Potting media for growing transplants is usually a mix of peat moss, vermiculite or perlite and compost or organic fertilizers that can provide a suitable environment with sufficient water holding capacity, nutrient content and aeration for plant growth and development. For organic growers, like conventional growers, peat (or coir), vermiculite/pearlite provide water-holding and aeration. But, because organic nutrients are supplied slowly over time it can be difficult to meet seedling nutrient needs.
Recently, we informally compared a number of organic potting mixes at the Seed Farm, New Farmer Training site in Emmaus, PA. It was interesting to see that with no additional fertilization from fish emulsion or other foliar/ drench some potting mixes did not have enough nitrogen to support healthy transplants (see photo below).
This is not surprising based on recent research from Cornell's Anu Rangaragan which showed that compost based organic potting mixes performed best when a small amount of blood meal, peanut meal or other highly available organic nutrient source was added. This is because the nutrients in compost are slowly available as the good microbes in the soil break them down. Early in the season when it may be cool in the greenhouse the microbes are less active and release less nitrogen for your plants. Also the compost alone often does not contain enough nitrogen for transplants depending on how it was made.
If you plan to make your own organic potting media or purchase a commercial media consider the following hints, especially if the mix is compost based:
- Compost can be a huge source of weed seed - try watering some of your compost (or potting mix) in a warm place to see if a carpet of weeds comes up. You don't want to have to weed your transplants!
- Compost can be a source of disease - ask your supplier if they pasteurize the compost they use. Pasteurization uses steam to heat the material hot enough to kill pathogens and weed seed but not sterilize the soil. Sterilization would kill the good organisms too. If you do not have a way to pasteurize compost you plan to use, know there is a much higher risk for damping off.
- Compost, mushroom soil and other potting mix components can be high in salts. Salts will starve transplants for water. If you are working with a new potting mix, whether commercial or farm-made test it for salts. You can send it to a lab or plant a tray of susceptible plants to see how they react. See direction in the "Soil Media and Plant Propagation".
- Organic potting mixes have variable amounts of nutrients. To determine the nutrient content of your potting mix you can send a sample to the lab. Note this is not a soil test! This is a Greenhouse (soil-less) media test. For example see Penn State's Greenhouse Media Test. As a backup it is often a good idea to have fish emulsion or another organic product that can be foliar applied in your transplants look stressed on hand. Talk to other growers to see which mixes have worked for them. Multiple recipes are listed in the article "Soil Media and Plant Propagation".
- Make sure any ingredients you use to mix your own organic potting mix are OMRI listed. The Organic Materials Review Institute (OMRI) reviews all materials certified for organic production. You may be surprised to know that many peat products have wetting agents added to help them absorb water that are not allowed for organic production. Look for OMRI on the label and ask your certifier.
Compared to even a few years ago there are many commercial sources for organic potting mix. Vermont Compost, McEnroe, and Fertrell are among the common suppliers in our area.
For more information on potting mixes and transplant production see the following resources.