Reading the Bag: A Grower's Guide to What's Actually in Commercial Potting Mix

Walk into any garden center and the potting soil aisle is a wall of bags making the same promises. Fertile forest floor. Rich organic blend. Now with mycorrhizae. New and Improved! Turn the bags over, though, and most of them are built from the same few parts. You can't shop your way out of the confusion, but you can learn to read the label. The Guild uses two FoxFarm mixes, Happy Frog and Coco Loco, so this isn't a neutral lab report.

§ 01The four decisions in every bag

Every bag of potting mix is four decisions layered on top of each other. Once you can see them, the marketing copy matters a lot less.

The base. This is the foundation of the mix, and it provides most of its water-holding capacity (Evans, n.d.-b). In soilless mixes it's usually sphagnum peat, sometimes coir (Grubinger, n.d.), and many bags add aged bark or other forest products. It's often the largest share of the bag: Pro-Mix BX, for example, lists sphagnum peat at 75 to 85 percent by volume (Premier Tech, 2024). Everything else adjusts from this one big choice.

The aeration. Perlite and pumice are added mainly for drainage and air. Perlite increases the share of large pores, which lowers water holding and raises air space (Evans, n.d.-a). Pumice holds a little water, but its larger particles mostly provide drainage and air (Evans, n.d.-a). Vermiculite is the odd one out: it holds a lot of water and adds nutrient-holding capacity (Evans, n.d.-a; UConn Home and Garden Education Center, 2016). So a bag that leans on perlite or pumice is built to drain, and one that leans on vermiculite is built to hold.

The charge. How much fertilizer is already mixed in. Some bags carry only a starter charge, and extension guidance is plain that those nutrients are soon used up, after which the plant needs regular feeding (UConn Home and Garden Education Center, 2016). Others carry fertilizer their makers say lasts longer: Miracle-Gro says its potting mix feeds for up to six months (ScottsMiracle-Gro, n.d.-a). For comparison, the controlled-release fertilizers used in container growing are rated to last from a few months to more than a year, and they release faster when it's warm (Pasian, 2013). Where the maker prints a guaranteed analysis (the three numbers, such as 0.30-0.30-0.05, in the table below), it tells you how much nitrogen, phosphate and potash the maker promises, so you can compare one bag's charge with another's.

The biology and pH. Mycorrhizae, beneficial bacteria, kelp, humic acid, plus lime. Unamended sphagnum peat runs at pH 3.0 to 4.5, so for most crops peat-based mixes are amended with ground limestone before plants go in (Evans, n.d.-a). The biology is the corner of the bag with the most marketing and the least disclosure, and it's where the next two sections spend most of their time.

That's the whole framework: base, aeration, charge, biology. Two terms are worth knowing, since the primer uses them. EC (electrical conductivity) measures the dissolved salts in a mix, which reflects either the fertilizer available or salts that have built up (Camberato et al., n.d.). Air-filled porosity is the share of the pot that's air after the mix drains, which is what roots breathe. If you want the physics under all of this, the root-zone primer goes deeper.

§ 02The clearest lesson on the shelf: peat versus coir

FoxFarm makes a handy comparison. Happy Frog is built on aged forest products and sphagnum peat (FoxFarm Soil & Fertilizer Company, n.d.-a). Coco Loco is built on coconut coir with aged forest products (FoxFarm Soil & Fertilizer Company, n.d.-b). Same company, same pitch, two different bases, sitting side by side on the shelf.

Start with the water. Sphagnum peat holds 60 to 68 percent of its volume as water (Evans, n.d.-a). Its well-known weakness is that it naturally repels water, and that shows most once it dries out. That makes rewetting a real problem, which is why some suppliers add a wetting agent (Robbins, 2018). You've seen this if you've ever watered a bone-dry pot and watched the water run straight out the bottom. In a lab rewetting test, dry peat with no wetting agent stayed far short of its capacity, though the authors note their peat sample may have been more water-repellent than usual (Fonteno et al., 2013).

Figures for coir vary between sources. Evans (n.d.-a) puts typical coir at 73 to 80 percent water by volume, slightly more than peat, while one lab comparison measured 42.5 percent for the coir it tested against 62.8 percent for its peat (Woznicki et al., 2023). Coir is easier to rewet after drying than peat (Robbins, 2018), but not immune: coir dried hard and given no wetting agent took up little water in lab tests (Fonteno et al., 2013; Fields et al., 2014). Robbins (2018) says coir's higher lignin content (lignin is the tough, woody part of plant fiber) makes it more resistant to microbial breakdown, so it may shrink less. That said, coir and peat were both fairly stable over three strawberry crops in the Woznicki et al. (2023) trial. Raw coir also carries potassium, sodium and chloride, which leach out easily (Evans, n.d.-a), and it's commonly washed, often with calcium nitrate, before use (Altland & Bugbee, 2026). FoxFarm says Coco Loco is triple-washed to reduce salt content (FoxFarm Soil & Fertilizer Company, n.d.-c).

