The minerals, and the buffer

Module I's recap said a plant runs on four inputs: light, water, air, nutrients. Modules II and III covered the first two, and Lessons 1 and 2 of this one covered air. This is the fourth. Substrate is the thing that holds nutrients between feedings, so this is the right place to meet them, even though how and when to feed is Module VI's job.

The 17, and where they come from

Plants need 17 essential nutrients. Three of them, carbon, hydrogen and oxygen, come from air and water and make up the bulk of the plant: a Purdue guide gives 89% of dry weight, a Montana State module about 95% of biomass (England et al., 2012; Jones & Olson-Rutz, 2016). Those three never come out of a fertilizer bag (England et al., 2012). The other 14 are mineral nutrients, taken up by the roots from the root zone (Shober & Denny, 2025). Six are macronutrients, needed in larger amounts: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S). Eight are micronutrients, needed in small amounts: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn) (Shober & Denny, 2025; England et al., 2012). Small amounts doesn't mean unimportant; Lesson 3's iron story is a micronutrient story.

Mobile and immobile, and why it shows where it shows

This is the idea Modules VII and IX will lean on hardest, so take a minute with it. A nutrient is mobile if the plant can move it from one part of itself to another as needed, and immobile if it can't (Shober & Denny, 2025). When a mobile nutrient runs short, the plant pulls it out of old leaves to supply new growth, so the shortfall shows first in the oldest, lowest leaves. When an immobile nutrient runs short, the old leaves keep what they have and the new growth goes without, so the shortfall shows first at the top (McCauley et al., 2011; Jones & Olson-Rutz, 2016). Mobile nutrients travel in both the xylem and the phloem, the two pipelines from Module I; immobile ones ride the xylem up and don't come back down (Vashisth & Oswalt, 2020).

Which is which? The University of Delaware's fact sheet lists nitrogen, phosphorus, potassium and magnesium as mobile, with deficiencies appearing first in old growth, and calcium, copper, boron, iron, manganese and zinc as immobile, appearing first in new growth (Shober & Denny, 2025). Extension sources agree on that core and differ at the edges: sulfur is listed as mobile by Florida, immobile by Kansas State, and "between the two" by Montana State depending on how severe the shortage is; molybdenum lands on different sides too; and Montana notes zinc is only somewhat immobile, with symptoms starting on the middle leaves (Vashisth & Oswalt, 2020; Ruiz Diaz et al., 2011; Jones & Olson-Rutz, 2016; McCauley et al., 2011). Research on crop plants has found calcium and manganese, usually counted as poorly mobile, being moved out of leaves in wheat and barley, so even the categories are "generally," not "always" (Maillard et al., 2015).

Fig. 7. When a mobile nutrient runs short, the shortfall shows first in the oldest, lowest leaves; when an immobile one runs short, it shows first at the top (McCauley et al., 2011; Jones & Olson-Rutz, 2016). Delaware's fact sheet lists nitrogen, phosphorus, potassium and magnesium as mobile, and calcium, copper, boron, iron, manganese and zinc as immobile (Shober & Denny, 2025). Treat where yellowing starts as a first clue, not a diagnosis.

So treat old-versus-new as a first clue that narrows the field. Cornell's guidance is the right register: leaf symptoms are only clues, slight deficiencies are hard to see, different deficiencies can look alike, the same species may respond differently, and things that have nothing to do with fertility produce leaf symptoms as well (Colucci, 2022). Module IX teaches the method that turns a clue into a test.

What a substrate holds: cation exchange

Now the substrate's part. Many nutrient ions carry a positive charge; chemists call these cations, and potassium, ammonium (a form of nitrogen), calcium and magnesium are all cations. Some substrate particles, clay, organic matter and vermiculite among them, carry fixed negative charges that attract and hold positively charged ions, then release them for uptake (Owen & Lopez, 2015). A substrate's capacity to do that is its cation exchange capacity, or CEC (Argo, 2004). Nitrate, the other common form of nitrogen, is negatively charged, so it isn't held this way and leaches easily; ammonium is positively charged and is held by substrate particles (Pennisi & Thomas, 2023).

