertilize your lawn?
• What does N-P–K on the fertilizer bag mean?
• How do I choose a fertilizer?
• What are the benefits of a soil test?
• How can I use the lawn fertilizer calculation?
• Considerations for the environment

Why fertilize your lawn

The grass needs certain essential nutrients to grow normally and healthy, just as we humans need a healthy diet. After you have spent so much time sowing grass seed and laying sod , ensuring that your lawn has enough nutrients will allow it to grow stronger, greener, and more resilient to drought and disease.

These are the three most important nutrients that you must pay attention to in order to have healthy grass growth.

• Nitrogen (N)
• Phosphorus
• Potassium (K)

Nitrogen – This is the most important mineral nutrient for plants. A deficiency in nitrogen can cause yellowing of the leaves.

Phosphorus plays an important role in photosynthesis, energy metabolism and structural integrity. A phosphate deficient grass might have slower growth and unusually dark green or purplish-colored leaves.

Potassium has multiple functions related to photosynthesis and respiration. It also maintains osmotic potential within plant tissues. A lack of potassium can cause yellowing and browning of the margins and leaf tips, as well as weaker stems.

Other nutrients are also required by grass, such as calcium or magnit. These nutrients may not be present in sufficient amounts to support the soil. Supplementing nutrients with additional nutrients can be beneficial if they are not. It is possible to add calcium by using lime (which raises pH) and gypsum.

It is important to be familiar with the specific species of grass that you wish to grow. This will allow you provide the best nutrients possible for the species. It may prove profitable for farmers to identify the right grass types and legumes in order to increase the number of cattle per acre .

What does N-P–K on the fertilizer bag mean?

A fertilizer label will list the N-P-K numbers. These numbers tell you how much of the bag’s weight is made up of nitrogen (N), potassium (K20) or phosphate (P205).

If the fertilizer is not phosphate- or potassium-free, the label will indicate the hypothetical phosphate and potassium levels that would be required to provide the same amount.

A bag of kelp fertilizer with NPK label 1-0-2 would have the same amount as a bag containing 2% potash.

How do you choose the right fertilizer type?

Look for fertilizers with slow release nitrogen. Sometimes called ‘controlled-release’,’slowly accessible’, or both. There are many benefits to slow-release nitrogen:

• Slowly, the nitrogen will be available to the grass. This will allow the lawn to stay well-fed longer.
• It is less likely that you will overfertilize or burn the grass, creating brown patches.
• They help reduce water pollution caused by nitrate leakage. This occurs when too many nitrogen are released at once.

Slow-release is a common feature of organic fertilizers. High-quality fertilizers may contain both slow-release and quick-release nitrogen. This gives your lawn an instant boost without causing any burn.

Keep in mind, however, that fertilizers in colder areas, such as organic fertilizers and some pesticides, must be broken down by soil microbes to release nitrogen. These will not provide nutrients as efficiently in cold soils.

For advice regarding the NPK ratio, consult your local lawn professionals, nursery, or university extension. The NPK fertilizer calculator will give you the best results if your fertilization plan is tailored to your soil, grass species, climate, and other factors.

How do I calculate fertilizer application rates per annum?

How much fertilizer do you need per acre if you were recommended a fertilization rate by your local lawn expert?

Although recommendations may be expressed in different units (e.g., pounds per 1000 feet, pounds per an acre or kilograms per an hectare), the calculations are the same regardless.

1. Calculate fertilizer application rate

The formula below can be used to calculate fertilizer application rates:


fertilizer application rates = N desired rate / (%N labeled / 100

If you were to calculate how much fertilizer you needed to achieve a N application rate 43 lbs/acre for a fertilizer that had an NPK ratio 28,0-6, then you could do this:


43 lbs N/acre / (28/100) = 154 lbs fertilizer/acre

2. Calculate the amount of fertilizer that you will need to maintain your lawn.

Then multiply the fertilizer rate and the area of your lawn to determine the amount to apply.


Fertilizer weight = Rate of fertilizer application * Lawn area

To find your lawn area, you can use our area calculation. It is important that your lawn area units match the application rate units. However, you don’t have to worry about conversions if you use our fertilizer calculation!

We would need to have:


1.5 Acres * 154 lbs fertilizer/acre = 231 lbs of fertilizer

You can also use the fertilizer calculator to calculate how many bags you need.

Why should I have a soil test?

Although it might seem simpler to follow the general fertilizer recommendations, a soil test can give you a better picture of how much each nutrient is in your soil. There are four main advantages to this method:

1. Your grass needs should be matched

You can create a fertilization program that meets the nutritional requirements for each grass species by using the soil test.

2. Save Money

Knowing how many nutrients your lawn has will help you save money, and avoid over-fertilization.

3. Avoid problems

Too much fertilizer can lead to faster lawn growth. This means you will have to mow your lawn more frequently. Excess fertilizer can cause grass to become brittle.

4. Monitor pH

You can also test your soil to determine if it is too high or low in pH. A pH of between 6.5 to 7.0 is ideal for lawn grass. Low pH can cause chemical reactions that make it difficult for grass to absorb enough K and P.

Supplements can be used to adjust the soil pH. You can increase the pH with lime or decrease the pH by using granular sulfur.

What is the best time for fertilization?

For seasonal climates that have winters below 55 degrees F (13 degrees C), the best season to fertilize is the fall. Fall fertilization allows grass to grow strong roots so that it can rebound quickly in spring. The best time to fertilize your grass is in fall, according to most experts.

Get the right advice

The best time and rate of lawn fertilization depends on the climate, species and location. Asking for advice from local grass experts such as a nursery or a landscaper will give you more confidence in your plan. Many university extensions provide detailed advice to the public.

Some environmental considerations

Remember that fertilizer can cause damage to aquatic life in local waters if it is introduced to them. How can this happen?

1. Exceed fertilizer from lawns or farms is leaked from the soil and then enters storm drains, streams and rivers.
2. Algae can be found in the water as fertilizer. Large algal blooms can be sometimes seen in satellite images.
3. Some algal species produce toxic substances that can be harmful to wildlife and humans. Algal blooms can also reduce oxygen levels in the area, causing “dead zones”, where fish and other aquatic wildlife are unable to survive.

Here are some ways you can prevent fertilizer pollution:

• To prevent soil leaching, only use what is necessary.
• To ensure that you aren’t adding nutrients to soil, test it.
• To prevent fertilizer from falling into storm drains, sweep up any fertiliser on driveways and sidewalks.
• You might consider spreading fertilizer in several applications throughout the year, but in smaller amounts.
• Organic fertilizers are less likely to leach and can improve soil structure.

A simple example of how to use the lawn fertilizer calculation

How many bags of fertilizer are needed to produce 1 lb nitrogen per 1000 sqft (43.56lbs per an acre) with a 25-5-10 fertilizer. These steps can be followed using the lawn fertilizer calculator.

1. Enter the Lawn Area. Let’s say it is 50 sq. meters.
2. Enter the Fertilizer number numbers with 25 N 5P and 10K.
3. Enter the N Application Rate at 1 lbs/1000 ft.
4. You can see the result: 4 lbs is the fertilizer weight required for a 50-square-meter lawn.

This application will also add 0.2% P2O5 to 1000 sq ft and 0.4% K2O.

Reference