Then there's where the base comes from. Northern sphagnum peatlands build up only about 1/85 to 1/36 inch (0.3 to 0.7 mm) of peat a year, so a layer about 3 feet (1 m) deep taken from one bog would take 1,400 to 3,300 years to replace at that spot (Altland & Bugbee, 2026, citing Vitt). Peat soils hold up to 44 percent of all soil carbon (International Union for Conservation of Nature [IUCN], 2021). The picture isn't one-sided, though. The same review calculates that Canada's peatlands as a whole grow peat far faster than horticulture harvests it, and concludes North American horticultural peat use is sustainable at current rates (Altland & Bugbee, 2026). Coir comes with its own costs: its processing uses a lot of water that's discharged to waste, and in the three life-cycle studies (which tally a product's environmental footprint from production to use) that measured broad environmental impact, coir pith came out with a higher impact than peat (Altland & Bugbee, 2026).

If you take a bagged mix indoors, expect fungus gnats to be a concern with any moist, organic mix, whatever its base (see § 05). Our suggestion is to open it up with coarse material such as pumice or perlite, or chunky bark or coco chips, before potting a houseplant.

§ 03The badge that doesn't earn its hype

Many premium bags now list added mycorrhizae. These are fungi that form partnerships with roots and help them take up water and nutrients. One common group, called arbuscular mycorrhizae, is especially good at taking up phosphorus (Epiphan, n.d.). Happy Frog's label lists 15 mycorrhizal species (FoxFarm Soil & Fertilizer Company, n.d.-a). Espoma adds its own blend, sold as Myco-tone (Espoma Company, n.d.). E.B. Stone's Recipe 420, Pro-Mix BX AGTIV REACH (the mycorrhizal version of BX) and Coast of Maine's Stonington Blend all list mycorrhizae too (E.B. Stone & Son, 2024; Premier Tech, n.d.; Coast of Maine Organic Products, n.d.-b). The underlying biology is real.

The problem is what's in the bag. A 2025 meta-analysis in New Phytologist pooled 302 inoculant trials and found that only 12 percent of the trials of commercial products both increased crop growth and showed good root colonization; most commercial inoculants gave minimal or no colonization (Koziol et al., 2025). A separate test of 23 inoculants (commercial products alongside lab-grown fungi and field soil) found that eight of the commercial products produced no colonization in any pot. For all but one product, the spore count ran 71 to 100 percent below the propagule count (the maker's tally of fungal units) on the product listing (Koziol et al., 2024). The same test found that inoculant products carrying more fertilizer of their own produced less colonization (Koziol et al., 2024). In an older study of bedding plants grown in peat-based media, inoculation did colonize the roots but didn't increase growth or phosphorus uptake (Koide & Besmer, 2000).

None of this is a mark against the mixes that carry mycorrhizae. It means "now with mycorrhizae" belongs near the bottom of your buying checklist, not the top.

§ 04What's in the premium bags

Strip away the label copy and the premium bags look a lot alike. Here's the lineup, taken from each maker's current label, product page or safety data sheet, and for Black Gold its organic listing certificate. We've left prices out on purpose, since cost per cubic foot swings by retailer, region and bag size, so check it at the shelf.

Label sources: (FoxFarm Soil & Fertilizer Company, n.d.-a; FoxFarm Soil & Fertilizer Company, n.d.-b; FoxFarm Soil & Fertilizer Company, n.d.-c; FoxFarm Soil & Fertilizer Company, n.d.-d; FoxFarm Soil & Fertilizer Company, n.d.-e; Espoma Company, n.d.; Premier Tech, 2024; Premier Tech, n.d.; E.B. Stone & Son, 2024; Coast of Maine Organic Products, n.d.-a; Sun Gro Horticulture, n.d.-a; Organic Materials Review Institute, 2021; ScottsMiracle-Gro, n.d.-a; ScottsMiracle-Gro, n.d.-d).

OMRI is the Organic Materials Review Institute, a nonprofit that reviews products meant for use in certified organic production (Organic Materials Review Institute, n.d.-a). An OMRI Listed seal means the product was reviewed and allowed for organic use; it isn't an organic certification of the product itself, which needs a separate certificate (Organic Materials Review Institute, n.d.-b). Black Gold's Natural & Organic mix in the table is one that carries the seal (Sun Gro Horticulture, n.d.-a; Organic Materials Review Institute, 2021).

The pattern jumps out once it's in a row. Almost all of them are a base, plus an aerator, plus a charge of meals and castings, often with a biology garnish. They differ at the edges: which base, how much feed, which meals. Two break the mold. Pro-Mix BX is a general-purpose peat mix with only a starter charge, and its maker says to begin feeding within 7 to 10 days after planting (Premier Tech, n.d.). Miracle-Gro is the other extreme, a mix its maker says feeds for up to six months (ScottsMiracle-Gro, n.d.-a).