Fig. 8. Some substrate particles carry fixed negative charges that attract and hold positively charged ions, then release them for uptake (Owen & Lopez, 2015). Potassium, ammonium, calcium and magnesium are cations; nitrate is negatively charged, so it isn't held this way and leaches easily (Pennisi & Thomas, 2023).

Ingredients differ a great deal. Peat, coir, vermiculite, clay and most composted organic matter have high CEC; sand, perlite, polystyrene, rock wool and uncomposted hulls have low CEC (Owen & Lopez, 2015), and perlite in particular has essentially none (Pennisi & Thomas, 2023). The numbers themselves move with the method and the sample, which is why we don't put a single figure on anything. A greenhouse course gives sphagnum peat 90 to 140 and coir 39 to 60 milliequivalents per 100 grams, a unit of charge where higher holds more (Evans, n.d.). One 2026 study measured ingredients the same way after a rooting trial and found coir at about 103, pumice about 15, perlite about 13 and the zeolite it tested about 10; mixing any of those minerals into coir at a quarter or more of the volume lowered the mix's measured capacity (Monsalve Camacho et al., 2026). Different methods, different numbers, same ranking: the organic ingredients hold a lot, the mineral aggregates little.

How much does CEC matter in a pot? The sources disagree

Here we have to show you a disagreement rather than hide it. William Argo, writing in 2004 in an orchid society journal, says that unlike field soils, research has shown the exchange capacity of peat, coir or bark substrates "has little effect on resisting change in pH or in supplying nutrients" (Argo, 2004). Purdue's substrate guide says to look for at least 6 to 15 milliequivalents per 100 cubic centimeters in a greenhouse substrate, and that substrates with low CEC need more frequent fertilizing (Owen & Lopez, 2015); a 2026 review credits peat's higher CEC with less fertilizer leaching and less pH drift than coir and other components (Altland & Bugbee, 2026); and a bark-substrate study notes CEC affects how much ammonium and potassium leach and how pH drifts (Altland et al., 2014). Read together, our summary is this: a substrate's exchange capacity is small compared with what you feed, so feeding does most of the work; the sources that measure it say it still affects how much ammonium and potassium stay in the pot between waterings and how pH drifts, and Argo's reading of the research is that the effect is slight.

What nobody disputes is where the nutrients come from. Soilless substrates don't contain enough nutrients to grow a plant well, and they bind nutrients less tightly than soil does, so nutrients leach readily and growers supply them with water-soluble or controlled-release fertilizer; the "starter charge" in a bag isn't enough to carry a plant, and a fertilizer program should start soon after planting (England et al., 2012). One review goes as far as calling soilless growing a kind of hydroponics (growing in a nutrient solution rather than soil): the plants are fed a dilute nutrient solution each time they're watered (Altland & Bugbee, 2026). That's the practical answer to the pumice-and-perlite worry. A mix with little exchange capacity isn't a mix that can't feed a plant; it's a mix that holds less in reserve, so it relies on you.

A note on zeolite

Zeolite comes up in our recipes, so it belongs here. Clinoptilolite, the natural zeolite growers use, holds potassium and ammonium strongly on its exchange sites; its measured capacity is modest among zeolites, about 2.2 milliequivalents per gram, with a selectivity order that puts potassium and ammonium near the top (Mumpton, 1999). Zeolites also hold water inside their framework, up to 30% of their weight for zeolites in general (Grifasi et al., 2025), and measuring the real exchange capacity of a natural zeolite is difficult because of impurities and other minerals that vary with where it came from and how it was purified (Grifasi et al., 2025). Some growers add 2 to 15% clay-type amendments such as zeolite to soilless media, which may improve nutrient retention and physical properties (Robbins & Evans, 2005); in one forest-nursery trial with peat, 5% zeolite gave the best European beech seedling growth and 20% reduced root growth, which the authors attribute primarily to a potassium shortfall from calcium and magnesium competing for uptake (Kučera et al., 2025). We use zeolite in some recipes as our own choice, for its hold on potassium and ammonium. We don't claim it raises a finished mix's exchange capacity or air space by any set amount, and the 2026 study above is one reason not to.