A lower price per bag does not establish a cheaper substitute for a greenhouse fertilizer. First put both products on the same nutrient and mass basis. Then calculate the quantity needed to match one explicitly chosen contribution and show everything else that quantity supplies.
The result is a comparison for the nutrition adviser. It is not permission to substitute a product, change a feed target or prepare a stock solution. Correct arithmetic can expose a difference that makes the cheaper offer unsuitable.
Copy the guaranteed analysis before comparing prices
Use the current manufacturer label for the exact product, together with a dated delivered quotation. Record package net mass, currency, delivery charges and the analysis basis. Keep nitrogen forms, secondary nutrients and micronutrients visible, even if the first comparison focuses on one major nutrient.
Virginia Cooperative Extension’s greenhouse fertilizer guide explains that the familiar three-number analysis expresses nitrogen as N, phosphorus as P2O5 and potassium as K2O on a weight basis under that labeling convention. Do not assume every international label uses the same convention; copy what the actual label states.
P2O5 and K2O are reporting bases. They should not be entered as elemental P and K without conversion. Likewise, a figure described as total nutrient content should not silently become a claim about a particular soluble fraction or chemical form.
For a liquid, resolve whether the analysis is weight per weight or weight per volume. Establish the package’s usable mass basis, using verified density information when a volume-to-mass conversion is required. If the basis is unclear, stop the comparison and ask the supplier. A liquid liter is not automatically a kilogram.
Normalize P and K once, with the conversion visible
The Virginia Tech fertilizer conversion table gives the following factors:
- Elemental P percentage = P2O5 percentage × 0.4365
- Elemental K percentage = K2O percentage × 0.8301
If the label already reports elemental P or K, retain that value rather than converting it again. Keep the original label analysis in one field and the normalized value in another, with the source of the conversion recorded.
For a confirmed weight-based analysis, nutrient mass in a package equals net product mass multiplied by the nutrient percentage divided by 100. Use matching units. A verified zero and an unspecified nutrient are different entries; do not turn “not stated” into zero.
Match one contribution and expose the other differences
Illustrative arithmetic only: consider anonymous weight-based analyses A, 20-10-20, and B, 15-5-30, using the N–P2O5–K2O convention. To account for 1 kg of N, the arithmetic requires 5 kg of A or about 6.667 kg of B. These are comparison quantities, not application rates.
| Contribution | A at 5 kg product | B at about 6.667 kg product |
|---|---|---|
| N | 1.000 kg | 1.000 kg |
| Elemental P | About 0.218 kg | About 0.146 kg |
| Elemental K | About 0.830 kg | About 1.660 kg |
On this matched-N basis, B requires one-third more product: its comparison quantity is 4/3 of A’s. Its delivered price per kg must therefore be below 75% of A’s to reduce the allocated product cost. At exactly 75%, the allocated product costs are equal. This threshold follows from the illustrative analyses; it is not a quoted fertilizer price or a substitution recommendation.
The matched N does not make the products equivalent. The adviser still needs to assess the different P and K contributions, nitrogen forms and all other relevant ingredients. Do not “fix” the difference by adding another product in this purchasing exercise.
The full cost of each comparison quantity is product mass multiplied by its verified delivered cost per unit mass. If purchasing requires whole packages, show the cash order quantity and unused balance separately from the allocated cost of the quantity being compared.
New Mexico State University’s fertilizer-cost guide explicitly addresses the complications of products containing several nutrients. Avoid adding N, P2O5 and K2O as interchangeable units or claiming several independent savings by allocating the same purchase cost repeatedly.
Use the contribution calculator and equivalence card
Download the two-product contribution calculator and the product-equivalence review card. Import the calculator into a spreadsheet with formula evaluation enabled. Do not sort or move its rows, or overwrite formula cells. The calculation requires a confirmed weight-based analysis and net product mass.
Enter a 20% analysis as 20, not 0.20, in the declared-nutrient fields. Fill the blank Product A and Product B input cells for identity, label, quote, net mass and analysis; enter the shared currency, chosen matching nutrient and matching quantity in Product A only. Product B’s shared fields link automatically. Normalized P/K, package nutrient masses, comparison quantities, allocated costs and accompanying nutrients are calculated outputs. Keep the fixed conversion factors unchanged and complete the remaining evidence fields separately.
Choose the same matching nutrient and contribution for both products. The calculator shows normalized P and K, nutrient mass per package, comparison quantity, allocated product cost and accompanying N, P and K. Use the card for the evidence a simple calculation cannot decide.
| Question | Record for both products |
|---|---|
| Are the labels comparable? | Exact product, revision, guaranteed analysis, element or oxide basis, and mass basis |
| Are costs comparable? | Delivered quote, date, currency, package mass, order constraints and excluded charges |
| What contribution is matched? | Chosen nutrient, accounting quantity and resulting product quantity |
| What else changes? | All accompanying nutrients, nitrogen forms, micronutrients and relevant formulation information |
| Can it be used in the intended system? | Supplier restrictions, solubility information and adviser compatibility review |
| What is the outcome? | Comparable for adviser review, materially different, or evidence missing |
“Comparable for adviser review” means the evidence is organized well enough to assess. It does not mean the two products have been approved as interchangeable. Keep the decision-maker and supporting response attached to the record.
Know when the price comparison must stop
Hold the calculation when the liquid basis is unresolved, a label is missing, the required nutrient is absent, or costs use different currencies without a documented conversion. Return an unavailable result when the selected nutrient fraction is zero; do not divide by zero or substitute a guessed value.
Mark a material difference when the matched contribution brings an unapproved change elsewhere in the formulation. Compatibility remains a separate gate, as discussed in the fertilizer stock compatibility guide. A correct nutrient total cannot prove that products behave identically in storage, preparation or delivery.
Only an adviser-approved comparison using real offers and complete labels can support a purchasing conclusion. Keep the original inputs and arithmetic with that decision. The value of the exercise is knowing what the lower price buys, including the differences that a headline analysis can hide.
