Potting mixes should support developing seedlings. Most potting mixes are soilless to avoid soilborne diseases and promote good drainage. 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.
Test Your Mix Before You Use It
If you are unfamiliar with your mix or have received a new batch, perform a greenhouse soil test. Mixes made with compost can be high in salts, which can inhibit germination. To test your mix, send it to a reputable lab for greenhouse testing. Remember, this is different from a soil test. For example, Penn State's Ag Analytical Services Lab has a "Greenhouse Soilless Media" test that will analyze your media's pH, soluble salt (electrical conductivity), and nutrient content. Media sent in as a soil sample is tested differently and results will not make sense.
Premium potting mixes tested with the saturated paste method recommended for greenhouse media will have a pH between 5.5 and 6.5, soluble salts between 1.5 and 3 mmhos/cm, nitrate nitrogen (NO3) levels between 75 and 150 mg/L, phosphorus (P) levels between 5 and 20 mg/L, potassium (K) between 150 and 300 mg/L, calcium between 100 and 200 mg/L, and magnesium between 50 and 100 mg/L, with sodium contents falling below 160 mg/L (Warncke 1995).
Consider pretesting your potting mix by doing your own greenhouse bioassay. To do a bioassay, grow cress, oats, beans, lettuce, or another fast-growing crop with a high germination rate in your soil mix. If there is a problem with the mix, you will see it in reduced germination or poor seedling growth (see sidebar). You may also compare your new mix to a mix that you are satisfied with.
Recently, handheld EC (electrical conductivity) meters have become more popular and available at reasonable prices. See Saline Soils and Plant Growth (Sanchez 2010) for more information on how to test for salts using an EC meter.
How to Avoid the Effects of High Salt Levels or Herbicide Residue Steps
1. Fill a flat with potting mix.
2. Count out 25 seeds of cress, lettuce, or other fast-germinating crop.
3. Seed flat.
4. Wait 5-7 days.
5. Count number of seedlings. If less than the legal germination rate (for lettuce, 80 percent), you may want to test your media for salts.
Making Your Own Mix
Even when making your own potting media, it is still important to ensure that the individual components of the media are specifically approved for certified organic production (see sidebar, next page). If you are purchasing compost to add to your homemade potting mix, most certifiers will require this compost to be reviewed (e.g., PCO requires an ingredient list from the source and a compost log in cases when the raw manure restriction is applicable). Fertility amendments, peat, coir, and other components must also be approved. Check for the OMRI label and talk with your certifier.
When you first start making your own potting mix, it's a good idea to try several different recipes that have worked for other growers and compare how they do on your own farm. A list of common potting mix recipes is provided at the end of this fact sheet.
Many organic potting mixes contain compost, which can provide many benefits. Compost adds organic matter to the mix and supports diverse microbial populations that can suppress soilborne-disease causing organisms (Klein and Hammer 2006). Microbes break down organic material, releasing plant-available nutrients that are slowly available for your seedlings.
Supplemental Fertility
If, after all possible precautions, your transplants are stressed due to nutrient-deficient media, you may need to use supplemental fertilizers such as fish emulsion. Organic sources of supplemental fertilizer include fish emulsion, soluble fish powder, kelp extracts, worm casting or compost tea, or other OMRI-approved products; see Using Organic Nutrient Sources (Sanchez and Richard 2009). These fertilizers can be applied to the soil by fertigation or foliar spray. Be careful with supplemental fertility. If you produce transplants in an area that is later used for in-ground production, leached fish emulsion or other products can build up soil nutrients to levels exceeding crop needs.
