According to a report from Penn State University, a mix of peat moss, vermiculite, or perlite, and compost or organic fertilizers, can provide a suitable environment with sufficient water-holding capacity, nutrient content, and aeration for plant growth and development. However, because organic nutrients are supplied slowly over time, meeting seedling nutrient needs can be difficult.
Numerous commercial mixes are available for organic growers. Make sure you know what the ingredients are in a commercial mix and check to see if it is listed by the Organic Materials Review Institute (OMRI). It should state “OMRI listed” on the packaging. If not, check the OMRI website to see if it is listed. It is always best to call your certifier to ensure that using the mix will not compromise your certification. Many commercial mixes contain wetting agents to facilitate water absorption by peat moss. Synthetic wetting agents are not allowed under organic production standards.
Depending on the certifier, a producer can also request that the certifier review a specific product/ media. OMRI has not reviewed all products. For example, Pennsylvania Certified Organic (PCO) has an internal materials review and publishes a list of materials that they have reviewed and approved. Members of PCO can request a review of any product free of charge. To prevent delays in your certification, be sure to have material input preapproved by a certifier.
For more information, check out the complete report from Penn State.