Mixes age

Everything in Lessons 2 through 6 described a mix on the day you made it. Pots don't stay there. This lesson is about the four ways a substrate changes with time, and the signs that tell you it has.

It gets finer, and wetter

Organic ingredients decompose during a growing cycle, and that changes the air-and-water ratio of the mix (Bilderback et al., 2005). The mechanism is Lesson 2 in reverse: decomposition creates an overabundance of small particles, which hold excessive water and leave limited air (Bilderback et al., 2005). The measurements agree. In peat substrates, air space fell and easily available water rose over 14 months, with no net change in total porosity (Allaire-Leung and colleagues, as summarized in Altland et al., 2011). In Douglas fir bark, with or without pumice, air space fell over 48 days and again over 382 days while container capacity rose, and a plant growing in the pot tended to make the loss of air worse (Altland et al., 2011). That last point matters: roots fill pores too, so even a mix that isn't decomposing much loses air as the plant fills it.

Unstabilized organic ingredients can go fast. The nursery guide warns that un-aged components can decompose quickly enough to leave a container that started full only three-quarters full within weeks or months (Bilderback et al., 2013). Over long production cycles, the same review says, "substrate in large containers becomes spongy," and the greatest disadvantage of decomposition is that air space may drop too far, which calls for careful irrigation to avoid waterlogging (Bilderback et al., 2005). A mix that has broken down this way holds more water and less air after each watering than it did fresh, so the old routine can now keep it too wet; check the mix before watering rather than keeping to the schedule.

It settles and shrinks

Aging mixes drop in the pot. Purdue's guide says a substrate should ideally keep a consistent structure with minimal decomposition, and that if the volume drops significantly during a crop it can be detrimental (Owen & Lopez, 2015). Shrinkage also happens from drying: a high-peat mix left unwatered loses water from the surface, shrinks, and pulls away from the pot wall so that water channels down the gap instead of wetting the root ball (Owen & Lopez, 2015). If you see a gap between mix and pot, or water that runs out the bottom almost as fast as it went in, that's what you're looking at.

It gets saltier, and its pH moves

Salts from tap water build up in a pot as water is taken up by the plant and evaporates, leaving the dissolved minerals behind (NMSU Southwest Yard & Garden, 2003). Fertilizer adds more; in extreme cases excess fertilizer salt shows up as a crust on the surface, and watering thoroughly so that excess runs out the bottom flushes it (Steil, 2024). Module III, Lesson 7 covered the leaching routine. Over the same months, the acidity the organics started with fades: as peat and bark break down, their ability to impart acidity can decrease (Pennisi & Thomas, 2023), which is one more reason an old pot's pH can sit somewhere different from where it began.

Reading an old mix

The signs, then. A mix that's old, broken down and compacted holds more water than it used to; Iowa State's advice for a plant in that state is to repot into fresh, well-drained mix (Steil, 2024). New Mexico State makes the same call from the other direction: potting soils with compost, peat or other organic matter decompose over time, which can change how the pot drains, so check the roots and repot before the plant declines rather than after (NMSU Southwest Yard & Garden, 2003). Unhealthy roots can smell foul (Pennisi, 2022), and we treat a sour smell from the mix as a sign it's staying too wet. A white crust at the surface or on the pot rim is salt (Pennisi, 2022). A gap at the pot wall is shrinkage. And a root-bound plant (one whose roots have filled the pot) shows it differently: Penn State lists a mix that dries out very quickly even in cool weather, roots growing out through the drainage holes, and growth slowing despite feeding, and says to repot if roots circle the root ball or fill its bottom third (Concklin & Marquesen, 2026). Notice that an old mix and a root-bound plant send opposite watering signals, stays wet versus dries in a day, and both end in the same place.

Fig. 11. The same mix fresh and aged. As organic ingredients decompose into smaller particles, a mix holds more water and less air (Bilderback et al., 2005); it settles and shrinks (Owen & Lopez, 2015); a mix that dries out can pull away from the pot wall (Owen & Lopez, 2015); and a white crust at the surface or on the pot rim is salt (Pennisi, 2022).

That place is a repot into fresh mix, which is the whole of Module VIII: when to do it, what to move the plant into, and how to nurse the recovery. One piece of it belongs here because it's about the substrate: when you repot, remove much of the old mix, because that's where the accumulated salts are (NMSU Southwest Yard & Garden, 2003). For now, the skill is recognizing the mix has aged before the plant has to tell you.