Today, we start another three part series featuring another of my favorite topics: container media (a.k.a. potting soil). I have played around over the years, both out of necessity and nerdiness, with potting soil. In this exciting blog series, we will cover potting soil basics, including characteristics and types of soil, making your own, and we will also discuss Wellfield’s product choices.

We’ll start this discussion with potting soil basics. Soils in the ground and in a garden container serve the same purpose for the plant, including:

• A reservoir for water,
• A place to hold nutrients,
• Space for root gas exchange, and
• An anchor point, something for root hairs to grab on to.

Key potting soil characteristics include:

• Bulk Density-relates to the ability of the plant to anchor. The soil needs to be substantive enough to prevent the plant from pitching over at its lightest, but not so heavy it is hard to handle/transport.
• Moisture Retention and Aeration-another balancing act between finding air to breathe, while being able to hold as much water as possible (the roots in a pot cannot keep growing to find more water). Different potting ingredients balance this differently.
• pH-is key to nutrient availability. The “ideal” range for most plants in soil-based media is 6.2 – 6.8, whereas 5.4 – 6.0 works best in soilless media like peat moss.
• Nutrient Holding Capacity (Cation Exchange Capacity)-is a measure of the ability of soil to attract and hold positively charged nutrient molecules for plant roots.

There are two basic types of potting soil (as hinted above): soil-based and soilless. A mineral component serves as the main ingredient in soil-based potting, hence the name. As you might guess, soilless potting soil lacks a mineral component. Some type of carbon-based material such as peat moss, bark, coconut coir or compost serves as the main ingredient in soilless media.

Why the two types? Glad you asked. Soilless media is a pretty recent innovation. Historically, gardeners growing plants in containers or greenhouses had to rely upon locally available field soil. They would dig it up, sometimes mix in other ingredients, and put it in a pot. But there are several issues with soil-based media including weight, sanitation and consistent performance. Soil-based media was fine until the horticulture industry wanted to ship plants long distances. Field soil is a little heavy. Also, weed seed and soil borne pathogens possibly existing in field soil pose a risk. Mineral soil must be sterilized in some fashion. Lastly, plant performance in soil-based media was hard to predict. The soil needed to be mixed with other ingredients, and getting those mixes consistent (since soil can be so variable) was difficult. Field soil removed from it context tends to lose its structure. Drainage is proportional to soil depth above the water table, which in this case is the bottom of the pot. Moisture content in a 7-inch pot is similar to soil two inches above the water table. There is little room left for aeration. Incorporating bulking agents to fluff things up a bit increases aeration and drainage. Mineral soils exhumed for potting purposes also lose connection to the humus and soil food web which naturally support garden grown plants. This, in turn, contributes to greater loss of structure.

All these disadvantages brought about the need for alternatives, and thus soilless media was born. Soilless media is much lighter and cleaner, and it is comprised of more consistent ingredients. Among most soilless medias the drawbacks include low inherent nutrient content and sterility. Field soil, being mineral based, might come with all those fabulous vitamins plants love like calcium, magnesium, and iron. Field soil also possesses a greater buffering capacity against changes in irrigation water pH compared to soilless media. As for sterile media, it is a double-edged sword. Sterile potting media means less pathogen pressure, but it also means no help from the microbe good guys. Thus, a container grown plant is totally reliant upon the chemical swirl the grower must provide. A container plant destined for the garden may potentially have greater difficulty adjusting to their permanent home once transplanted compared to potting soil infused with microbes like mycorrhizae. However, it should be said further research must be done to confirm to what beneficial extent mycorrhizae potting mixes do have upon containerized plants.

Next time, (if I have not buried you yet) we shall discuss the joys and frustrations of making your own potting soil.
Josh Steffen, Horticulture and Facilities Manager

Photo credit: Gabriel Jimenez (https://unsplash.com/@gabrielj_photography)