The organic ingredients

The ingredients that hold water in most mixes are organic: they were plants once. That's what makes them good at the job and what gives each one a habit you need to know. Lesson 6 covers the minerals.

Fig. 9. The organic ingredients side by side: sphagnum peat, coir, composted pine bark and worm castings. These are illustrations, some from our glossary set and some newly generated, not photographs of our own stock.

Sphagnum peat

Peat is partly decomposed sphagnum moss. A 2015 review explains why it has led the organic ingredients in growing media across much of the world for half a century: it was abundant, it holds air and water well, it's low in pH and salts, and it's free of pests (Carlile et al., 2015). A greenhouse course puts its water holding at 60 to 68% of its volume, its unamended pH at about 3.0 to 4.5, and its exchange capacity at 90 to 140 milliequivalents per 100 grams (Evans, n.d.). Because it's so acidic, limestone is added to peat mixes to set the starting pH (Lesson 3). It decomposes slowly, and it makes up 30 to 80% of many bagged potting mixes (Smith & Cox, 2015).

Its famous habit: dry peat repels water. Peat is hydrophobic (water-repelling) when dry, so substrates that contain it carry a wetting agent, a surfactant that helps water soak in rather than bead up (Owen & Lopez, 2015). The same guide describes the failure you'll eventually see: a high-peat mix left unwatered shrinks as water evaporates from the surface, pulls away from the pot wall, and irrigation water then channels down the side instead of wetting the mix (Owen & Lopez, 2015). Two refinements. Hydrophobicity is a dryness threshold, not a permanent property: in one study peat repelled water at 40% and 50% moisture by weight and wetted readily, like every other material tested, at 60% (Durand et al., 2021), and the same review that calls peat the dominant ingredient also calls peat and coir superhydrophilic materials, renowned for absorbing and holding water (Carlile et al., 2015). The lesson for the sill is simple: don't let a peat mix dry all the way down, and if it does, rewet it slowly.

And the question people ask about peat. Peatlands are a carbon store (Carlile et al., 2015). A soil scientist in Oregon State's horticulture department says peat harvesting could be called renewable, since some bogs are replanted, but that full recovery takes centuries (Oregon State University Extension Service, 2022). In Europe, low-humification sphagnum peat has become too scarce to extract in many places, and peatland policy there now favors protection, restoration and rewetting (Glatzel & Rochefort, 2017). Our own mixes leave peat out. That's our preference, on those ecological grounds, and not a scientific verdict on peat as an ingredient; a well-made peat mix grows plants well, and the recipes you meet elsewhere that use it aren't wrong to.

Coir

Coir (coconut coir, coco peat) is the pith and fiber of coconut husks, and it's the usual substitute for peat. It starts with a higher pH than peat, with extension sources giving about 5.5 to 6.8 or 5.8 to 6.9 (Robbins, 2018; Evans, n.d.), so it needs less lime (Owen & Lopez, 2015), and in one study it was used with none (Langenfeld et al., 2024). One greenhouse course gives its exchange capacity as 39 to 60 milliequivalents per 100 grams, lower than sphagnum peat's, and its water holding as 73 to 80% of its volume (Evans, n.d.). It takes up water faster and drains faster than most peats, and the wetting agents routinely added to peat media aren't needed in media that are mainly coir (Carlile et al., 2015). One extension guide says it's more resistant to microbial breakdown than peat and so may shrink less (Robbins, 2018).

Its quirk is salt. Coir pith can carry high sodium, chloride and potassium, so it is washed, or leached with calcium nitrate to make what's sold as buffered coir, before horticultural use (Carlile et al., 2015); improperly processed coir can push a substrate's salt reading past recommended levels (Owen & Lopez, 2015). Purdue adds that coir-based substrates need more calcium, sulfur, copper and iron and less potassium than peat-based ones, and that growers report more tie-up of soluble nitrogen in coir than in peat (Owen & Lopez, 2015). Buy washed or buffered coir, or rinse it yourself, and you've handled most of that.

Bark

Composted or aged pine bark is the predominant substrate component in nurseries (Owen & Lopez, 2015), and it's the coarse organic ingredient in our own chunky mixes. Its job is air. Fresh pine bark in one study had 39% air space, above the 20 to 30% suggested for bark alone, with few water-holding pores and only 49% container capacity against 61% for aged bark (Bilderback et al., 2005). Bark is usually aged or composted before use, which removes its tendency to repel water, breaks down compounds in it that inhibit microbes, and reduces nitrogen tie-up; composted bark has lower air content and holds water better than fresh (Carlile et al., 2015). Nursery growers prefer bark with little white wood in it, though research reports that white wood isn't harmful to growth (Owen & Lopez, 2015).

Its quirk is nitrogen. As bark decomposes in the pot, the microbes doing the work use nitrogen the plant would otherwise take, which can show as yellowing and stunting; the Arkansas guide puts bark's carbon-to-nitrogen ratio around 300 to 1 and notes that growers using fresh bark add extra nitrogen to compensate (Robbins & Evans, 2005). NC State's advice is that bark in a greenhouse mix should be composted, not green, because aged-but-uncomposted bark causes nutritional imbalances as the decomposers outcompete the plant (Fonteno et al., 1995). In bark's favor, Purdue reports that growers see less shrinkage from pine bark mixes than from peat-based ones when a crop stays in the pot a long time, cyclamen and foliage plants among them (Owen & Lopez, 2015), and UMass notes that bark products dry out faster than peat (Smith & Cox, 2015). More air, more watering.

Compost and worm castings

Compost brings nutrients and biology, and it varies a great deal from batch to batch. The nursery guide says substrates with up to 25 to 50% compost by volume are generally acceptable, that some composts lack the coarse particles needed for aeration, and that composted animal wastes and mushroom compost are high in salts and are limited to 10 to 20% (Bilderback et al., 2013). UMass's greenhouse guidance gives about 30 to 40% as the accepted recommendation, noting that most composts are heavy, hold either too much or too little water, or start out too salty (UMass Extension, n.d.), and warns home gardeners that composts vary batch to batch and some carry nutrient levels high enough to injure roots in a container (Smith & Cox, 2015). Florida's guide to compost-based foliage media gives the specification a good compost should meet before it goes in a pot (Chen & McConnell, 2020). Worm castings (vermicompost) are used in smaller doses: one extension presentation suggests one part castings to four parts potting media for seedlings, and a couple of tablespoons per quart of mix for established pots (Sherman, 2021). For houseplants we treat compost and castings as a minor fraction for the same reason the nursery guide does: fine, rich material lowers air and raises salts, and a pot has no room to spare for either.