One of the most critical aspects of starting seeds or potting up plants is to use an appropriate growing medium. A good quality potting soil/medium will provide aeration, water, nutrients, and support to the plants as they grow giving them an adequate foundation to thrive. However, different situations warrant different growing mediums; understanding the characteristics of the different mediums will aid in choosing what type is best in a given scenario.

Potting soil is important for plants because it creates pockets of “empty” space around the roots for air allowing them to breathe; it serves as a holding tank of moisture and nutrients for the roots; and it provides support to plants by giving the roots a substance to anchor into.  Starting a project with either a poor quality potting media or a well suited medium greatly impacts the growth of the plants.

Common Potting Medium Materials

So what makes a good potting “soil” then? With so many options available for purchase it’s easy to get overwhelmed with all the choices and be unsure what material(s) to choose. Here’s a quick rundown of the most common:

Peat moss and other peat materials form due to the accumulation of plant materials in areas such as bogs that drain poorly. Sphagnum moss is the most commonly used form of peat moss; it is light in nature and can retain 10 to 20 times its weight in water while simultaneously draining well. Fungistatic properties inhibit the growth of fungi too.

Coconut coir is similar to peat moss, and can hold approximately 8-10 times its weight in water. It is composed of the brown and white fibers found between the shell and the outer coating of a coconut seed. Coconut husks are soaked in water to remove the coir, and then the coir is allowed to dry and formed into bales. 

Wood residues such as sawdust, pine bark, and leaf mold are readily used for growing media since they are easily sourced. All are products of the lumber industry and are readily available, typically at an inexpensive price. The wood residues have similar characteristics to peat moss.

Perlite is a silica based mineral with volcanic origins. To be used as a growing media, high grade materials are crushed and then heated until the internal water is vaporized, creating a light, airy substance. Perlite is excellent at increasing aeration and drainage because of its light, airy texture.

Vermiculite is formed from mica, another silicate mineral, that has a distinctive layered structure. Massive heat is applied to the mineral causing the layers of thin plates to expand into accordion like structures. The expanded particles have a very high water holding capacity and help to improve structructure and drainage.

Expanded clay is typically used to increase the large pore space in a potting media, improving drainage and aeration by reducing the water holding capacity. Montmorillonite clay minerals are heated to a high temperature creating pottery like particles.

Rockwool cubes are a dense mat of natural fibers that have been spun together. Basalt rock and chalk are mixed together and then melted to form a lava; the lava is then blown into a spinning chamber to create long, intertwined fibers of an inert nature. When the fibers are spun it creates a structure that is perfectly suited to retain water while holding more oxygen than typical soil mediums. 

These materials can be purchased to use separately or may be mixed together to create a potting soil that has a variety of characteristics and benefits.

Commercial potting soil mixes are a blend of ingredients, typically comprised of peat moss or coconut coir, pine bark, and either perlite or vermiculite. The peat moss or coir give the potting mix the ability to retain water, pine bark provides pore space for oxygen while preventing the mix from breaking down too quickly, and the perlite or vermiculite provides additional air space while lightening the overall weight of the mix.

Special Growing Situations

As previously mentioned,plants have different needs in their life, depending on the growing cycle they are in. When seeds germinate they have very different requirements that plants that are bigger and more established. Due to this, the potting soil needed in these life phases is different as well.

Starting Seeds

When germinating seeds, care needs to be taken to use a growing media that retains the moisture needed for germination but also drains well so there is room for oxygen in the pore spaces. Seeds need water to germinate but can experience detrimental problems if they become waterlogged. If they get too much water as the seed coat breaks down the entire seed can rot instead of germinating; seedlings can “drown” if all of the pore space is filled with water and the roots have no access to oxygen. 

There are some really great soilless media available for use when starting seeds and some that should be avoided if at all possible.

Peat based soils have excellent water retaining properties, which is beneficial when starting seeds but solely using peat moss as a growing media can expose the seeds to too much water and inhibit germination.

Coconut coir is used by some growers as the sole component when starting seeds but it gets mixed opinions on how well it’s successful overall. If you choose to use it be careful to not overwater and cause damping off, but make sure the excellent drainage properties don’t leave the top layer too dry to support germination.

Wood residues hold too much water to use by themselves (sawdust especially, although the fine particles would seem to be a perfect medium for starting seeds), causing mold and mildew to form. This fungal growth could be detrimental to your seeds.

Perlite and vermiculite are made to increase the water holding capacity of potting mixes, while allowing the “free” water to drain quickly. Their particle size and shape works well in mixes but can be too large for seedlings to stay securely nestled in the mixture.

Expanded clay can be used to germinate seeds but the larger particle size won’t provide good contact with the seeds. Rinse clay pebbles well before using them to remove the dust and then crush the clay pebbles slightly to create a finer medium to work with. Make sure that the pebbles do not dry out.

Rockwool cubes are quite popular for germinating seeds in because of their excellent moisture retention; they provide the water needed but don’t let seeds sit in a waterlogged environment. The drawback to using rockwool cubes though is the prep work that needs to be done to adjust the pH to a proper level before using them.

Seed and cutting compost is a unique product specifically formulated to provide ideal conditions for germinating seeds, as well as establishing seedlings and plant cuttings. It contains find grade compost to ensure close contact with the seed, vermiculite to improve drainage and aeration, and unique plant extracts to promote strong seed germination and healthy cutting growth.

Potting Up Seedlings

When plants are started as seeds, there will come a time when they are too big for the container or cell they are started in. Often times it is still too early in the season to move them outdoors, or they need to be transferred into containers to be sold to customers as individual plants. This process of moving seedlings to a larger container is also known as potting up. Just as when starting seeds the growing medium used greatly impacts the growing conditions and health of the plants.

Seedling also have slightly different requirements when it comes to the potting media they need. The seed no longer provides food for nutrition so the potting media must hold onto fertilizers to let roots absorb nutrients; the more robust root system and plantlet can also withstand a courser media and higher water holding capacity.

Peat soils aren’t typically used by themselves for growing plants because of their low nutrient holding capacity. Its acidic nature can be problematic as well, unless growing acid-loving plants. Most growers prefer to mix it with other materials.

Coconut coir is gaining popularity in terms of being used by itself for potting up plants. Growers like it because it is inexpensive, renewable, and has a much more appropriate pH level than peat products.

Wood residues should be avoided as a sole potting media for potting up plants. Being high in carbon, all available nitrogen gets tied up by organisms tryings to break down the carbon.

Some commercial growers will pot seedlings into straight perlite, vermiculite, expanded clay, or rockwool cubes when potting them up because of their water retention capabilities. All of these media are inert though, so they are void of any nutrients for plants to use. To be used for potting up it would be necessary to create a nutrient solution -- such as in a hydroponics setup -- to supply plants with the food they need.

Many commercial and home growers will combine 2 or more of these materials to create a unique blend to use as potting soil for potting up seedlings. This creates a mixture that retains moisture, drains freely, and can hold onto nutrients for plant availability. One of the most commonly used potting “soils” for potting up seedlings is a mix of peat or coir, some sort of organic matter (pine chips or bark is commonly used) and either perlite or vermiculite to aid in drainage. Commercial mixes are available in a range of varieties based upon the plants you are working with, and it’s quite easy to mix your own to customize a formulation.

Conclusion

Using the correct potting medium when planting seeds or potting up seedlings is a critically important part of maintaining good plant growth. The correct potting medium will provide aeration, water, nutrients, and support to the seeds/plants, providing them the optimal growing space. Since plants have different needs that correspond with points in their life cycle, its best to tailor the potting mix to plant requirements.