Where the makers are

A few of these companies lean on ingredients that fit where they're based. Coast of Maine, headquartered in Portland, Maine, puts lobster and crab shell meal and kelp meal in its Bar Harbor Blend (Coast of Maine Organic Products, n.d.-a; Coast of Maine Organic Products, n.d.-c). Premier Tech, which makes Pro-Mix, is headquartered in Rivière-du-Loup, Quebec, and harvests peat in several Canadian provinces (Innovation, Science and Economic Development Canada, n.d.). FoxFarm (Arcata, California) and E.B. Stone (Suisun, California) both build on forest products and guano (FoxFarm Soil & Fertilizer Company, n.d.-d; E.B. Stone & Son, 2024; E.B. Stone & Son, n.d.-b). A company's address isn't where every ingredient comes from, though. India and Sri Lanka are the main coir exporters, whoever's name is on the bag (Austin et al., 2026).

§ 05The gnat question

If you've used a living organic mix, you've probably met fungus gnats. Their larvae usually stay within the upper 2 to 3 inches of the mix, depending on how moist it is, and feed mainly on fungi, algae and decaying plant matter. The adults are strongly drawn to moist growing media (Cranshaw & Cloyd, 2003). That makes any moist, organic mix a candidate, whatever the brand. Switching bases doesn't solve it: fungus gnats live and breed as well in a coir-based mix as in a peat-based one (Cloyd, 2010), and in lab choice tests the number of eggs females laid in a coir mix didn't differ significantly from two other commercial mixes (Meers & Cloyd, 2006).

Some makers sell around the problem. Miracle-Gro says its Indoor Potting Mix is "designed to be less prone to gnats (contains no compost or bark, which are known to shelter fungus gnats)" (ScottsMiracle-Gro, n.d.-b). The most important thing you can do with any mix is let the top 1 to 2 inches dry between waterings (Cranshaw & Cloyd, 2003). Top dressings, a layer spread over the surface of the mix, are less certain than they sound: a layer of sand didn't stop adults emerging or females laying eggs (Cloyd, 2010), though a layer of recycled-glass aggregate reduced adult emergence over time (Cloyd, 2015). If they've already moved in, our fungus-gnats field guide has the full protocol.

§ 06Match the bag to the job

This is where it pays off. Four common jobs, and the bag we'd reach for.

Houseplants and aroids. The Guild's own houseplants grow in its mineral-forward mixes (that's the whole Substrate Library), not in bagged mix. If you want to stay with a bag, our suggestion is to start from Coco Loco or Happy Frog and mix in coarse material, such as orchid bark or chunky coco chips plus perlite or pumice, until it makes up at least half the mix, then let the top of the pot dry between waterings.

Container vegetables. Container vegetables need steady feeding once any starter charge is used up (UConn Home and Garden Education Center, 2016). For established transplants we'd reach for Ocean Forest. FoxFarm says it feeds for about three to four weeks (FoxFarm Soil & Fertilizer Company, n.d.-e), so plan to start feeding after that. For younger plants, our suggestion is Happy Frog, with feeding stepped up as they grow.

Raised beds. Extension advice is mixed. Minnesota Extension says potting soil isn't a good fit for raised beds because it dries out too fast, and suggests filling them with roughly half to two-thirds topsoil and the rest plant-based compost (Hoidal et al., n.d.). Maryland Extension, for a bed built on a hard surface, suggests compost and a soilless growing mix in equal parts (University of Maryland Extension, 2024). Cost is the other catch: buying enough prepared mix for a raised bed can be cost-prohibitive (Durham et al., 2018). Several makers sell blends labeled for raised beds, including E.B. Stone, Black Gold, Coast of Maine and Miracle-Gro (E.B. Stone & Son, n.d.-a; Sun Gro Horticulture, n.d.-b; Coast of Maine Organic Products, n.d.-d; ScottsMiracle-Gro, n.d.-c).

Seed starting and propagation. Here the rules change. Extension guidance is to germinate seeds in a sterile, fine-textured soilless mix (Peronto et al., 2021). Watch the salts: mixes made with compost can be high enough in salts to hold back germination (DuPont, 2025), and injury from too much fertilizer or high soluble salts can be mistaken for damping-off (Meadows, 2017). Damping-off itself is caused by several fungi and water molds, among them Pythium, Phytophthora and Rhizoctonia. It can attack a seedling before it comes up, or rot the stem where it meets the soil, or just under it, after it does (Mazzone, 2026). A bag labeled for seed starting is built for this job.

§ 07The bottom line

Brand matters less than you'd think. What you're buying is a base and a charge for a specific job. Once you can read those off the back of the bag, the aisle gets a lot simpler, and you stop paying extra for stickers. Read the bag, match the job, and watch how the plant responds.

Go deeper in the free Apprenticeship: Module IV, What Substrate Is →

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