In wholesale fertilizer purchasing, the document that separates a professional buyer from a lucky one is the Certificate of Analysis (COA). It is the only objective record that the product you are paying for actually contains what the label promises. Here is a practical checklist for reading one before you commit to a bulk order.
Why the COA matters in B2B fertilizer buying
A fertilizer label states guaranteed nutrient levels, but the label is not proof. The COA is the lab-measured result of a specific batch. If a supplier cannot produce a COA for the batch you are ordering, you are buying on faith — and in agriculture, faith does not feed plants.
Six things to check on every COA
- Measured vs declared nutrients. Compare the measured N-P-K (and secondary/trace elements) against the label guarantee. Small deviations are normal; large gaps or values that conveniently match the label exactly every time deserve suspicion.
- Heavy metals and contaminants. Check that lead, cadmium, arsenic, and mercury are tested and within the limits of your target market’s regulations. This matters especially for food crops and for markets with strict import control.
- pH and EC of the solution. These determine compatibility with your fertigation and hydroponic systems. A product that shifts pH heavily or spikes EC may not suit your program, no matter how high its NPK.
- Solubility and residue. For drip irrigation and hydroponics, low residue and fast dissolution are critical. The COA or the attached technical sheet should show solubility and insoluble matter.
- Batch number and production date. Traceability is the difference between a controlled supply chain and a lottery. The batch number should match the bags you receive.
- Test method and laboratory. Note whether the test was done by an independent lab or in-house, and which standard was used. Independent results carry more weight in a dispute.
Red flags when reviewing a COA
- The same COA is reused across multiple batches (check dates and batch numbers).
- No batch number, no production date, or no test method stated.
- Results are always exactly at the label guarantee — real production has natural variation.
What to ask your supplier
- Request the COA for the specific batch before payment.
- Ask if retained samples are kept for independent third-party testing.
- Confirm the testing standard matches your import market’s requirements.
At Trelfey, batch-level COAs are available on request across our water-soluble NPK, hydroponic nutrient, amino acid, and humic acid series. If you are comparing suppliers, ask for their COA first — it tells you more than the price list.