Input source verification: Tracing food one step back

Produce growers who are looking to become GAP compliant need to document inputs are free of foodborne contaminants. Know which input suppliers are most critical and what’s needed in implementing input source verification.

The theory behind due diligence in food safety for produce is being able to track the produce one step forward and one step back. In the case of growers, this one step back is all the suppliers you purchase inputs from. Most Good Agricultural Practices (GAP) audits require that you have a letter from these suppliers stating that the supplier took all precautions necessary to limit contamination of the input to mitigate the risk of contamination.

One section of your food safety plan should be devoted to cataloging these documents that are commonly in the form of a letter from the supplier. Listed below are some of the input suppliers from whom documentation may be required. Some or all may apply to your operation.

Potential input source suppliers

  • Seed dealers
  • Transplant dealers
  • Nursery stock dealers
  • Fertilizer suppliers
  • Chemical suppliers
  • Biosolids suppliers
  • Manure suppliers
  • Compost suppliers

If a grower has specific questions about input source verification or has difficulty tailoring GAPs to their farm, they are welcome to contact the Agrifood Safety Work Group at or 517-788-4292. To obtain a sample supplier input source verification letter, ask for guidance document AFSM005-01.

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