A good seed starting mix is different than potting soil because it is especially composed to be a neutral medium in which seeds can germinate. Whether commercially prepared or homemade, the best seed starting mix should be sifted so that the particles are very fine, allowing excellent drainage. It should be sterile and free of fertilizers and other contaminants.
While potting soils are often composed of fertilizers and enriched soils, these mixtures can be too strong for germinating seeds and young seedlings. Potting soil can also contain material that might be contaminated with plant diseases or fertilizers that can feed the diseases. This might not affect strong, established plants but can be deadly for new seedlings. Most seed starting mixes are sterilized and do not contain any organic materials that could harbor disease. Choose a mix that is designed especially for starting seeds, not for feeding or transplanting bedding plants.
Consider the texture and weight of the medium's particles. A good seed starting mix will be very fine and preferably pre-sifted to eliminate clumps. A finely textured medium allows immediate drainage after watering, as an oversaturated planting medium could be a breeding ground for disease, promote rot, and drown the roots by cutting off air circulation. The tiny roots of a new plant are very weak and fragile, and a fine soil makes it easier for the roots to spread out and grow stronger.
Fertilizer is also a component of some potting soils and other soil mixes that should not be present in a high-quality seed starting mix. A seed contains all of the nutrients it needs to germinate and initiate the growing cycle, and any additional nutrients in the planting medium can actually be detrimental. In fact, certain fertilizers can burn young roots or cause an imbalance that can severely damage the plant or even kill it. As plants become more established and strong, they will require light feedings, but it is important to choose a seed starting mix that is free of these nutrient additives.
Another factor to consider is sourcing. A good mix is sterile with no nutrients, so it does not matter whether the mix is organic. It will not contain any live components. It can be difficult to sterilize and sift homemade seed starting preparations, but this may be more cost effective if large quantities of planting media are necessary. Additionally, environmentalists recommend using coir or vermiculite instead of peat moss, as harvesting moss can devastate peat bogs which are the natural habitats of many types of wildlife.