Transplant production: Potting mix

The right media is essential in order to grow healthy, productive transplants. Whether it is purchased or made on the farm, the right potting mix can be the start to a successful season.

High quality potting mixes provide adequate nutrients, water, and air to seedlings, resulting in productive growth and improved plant health. Photo credit: Collin Thompson, MSU

High quality potting mixes provide adequate nutrients, water, and air to seedlings, resulting in productive growth and improved plant health. Photo credit: Collin Thompson, MSU

Take a walk through the garden center of a large retailer and you will be overwhelmed with the numerous potting mix options. There are organic mixes, soil-less mixes, mixes with mycorrhiza – the list goes on and on. The presence of so many options can make it hard to know where to start. This article will lay out some of the primary components of potting mix and help you understand how to select a potting mix, or perhaps make your own.

When selecting a potting mix for germination, we need to consider texture, water and air holding capacity, pH and nutrient content and mix integrity (absence of weed seeds/diseases). Good potting mix will be light, hold water and air well, provide enough nutrients for healthy plant growth, and be free of diseases and weed seeds. This is driven largely by potting mix components:

Structural Components

  • Peat, coconut coir, rice hulls and compost are often added to potting mixes to provide good structure to the mix to increase the water and air holding capacity of a mix. Compost can be used to improve structure, as well as provide food for growing seedlings.
  • Perlite and vermiculite are commonly added to increase air and water holding capacity.
    • Perlite is a silica-based volcanic rock that is expanded through a heating process. It increases water-holding capacity while also improving aeration and loft in a mix.
    • Vermiculite is a micaceous mineral that also expands during a heating process to improve water-holding capacity in a mix. It can also be used on top of seeded trays to improve germination by maintaining adequate moisture.

Proper structure will allow for adequate drainage, while still maintaining adequate moisture to improve root penetration and growth. For smaller containers, the media should be finer-textured, while larger containers can accommodate coarser media. Regardless, the potting mix should consist of 10-20 percent air space, and 40-60 percent water when moistened.

Fertility Components

Providing proper levels of nutrients for growing seedlings is crucial for productive growth. While potting mixes need not be nutrient powerhouses, given the short amount of time it needs to support plant growth, inadequate nutrients can lead to stunted growth, reduced productivity, and susceptibility to pathogens and pests.  

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Inadequate fertility in a potting mix can lead to deficiencies. These tomatoes are suffering from a potassium deficiency and will likely take longer to rebound once planted. Photo credit: Collin Thompson, MSU

Potting mixes are widely variable in their nutrient makeup, though there are some guidelines that help growers make decisions on which mix to use:

  • Ideally, pH should fall between 5.5 and 7, available N between 130 and 220 ppm, available P greater than 3 ppm, and available K greater than 25 ppm.
  • Additionally, Ca should be present at a minimum of 30 ppm and Mg at a minimum of 10 ppm.
  • Greater nutrient loads will be required of seedlings that will be in the container for longer periods of time, while short-lived transplants can be grown using lower-rates of fertility.

Many premium mixes are commercially available and can be trusted to produce high-quality transplants. However, many growers decide to develop their own potting mix recipe to cut costs or to match their production needs. Special care should be taken when sourcing inputs to ensure limited risk of disease and weed seeds.

It’s important to protect yourself when handling, mixing, or making commercial or homemade potting mixes. They contain small particles that can irritate your lungs, so it is encouraged to work in well-ventilated areas and use personal protective equipment, such as dust masks or respirators, to reduce the risk of harm.

Whether you are buying potting mix or making your own, having the right structure, fertility and integrity in your mix can help you get your production off to a good start.

Be sure to keep an eye out for part three: Transplant production: Timing and crop types.

Collin Thompson is the Farm Manager of The North Farm at the Michigan State University Upper Peninsula Research and Extension Center in Chatham, Michigan and a Community Food System Educator with Michigan State University Extension

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