In last month's newsletter, we discussed how to interpret soil test results from Dairy One's lab, along with a brief overview of how to calculate fertilizer requirements based on those results. However, two things became clear:

1. There are many different formulas for calculating fertilizer needs, and the right formula depends on the situation.
2. A refresher on how to calculate fertilizer applications would be useful for many of us.

So, this month's article is packed with math (yes, you read that right), to help walk you through the calculations needed to determine the right fertilizer applications for your farm. We'll cover everything from understanding fertilizer labels to calculating how much of each fertilizer to apply based on your soil test results.

Fertilizer products are labeled with three numbers in the format N-P2O5-K2O (or N-P-K). These numbers represent the weight percentages of Nitrogen (N), Phosphorus (P2O5), and Potassium (K2O) in the fertilizer. For example, a fertilizer labeled 15-10-10 contains:

• 15% Nitrogen (N)
• 10% Phosphorus (P2O5)
• 10% Potassium (K2O)

The purpose of this labeling system is to help farmers understand the nutrient composition of the fertilizer so they can calculate how much of each nutrient is present in the product.

Example 1: Calculating Pounds of Nutrients in a Bag

Let's say you have a 50-pound bag of fertilizer labeled 15-10-10. You want to calculate how many pounds of nitrogen (N), phosphorus (P2O5), and potassium (K2O) are in the bag. Here's how to do it:

The general formula to calculate the pounds of nutrient is:

Pounds of Nutrient = Weight of Product ?-- (Percentage of Nutrient/100)

Now, let's break it down:

• Nitrogen (N): 50 lbs ?-- 0.15 = 7.5 lbs of nitrogen
• Phosphorus (P2O5): 50 lbs ?-- 0.10 = 5 lbs of phosphorus
• Potassium (K2O): 50 lbs ?-- 0.10 = 5 lbs of potassium

So, in a 50-pound bag of 15-10-10 fertilizer, there are 7.5 lbs of nitrogen, 5 lbs of phosphorus, and 5 lbs of potassium.

Now that you understand how to calculate the nutrient content in a fertilizer, we can move on to calculating how much fertilizer you need to apply based on your soil test results.

Example 2: Meeting Potassium Needs with Muriate of Potash

Let's say your soil test shows you need to apply 80 pounds of K2O per acre. You're planning to use muriate of potash (0-0-60), which means the product contains 60% potassium (K2O).

To calculate how much muriate of potash is required, you can use the following ratio:

60 lbs K2O                                    80 lbs K2O

____________________        =       ____________________

100 lbs fertilizer                             X lbs fertilizer

In this case, the unknown is X, or the amount of fertilizer you need to apply to meet the potassium requirement. To solve for X, we can cross-multiply and solve the equation:

60?--X=80?--100

60X=8,000

Now, divide both sides of the equation by 60 to solve for X:

X=8,000/60=133.33lbs of muriate of potash per acre

So, you would need to apply 133.33 lbs of muriate of potash per acre to meet the potassium requirement of 80 lbs K2O.

In many cases, you may need to blend different fertilizers to meet multiple nutrient needs for your crops. Let's say your soil test recommends applying 80 lbs of P2O5 and 95 lbs of K2O per acre. You have access to two fertilizers:

• 0-46-0 (for phosphorus, P2O5)
• 0-0-60 (for potassium, K2O)

Here's how we can calculate how much of each fertilizer you need to apply.

Phosphorus (P2O5) Requirement

First, we calculate how much 0-46-0 fertilizer is needed to meet the phosphorus requirement. Using the same formula as before:

46 lbs P2O5                                 80 lbs P2O5

____________________        =       ____________________

100 lbs fertilizer                             X lbs fertilizer

Solving for X:

46X=80?--100

46X=8,000

X=8,000/46=173.9 lbs of 0-46-0 per acre

So, you need 173.9 lbs of 0-46-0 per acre to meet the phosphorus requirement.

Potassium (K2O) Requirement

Next, we calculate how much 0-0-60 fertilizer is needed to meet the potassium requirement. Using the formula:

60 lbs K2O                                    95 lbs K2O

____________________        =       ____________________

100 lbs fertilizer                             X lbs fertilizer

Solving for X:

60X=95?--100

X=95?--100

X=9,500/60=158.3 lbs of 0-0-60 per acre

So, you need 158.3 lbs of 0-0-60 per acre to meet the potassium requirement.

Total Fertilizer Requirements

To meet the needs of both nutrients, the total amount of fertilizer needed for each nutrient is:

• 0-46-0: 173.9 lbs/acre ?-- 20 acres = 3,478 lbs
• 0-0-60: 158.3 lbs/acre ?-- 20 acres = 3,166 lbs

To calculate the final nutrient analysis of the blended fertilizer, we need to find the percentage of phosphorus and potassium in the final mix:

For phosphorus:

80 lbs of P2O5

   _________________  x 100  =24% P2O5

173.9+158.3

For potassium:

95 lbs of K2O

                 __________________  x 100  =28% K2O

173.9+158.3

So, the final analysis of the blended fertilizer will be 0-24-28 (0% nitrogen, 24% phosphorus, and 28% potassium).

Liquid fertilizers are also common, and calculating their application rates is slightly different because they are measured by volume, not weight. Let's look at an example using a liquid fertilizer labeled 10-15-10, with a weight of 12 pounds per gallon and a tank size of 50 gallons.

Example 3: Nutrient Content in a 50-Gallon Tank

To find how many pounds of each nutrient are in the tank, we multiply the concentration of each nutrient by the weight of the product and the number of gallons:

• Nitrogen (N):

10 lbs N?--12 lbs/gal?--50 gal=60 lbs of N in the tank

• Phosphorus (P2O5):

15 lbs P2O5?--12 lbs/gal?--50 gal=90 lbs of P2O5 in the tank

• Potassium (K2O):

10 lbs K2O?--12 lbs/gal?--50 gal=60 lbs of K2O in the tank

Example 4: Gallons Needed to Apply Nitrogen

If you want to apply 50 pounds of nitrogen per acre, you can calculate how many gallons of this liquid fertilizer are needed using the following formula:

                    60 lbs N                            50 gal

__________       =       _____________

50 lbs N                             X gallons

Solving for X:

60X=50?--60

X=2,500/60=41.7 gallons per acre

Thus, to apply 50 lbs of nitrogen per acre, you would need to apply 41.7 gallons of fertilizer per acre.

By understanding how to read fertilizer labels and perform these calculations, you can ensure that your crops get the right amount of nutrients without over- or under-applying. Whether you're using granular or liquid fertilizers, knowing how to calculate nutrient requirements and fertilizer application rates is essential for optimal crop growth and cost-effective farming.