When selecting and using a potting media for starting seeds or growing other plants, key factors to consider include which components you will want, how fine or coarse those components are processed, how the potting media is handled, and what type of container they will be placed in. Ultimately watering practices will dictate much of the success in growing plants, but considering all these factors can increase the odds of success and make watering less challenging. Uniformity of your potting media and how it is handled can make plant care easier (Figure 1).
Differences Between Soil and Potting Media
Soil functions great in the ground, but it often lacks some of the important traits we need on containerized plants. Since soil in the ground can be very deep, it provides a long column of area for drainage. When we put soil in a container, the lack of depth can deter drainage. The goal of a good potting media is to provide a lightweight and porous substrate that doesn’t predispose our plants to poor aeration but holds enough water for the plants as well. An excellent quality soil in the ground will be roughly 50% pores and 50% solids (Figure 2). But many high-quality potting medias are roughly 85% pores and are lightweight for handling. Even if we start out with a high-quality porous soil, once we dig it up and put it in a container, many of the larger pores that hold air will be destroyed. For these reasons, potting media is generally a better choice for containers.
Different Components of Potting Media
Potting media components are generally placed into two categories. Organic components are items like peat moss, bark etc., that are primarily made up of carbon. Given time, they will decompose, but often this will take many years. Inorganic components are made of mineral- or rock-like structures. These will not change much or decompose over time. Even though organic components will break down over time, they should be the base of most potting media mixes.
| Organic | Inorganic |
|---|---|
| Bark | Perlite |
| Coir | Sand |
| Compost | Soil |
| Peat moss | Vermiculite |
| Rice hulls |
Components Used Primarily to Hold Water
Peat Moss
Peat moss has traditionally been the base for most mixes, and it still is. The peat moss sold in large bags as a soil amendment is more decomposed and much finer than the horticulture-grade peat moss that is generally used in potting media. The soil amendment type of peat moss, often referred to as ‘black peat moss’ will generally retain too much moisture and have very few air-filled pores for it to be ideal for use in potting media. Course or chunk peat moss is minimally processed and is often used for plants like orchids since it can retain water, but the large pores between chunks can provide excellent aeration.
Coir
More recently coir, a co-product from the coconut industry, has been available as a peat moss replacement and can be used as such. Neither components should not be allowed to dry out as they become hydrophobic. Re-wetting them can be a major challenger and needs to be done several days prior to using them in potting media.
Bark
Bark is generally not used in mixes intended for starting seeds since its larger particle size can make seeding in small-celled trays difficult. Bark can also vary a lot. The bark that is generally added to potting media has been composted and processed to a ½ inch size or less. Bark used in orchid mixes is much coarser and tends to produce more air-filled pores. Avoid using bark mulches for potting media as it has not been processed and may not be from the ideal tree species.
Compost
Compost can be a component for potting media, especially if you are growing organically. With limited fertilization options, it can provide a low dose of nutrition, but supplemental fertilizer will likely still be needed. Using compost in potting media can increase some risk factors. Compost varies based on the source. It's often sold before it’s completely composted and will continue to decompose (often more quickly than peat moss) and may contain weed seeds or other bacteria and fungi that could be either positive or negative.
Soil
It’s also tempting to use soil in potting media. The challenge with using soil is that it loses most of its air-filled pores and compacts when we dig it up and put it in a container (Figure 3). These air-filled pores are especially important when putting plants in containers. Like with compost, weed seeds can be present in soil.
Vermiculite
Vermiculite is the shiny and flakey media component. It’s good at holding water and, to some degree, nutrients, but it tends to be one of the more expensive components. With a high-quality mix and a good nutrition program, it’s likely unnecessary. Some growers like to use fine vermiculite for covering seeds after sowing them in plug trays.
Components Used Primarily to Promote Aeration
Perlite
Some components are used to add air-filled pores into a potting media mix. Perlite is the most common and is a rock-like material that has been ‘puffed up’ kind of like popcorn. It provides long-lasting air-filled pores. Use caution when handling dry perlite since it can be very dusty.
Rice Hulls
As perlite costs have risen, rice hulls have also been used to provide air-filled pores. This can be an option for short term crops like starting seedlings for transplant, but for long term plants like a houseplant, they will decompose too quickly.
