Quick Guide to Calculating the Value of Nutrients in Fertilizers

The value of fertilizer nutrients can represent 25% to 40% of crop production costs. Knowing how to calculate the value of the major plant nutrients – nitrogen (N), phosphate (P ₂O ₅) and potash (K ₂O) – can be very useful.

This is easy to do when we know the cost and nutrient content of the fertilizer. Fertilizer labels give the percentage of nitrogen, phosphate and potash in the fertilizer. For example, the label for diammonium phosphate (DAP) is 18-46-0. This means there are 18 pounds of N, 46 pounds of phosphate and 0 pounds of potash in 100 pounds of DAP fertilizer.

In our examples, we will use spring 2021 prices for urea ($529), DAP ($729) and potassium chloride ($489).

Nitrogen from Urea (46-0-0)

Urea contains 46 pounds of nitrogen in 100 pounds of fertilizer. We calculate the nitrogen content in 1 ton of urea as:

(46 pounds of N ÷ 100 pounds of urea) × 2,000 pounds per ton = 0.46 × 2,000 = 920 pounds of N per ton of urea

Since a ton of urea costs $529, the cost per pound of N is:

($529 per ton of urea) ÷ (920 pounds of N per ton of urea) = $529 ÷ 920 = $0.575 per pound of N

Phosphate from Diammonium Phosphate (18-46-0)

Diammonium phosphate is the most readily available form of phosphate in West Virginia. When nitrogen has no value, we calculate the cost of phosphate from DAP using the same method as we did for nitrogen from urea.

(46 pounds of P ₂O ₅ ÷ 100 pounds of DAP) × 2,000 pounds per ton = 0.46 × 2,000 = 920 pounds of P ₂O ₅ per ton of DAP

When a ton of DAP costs $729, the cost per pound of P ₂O ₅ is:

($729 per ton of DAP) ÷ (920 pounds of P ₂O ₅ per ton of DAP) = 729 ÷ 920 = $0.7924 per pound of P ₂O ₅

However, the N in DAP has value that we should account for. There are 360 pounds of N in a ton of DAP (18 ÷ 100 × 2,000 = 360) worth $0.575 per pound. The value of the N in DAP is:

$0.575 × 360 pounds per ton = $207 worth of N per ton of DAP

We remove, or discount, this from the value of a ton of DAP.

$729 – $207 = $522 per ton of DAP, discounted for value of N

Then, the cost of P ₂O ₅ is:

$522 ÷ 920 pounds of P ₂O ₅ per ton of DAP = 522 ÷ 920 = $0.5674 per pound of P ₂O ₅

Potash from Potassium Chloride (0-0-60)

The value of potash in potassium chloride also is calculated using the same method as we did for nitrogen from urea.

(60 pounds of K ₂O ÷ 100 pounds) × 2,000 pounds per ton = 1,200 pounds of K ₂O per ton of potassium chloride

When potassium chloride costs $489 per ton, the cost per pound of K ₂O is:

($489 per ton of fertilizer) ÷ (1,200 pounds of K ₂O per ton KCl) = $0.4075 per pound of K ₂O

Why It Matters

The value of fertilizer nutrients has practical meaning when we look at the value of the nutrients in a ton of hay. Typical grass hay contains 32 pounds of N, 12 pounds of P ₂O ₅ and 45 pounds of K ₂O. When N costs $0.575, P ₂O ₅ costs $0.5674 and K ₂O costs $0.4075, then the total value of nutrients in a ton of hay is equal to:

(32 × $0.575) + (12 × $0.5674) + (45 × $0.4075) = $18.40 + $6.81 + $18.34 = $43.55 per ton of hay

If hay costs $100 per ton, the $43.55 worth of fertilizer in the hay is almost 44% of its total cost.

Knowing how to calculate the value of fertilizer nutrients enables the manager to know how to meet the fertilizer needs of their crops in the most cost-effective manner when alternative fertilizer sources are available. 

Author: Ed Rayburn, Retired WVU Extension Specialist – Forage Agronomy

Last Reviewed: March 2022

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