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Indoor Seedlings—Potting Soil for Sprouting Seeds

How to Pick the Right Soil Mix for Your Seedlings

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Indoor Seedlings—Potting Soil for Sprouting Seeds

Sphagnum moss forms the foundation of most seedling mixes, combined with perlite or vermiculite and lime to adjust pH and supply calcium and magnesium.

Photo © Jon VanZile

Although seedlings might sprout in any number of situations, you can give your seeds a greater chance of success by using a media designed for seedlings. In general, seedlings don’t like “hot” potting soil. In other words, potting mixes that are fortified with fertilizers and water crystals and other additives may be too strong and too slow-draining for many seedlings and reduce the germination rate. Instead, seedlings like a fast-draining, light and sterile mix.

Never use regular garden soil to start seedlings. Besides holding too much water (which can easily kill young seedlings by promoting damping off), there may already be seeds in the soil. You might end up sprouting weeds!

Most commercial seed-starting mixes are a simple combination of sphagnum moss and perlite or vermiculite that is then adjusted with dolomite lime to add calcium and magnesium and adjust the pH. Both Jiffy and Scott’s make seedling mixes that can be used to start most seeds.

If you choose to make your own seedling mix, you can use the following recipe:
  • 4 parts sphagnum moss
  • 4 parts perlite OR an equal mixture of perlite and vermiculite
  • 1/4 Tablespoon dolomite lime per gallon of mix to adjust for pH and add calcium and lime

Alternatively, many people start seeds directly in perlite, vermiculite, crushed clay aggregate, or (for hydroponics applications) rockwool cubes. The most important considerations at this time are drainage (aeration) and support for the new plants.

At this stage, when you’re still working with seedlings, avoid the temptation to add nutrients to the mix. Although some sources recommend adding worm castings or other highly diluted fertilizers to the seedling media, external nutrients are not necessary for the very early stages of seed growth. Nature has seen to it that seedlings will have enough food to sprout and carry them through the emergence of the first rounded leaves, called cotyledons. Feeding should begin with the emergence of the plants first “true leaves,” which will resemble adult leaves of that species.

For the next step, go to Providing Light For Your Seedling

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