“Soil” for seed-starting (and beyond)

Top: Potting Mix
Bottom: Germination Mixes
from different companies

“Soil” has three main functions: 1) provide physical support for the plants, 2) supply roots with nutrients, air, and water, and 3) allow maximum root growth. There are different formulations of this medium that are designed for starting seeds, potting up seedlings, and transplanting plants. Selecting the right mix (at the right time) is essential for raising healthy transplants.  

The best growth medium used for starting seeds is not actually soil. It is usually a blend of sphagnum peat, vermiculite, perlite, and lime, but may also include coir, sand, and other ingredients. This soil-less seed-starting mix (also called “germination mix”) can be purchased from any garden center or online retailer. You can also prepare your own germination mix at home. A simple recipe is 1 part Sphagnum peat moss, 1 part vermiculite, and 1 part perlite.

The purpose of the most commonly used components in germination mixes are described below:

• Sphagnum peat and coir are organic materials that provide seedling with a fine, yet sturdy medium for encouraging strong root growth
• Vermiculite is a lightweight and porous natural mineral that helps to retain moisture and aerate the mixture
• Perlite is a silica-based volcanic rock that helps to improve drainage and aerate the mixture
• Ground limestone (lime) is used to adjust the pH of the mixture

Germination mixes are not suitable for transplanting seedlings into larger pots (potting up) because they do not provide sufficient nutrients for the seedlings. Instead, it is better to first transplant seedlings into a potting mix that contains a nutrient-rich organic material, such as compost. 

Not only does potting mix provide food for the plant, it also helps the plant’s roots to get accustomed to a more dense growth medium. Once the seedlings are ready to be transplanted outdoors, they should have a strong root system that can quickly adapt to the more compact soil that is in the garden. 

By stepping up the seedling gradually to more dense soil, it will help reduce transplant shock and give the plant the best chances of success.