Sand
In the ground, sand is known for providing good soil drainage, but caution needs to be used extrapolating that to its performance when placed in a container. Water flow is held up in a container differently than in the soil, and sand does not function as well for aeration (Figure 3). One reason sand is still used is to provide ballast (weight) for tall crops.
Processing
As was mentioned above, not all components are created equal. The traits of peat and bark will vary based on the particle size and processing. The composted ½ inch minus bark will hold more water and less air than the bark used in orchid production. Peat moss also varies greatly. Peat moss harvested from the deeper parts of the bog will be somewhat decomposed, have a fine particle size, and will hold water. Horticulture-grade peat moss will still hold water, but when it’s processed to a larger particle size it will have more air-filled pores than when processed to a smaller particle size.
Handling
Like with soil, potting media can get compacted. It does not take a lot of pressure for potting media to become compacted, so proper handling is important. Sources differ on what is the ideal moisture level potting media should be at when filling containers. However, most sources indicate you should be able to squeeze it into a ball in your hand and it should remain that shape in your hands but easily break apart. Little to no water should be able to come out of the media when you are pressing it into your hand.
Filling containers and seed trays evenly is essential to success. Seed trays are often under filled in the cells along the outside edges of the tray. This may not seem like a big issue, but this creates issues with watering later in the life of the plants. Those outer cells will hold less water if there is less media in them.
Generally, we fill containers and seed trays with extra media on the top and brush off the excess. Tapping or pressing on the potting media is not advised because it packs the potting media in and reduces air-filled pores, which are in short supply if overwatering occurs (Table 2). Do not stack filled containers on top of each other, as this can also pack in the potting media and reduces air-filled pores. If you must stack due to space issues, place a thin sheet of plywood or paneling between the layers of pots or trays.
| Container Filling Method | Air-Filled Pores |
|---|---|
| Filled then brushed level | 15% |
| Filled and tap twice on the table | 9% |
| Filled, pressed down, topped off | 4% |
Containers
One of the more underappreciated aspects of potting media drainage is the container we select. There are some misconceptions about which containers promote drainage. Shallow containers are often used for plants that require good drainage. In one sense that can help since there is less media to hold water. But generally, for crops that need excellent media drainage, commercially we use taller pots and/or a media with components with more air-filled pores.
To better illustrate this, if we compare filling a short, medium, and tall pot with the same potting media, after watering to capacity, the media at the bottom of the container has less air-filled pores than at the top of the container, and we refer to that as a saturation zone (Figure 4). Both areas will have moisture in them, but due to pore size, that water will more closely saturate the bottom area. The smaller the pores, the higher that mostly saturated area will be. In our example, all the pots have the same potting media, so they will roughly saturate to the same height. The difference is how much potting media is above that saturated zone. With taller pots there is more area above the saturated zone than in shorter pots. As the potting media dries out, the saturation zone will decrease, but when plants are over watered, it persists.
To be clear, you can grow a plant in just about any container and potting media, but the challenge is how much management you want to provide, and the shorter the container the more challenging watering can be since the bulk of the plant roots are in that potentially saturated zone. Overwatering or watering too often increases the impact of that saturated zone, which limits rooting (Figure 5).
Seedlings grown in trays with small cells are prone to both overwatering and underwatering. When growing seedings in trays, consider using a deeper cell rather than a shallower one. This can provide more of a buffer from over or underwatering. Separating different species from each other in a tray can also help with management. For example, it’s common to sow tomatoes and peppers in the same tray but peppers generally need less water. Even if all other practices are ideal, it can be challenging to water these two crops separately, if they are not physical separated from each other.
Conclusion
If can be fun to buy different potting media components to tailor the mix to your individual plant needs. Keep in mind that if you are doing your own mixing, it’s essential that you get all the components evenly mixed. Most growers choose to buy their potting media pre-mixed to simplify the process and reduce the number of items needed to purchase. These pre-mixed potting media also often have things like wetting agents and lime added to the mix to make management easier. Before buying a mix, check the ingredient list to see if it has the traits you are looking for. Some products within a brand of potting media now list traits like drainage, particle size, and water retention to allow purchasers to better understand the differences between products to assist in selection.
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