Seeding Starter? Potting Soil? Potting Mix? Oh My.

I used up the last of our seed starter and potting soil with the last round of seeds that I planted, so we needed to purchase some more. Okay, no problemo, right? Wrong. I find the world of potting mixes to be extremely confusing. There are so many brands, each has several different types of potting soil or seed starting mix. Each claims to be the best for your plants. Which one do you buy? Should you spend more for a “higher quality” mix? What makes a mix higher quality?

At three stores near me – Cornell Farms (a local garden store up the road from us), Fred Meyer (Portland’s version of Kroger), and Home Depot – there were 30 different brands selling potting soil or seed starting mix and 57 individual products. I started making a spreadsheet of all of the options and pretty quickly realized this would be a frustrating endeavor.

It was difficult to find a complete ingredient list for many of the soils/mixes. BlackGold products contain “RESiLIENCE®” (which is some sort of proprietary silicone product, I think? Who knows.) Miracle-Gro potting mixes all contain “a blend of sphagnum peat moss, aged bark fines, perlite, plant food, and a wetting agent.” There were nine different Miracle-Gro potting soils on my list- with prices ranging from $0.31/qt to $2.36/qt. Hm.

The only objective criteria I can figure from all of this are the following:

  1. Ingredients: If you can find the complete ingredient list, you can choose a soil that either doesn’t contain an ingredient you don’t want or contains an ingredient you do want (but you probably won’t know how much of that ingredient is in the mix). For example, peat moss. Peat is a controversial potting mix component because it is harvested from peat bogs which are large carbon sinks and unearthing peat releases carbon dioxide (a greenhouse gas) into the atmosphere. There is a big push to get away from peat in gardening applications. So, if you wanted a peat-free mix, looking at the ingredients list could narrow your search down quite a bit.
  2. Fertilizer: Some potting mixes contain added fertilizer (either organic or synthetic) and the N-P-K levels should be published on the bag. If you want to control the amount of fertilizer in your pot by adding your own, avoid a mix with added fertilizer. Or if you want an all in one product, buy a mix with fertilizer.
  3. OMRI certification: OMRI stands for Organic Materials Review Institute. This is a non-profit organization that evaluates farming/gardening inputs to determine if they are appropriate for USDA-certified organic farming/gardening use. It’s not strictly necessary for organic farming. For example, E.B. Stone’s potting mixes are evaluated by the California Department of Food and Agriculture (CFDA), which uses the same standards as OMRI, so E.B.Stone doesn’t see the point in paying OMRI to also certify their products. If organic gardening is important to you, look for an OMRI stamp. I didn’t check every product on the list, but based on an initial survey, if a product said “Organic” in the label, it was OMRI (or CFDA)-certified.
  4. Price: This is an objective criterion, but also a bit difficult to evaluate. Most mixes were priced by volume, but some were priced by weight and you can’t convert weight to volume without knowing the composition of the product which….is impossible to figure out. Prices varied from $0.09/qt to $3.59/qt! Quite a range! And there didn’t seem to be any rhyme or reason. Organic mixes weren’t always more expensive than non-organic products. Without knowing the mix composition, I can’t tell which products are “higher quality” so I have no idea if price correlates with quality.

All of this is to say, I am as confused as ever about potting soil and seed starting mix. I have no advice for you other than maybe, Don’t waste your money? Buy whatever seems reasonably-priced and is organic if that’s important to you? The only way to really know what you’re getting is to mix your own potting soil, which I might end up doing at some point, but I’m not organized enough for that this year.

We ended up buying two kinds of potting soils – an organic kind from the Kellogg brand and Miracle-Gro’s Potting Mix.

The Kellogg mix has “Processed Forest Products, Recycled Forest Products, Arbor Fines, Perlite, Peat Humus, Sphagnum Peat Moss, Dehydrated Poultry Manure, Feather Meal, Composted Poultry Manure, Bat Guano, Kelp Meal, Earthworm Castings” whereas the Miracle-Gro product has a mysterious “blend of sphagnum peat moss, aged bark fines, perlite, plant food, and a wetting agent.”

The fertilizer composition of the Miracle-Gro product is 0.21-.0.11-0.16 compared to Kellogg’s 0.30-0.10-0.10. Kellogg has slightly more nitrogen and less potassium, but in the end, the two are not all that different, which is surprising given that Miracle-Gro is using synthetic fertilizers and Kellogg is using organic fertilizers. I’ve read that you don’t want to give little seedlings too much fertilizer or it may burn their roots, but how much is too much? Will these mixes be okay?

I’m going to be sowing tomato seeds this weekend (Hoooray! It’s Tomato Time!) and I will start some using the Miracle-Gro and some using Kellogg. We’ll see if there is any difference.

In other news, if you’ve managed to read this far, I came home from work yesterday to find this:

A squirrel has been digging in the pot in which I sowed Boston Lettuce seeds! Aaaargh! What a frustrating day.

One thought on “Seeding Starter? Potting Soil? Potting Mix? Oh My.”

  1. What a thorough review–including a spreadsheet! I don’t have anything to add, except that “Earthworm Castings” sounds delicious. Squirrels are the bane of Dave’s garden and our bird feeders. Why are there so many? Why are they so aggressive? Why are they cute?

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