Fertilizer Cost per Nutrient Unit (N, P, K)

When comparing fertilizer products, the price per bag or per ton can be misleading. A more meaningful comparison is the cost per unit of actual nutrient—how much you pay for each pound or kilogram of nitrogen (N), phosphate (P₂O₅), or potash (K₂O). This tool helps you make that comparison.

How Fertilizer Analysis Works

Fertilizer bags display a three-number analysis like 46-0-0 (urea) or 18-46-0 (DAP). These numbers represent the guaranteed minimum percentages by weight of:

• N (Nitrogen): The first number—elemental nitrogen content
• P₂O₅ (Phosphate): The second number—phosphorus expressed as phosphorus pentoxide
• K₂O (Potash): The third number—potassium expressed as potassium oxide

Why P₂O₅ and K₂O Instead of P and K?

By convention, fertilizer phosphorus is reported as P₂O₅ and potassium as K₂O, even though plants take up elemental P and K. This dates back to historical analytical methods. To convert:

• P₂O₅ × 0.44 = elemental P
• K₂O × 0.83 = elemental K

This tool uses the oxide forms as labeled on fertilizer bags, which is the standard for comparing products and matching soil test recommendations.

The Cost per Nutrient Unit Calculation

The formula is straightforward:

For example, if urea (46-0-0) costs $500 per short ton:

• N in one ton = 46% × 2000 lb = 920 lb of N
• Cost per lb N = $500 ÷ 920 = $0.54 per lb N

Why This Comparison Matters

Different fertilizer products contain different nutrient concentrations. A bag of urea (46-0-0) contains more nitrogen per pound than ammonium nitrate (34-0-0). By comparing cost per unit of nutrient, you can identify which product provides the most economical source of each nutrient you need.

Important Limitations

Cost per nutrient unit is just one factor in fertilizer decisions. Consider:

• Nutrient availability: Not all forms are equally available to plants. Ammonium vs. nitrate nitrogen, for example, behave differently in soil.
• Application costs: Liquid, dry, and anhydrous forms have different equipment and handling requirements.
• Soil conditions: pH, organic matter, and soil type affect which nutrient forms work best.
• Timing: Some products are better for pre-plant vs. side-dress applications.
• Secondary nutrients: Some products provide sulfur, calcium, or micronutrients not captured in N-P-K analysis.

Common Fertilizer Products

Using This Tool Effectively

1. Enter the products you're considering, including their N-P₂O₅-K₂O analysis and current price per ton (or other unit).
2. Compare the cost per unit of each nutrient you need.
3. Use this information as one input into your decision, alongside soil test recommendations, agronomic advice, and practical considerations.
4. Remember that the cheapest source of a nutrient is not always the best choice for your specific situation.