Reading a bag, reading a recipe

You now know what a substrate is for, how particles set pores, what pH and exchange capacity do, what the ingredients bring and how they age. This lesson is where that pays off at the garden center and at the mixing tub. (The Substrate Library on this site has a longer buyer's guide, Reading the Bag, that goes deeper into specific products.)

What a bag is telling you

Bagged potting mixes are soilless. They usually combine peat moss, coir, pine bark, vermiculite and perlite, and they're made to hold moisture, provide air space, and be free of weeds, insects and disease (Smith & Cox, 2015). The words on the front have specific meanings. Starter charge, or starter fertilizer, means a minimal amount of fertilizer that's mostly used up after two or three waterings (Smith & Cox, 2015). Continuous or controlled-release fertilizer means resin-coated prills (small coated granules) that release nutrients over weeks, faster in warm temperatures (Smith & Cox, 2015). A wetting agent lowers water's surface tension so it spreads and penetrates and thoroughly wets the mix; organic mixes may use yucca extract for the job (Smith & Cox, 2015). Moisture control mixes contain water-absorbing polymer gels or crystals that reduce how often you water, at the cost of a higher overwatering risk in cool weather, and their effect fades by mid to late summer (Smith & Cox, 2015). Limestone on the label is the lime from Lesson 3; commercial greenhouse blends typically arrive with it already incorporated, a nutrient charge added, and the pH adjusted to about 6.0 (UMass Extension, n.d.).

What the bag doesn't tell you is the part that matters most to a pot: particle size and air space. NC State's guidance to growers is to examine a mix's contents and particle sizes before choosing it (Fonteno et al., 1995), and for good reason: the same components in the same ratios give different physical properties when the particle sizes differ, and different again depending on whether they were blended dry or moist (Bilderback et al., 2005). Two bags with the same ingredient list can be two different substrates. The texture you see when you open the bag tells you more than the label does.

And a word in defense of the bag. UMass's advice to home gardeners is that general all-purpose mixes work fine for almost all annuals and mixed containers, with orchids the exception because general mixes hold too much water for them and lack air space (Smith & Cox, 2015). One Arkansas handout is less kind, calling commercial mixes often misleading as to content and suggesting you mix your own if you can (University of Arkansas Cooperative Extension Service, n.d.). Our view sits between: a good bagged mix is a fine place to start, and the moment it fails you is the moment to know what's in it.

Reading a recipe as jobs

A recipe is easier to read, and to adjust, if you stop reading the ratios and start reading the jobs. The Arkansas greenhouse course lays out the logic: start with a component that serves as the foundation of the substrate and provides most of its water holding, then make further adjustments to the physical properties with other ingredients (Evans, n.d.). In the language of this module: pick the water holder (peat, coir, composted bark, a measured dose of compost), then add the air (bark, perlite, pumice, expanded clay), and if you want the mix to hold more between feedings, add exchange capacity (coir, vermiculite, a little zeolite). Weight and anchorage ride along with sand or clay.

Two levers raise the air after drainage, and both are now familiar: a coarser mix, with bigger particles, and a taller container (Bilderback et al., 2013; Owen & Altland, 2008). Both have a price. A mostly coarse substrate holds relatively little water and lets nutrients leach more readily (Bilderback et al., 2013), and the higher the air-filled porosity, the more often you'll water (Robbins & Evans, 2005). Heavy, fine-textured mixes and those with a lot of peat hold more moisture than loose mixes of bark, sand and perlite (Shaughnessy & Pertuit, n.d.). So match the mix to the plant and the spot: more air for a plant that resents wet roots, for a short pot, or for a cool, dim room where the pot dries slowly; more water holding for a thirsty plant, a bright warm spot, or a grower who travels. And don't overpot: too large a container leaves roots in a zone that stays too wet (Pennisi, 2022; Criscione & Holliday-Goulart, n.d.).

That's also how to read our recipes, and anyone else's. The Guild's mixes lean mineral because we want air that lasts; they are one approach, not a requirement, and the trade-off is in the sentence above. If a recipe you like uses different ingredients to do the same four jobs, it's a good recipe.

Measuring air space at home

You don't have to guess. Purdue's fact sheet gives a method that needs nothing more than a pot, a watertight liner, a second container and a measuring cup (Nemali, 2018). Work in the pot the plant will actually live in, because the result depends on the container's height (Lesson 2). The steps, paraphrased from the Purdue sheet:

1. Know the pot's volume. (Fill it with water and pour that into a measuring cup if the maker doesn't say.)
2. Line the pot with a watertight liner, such as a polyethylene bag, taped so nothing can drain through the holes.
3. Fill the pot to the top with your mix, firmed the way you'd normally pot a plant. The mix volume now equals the pot volume.
4. Add water slowly to the top, keeping track of every cup, until the mix is saturated and a shiny film of water sits on the surface. Cover it, wait an hour, and top up if it will take more. The total you added is the pore space.
5. Set the pot in the second container, pierce the liner at the bottom, and let it drain for 10 to 15 minutes. Measure what drained. That volume is the air space.

Fig. 12. Measuring air space at home, adapted from the Purdue fact sheet: know the pot's volume, line the pot, fill it with mix, add water until it's saturated and keep count, then pierce the liner and measure what drains in 10 to 15 minutes. The total you added is the pore space; what drained is the air space (Nemali, 2018).

Divide the drained volume by the pot volume for air space as a share of the substrate, which is the basis the nursery ranges in Lesson 2 use (Nemali, 2018); divide it by the pore-space volume from step 4 instead and you have the share of pore space, the basis Purdue's earlier web article used (Nemali, 2017). Say which one you've got when you write it down. Purdue adds a caution worth repeating: pore space isn't constant, because organic ingredients swell and shrink with wetting and drying, decompose, and compact under watering and root growth, so a reading on fresh mix is a starting point, not a permanent fact (Nemali, 2018). Lesson 9's field exercise asks you to take one reading and compare it with the published bands.