The mental model
Substrate, then, as one system. Particles make pores. Pores hold water or air, depending on their size, and the pot's height decides how much of the wet band at the bottom the roots have to live with. The ingredients decide how long that balance lasts and how well the mix buffers pH and holds nutrients between feedings. And pH stands at the gate, deciding which of the nutrients in the pot the plant can actually use. Read the signals below, then go measure.
Fig. 13. Substrate as one system. Particles make pores; pores hold water or air depending on their size; the pot's height decides how much of the wet band the roots live with; the ingredients decide how long that balance lasts and how well the mix buffers pH and holds nutrients; and pH stands at the gate.
Read the signal, name the cause
Tap a symptom on the left, then tap the explanation on the right that fits it. Each one is a substrate story you've now read.
The symptom
What's actually happening
How this connects to the rest of the curriculum
Substrate is the supply side of nearly everything else:
- Module III (Water, Properly) is the other half of this module: the mix sets how wet each height of the pot is, and watering decides how often you refill it.
- Module V (Containers) takes the pot's side: height, material and drainage, and how much of the saturated band a given pot keeps.
- Module VI (Feeding) picks up pH, alkalinity and the mineral nutrients from Lessons 3 and 4 and turns them into a fertilizer routine.
- Modules VII and IX (Reading a Plant, Diagnosis) lean on the old-leaf-versus-new-leaf rule you learned here.
- Module VIII (First Repot) is what you do when Lesson 7's signs show up.
Eight things to remember and use
- A substrate has four jobs: support the plant, provide air, hold water, and hold nutrients. Air and water share the same pores, so more of one means less of the other.
- Particle size sets pore size. Narrow pores hold water against gravity; wide pores drain and fill with air. Finer isn't automatically better, and coarser generally means more air and more frequent watering.
- An air-space number without a pot height is half a number. In the nursery guide's example, the same mix leaves a saturated band of the same height in a tall pot and a short flat, so it takes a bigger share of the short one.
- Published air-space guidance describes a broad workable range, roughly 10–30% air space after draining, written for nursery and greenhouse production. There's no single right number; measure your own mix in its own pot.
- pH decides which nutrients are available. In soilless mixes iron goes unavailable as pH rises and shows as yellowing between the veins on new leaves. The water's alkalinity, not its pH, is what pushes a pot's pH up over time.
- Mobile nutrients (nitrogen, phosphorus, potassium, magnesium) show a shortfall in old leaves first; immobile ones (calcium, iron and most micronutrients) show it in new growth. It's a first clue, not a diagnosis.
- In soilless mixes the fertilizer supplies the nutrients. Exchange capacity helps a mix hold some between feedings, and sources disagree about how much that matters; a low-capacity mineral mix simply leans on regular feeding.
- Mixes age: organics break down, particles get finer, roots fill pores, air space falls, salts build and the mix settles. When a pot stays wet longer than it used to, or pulls from the pot wall, the mix is telling you it's time.
Measure the air in one pot
You've read the ranges. Now get a number of your own. Use the pot a plant will actually live in and the mix you'd actually use, because the answer depends on both. Follow the steps from Lesson 8 and record what you find. There's no pass mark; the point is to see where your mix sits and to feel how much water a pot of it really holds.
Fill in each line as you go. Use the same unit (cups or milliliters) throughout. The example column is one made-up run, so you can see the arithmetic.
| Step | What to record | Example | Your pot |
|---|---|---|---|
| 1 | Pot volume (fill the empty pot with water and pour it into the measuring cup) | 6 cups | |
| 2 | Mix used (name it, and note the texture: fine / medium / chunky) | Bagged mix, medium | |
| 3 | Water added to saturate (liner in, mix firmed, add slowly until the surface shines; cover, wait an hour, top up) | 3 cups | |
| 4 | Water drained in 10 to 15 minutes after piercing the liner | 1 cup | |
| 5 | Air space as a share of the pot: step 4 ÷ step 1 | 1 ÷ 6 = 17% | |
| 6 | Air space as a share of pore space: step 4 ÷ step 3 | 1 ÷ 3 = 33% | |
| 7 | Water held after draining: step 3 minus step 4 | 2 cups | |
| 8 | Where does step 5 sit against the nursery band of 10 to 30%? Does that match how this mix behaves for you? | Inside it; dries in about a week |
Notes: Firm the mix the way you'd pot a plant, not harder. Measure in the pot the plant will live in; a shorter pot will give a lower number for the same mix. If you have two mixes, run both and compare.
Look at step 7 for a moment: that's how much water your pot carries after a good watering, and every bit of it has to leave through the plant or the air before the roots get their pores back. Now you know what the watering can is really doing.
"I've read this module, and I've measured the air space of at least one mix in the pot it lives in."
This marks Module IV complete and carries your progress forward. The work is done by your own honest account. No points, no badge, no leaderboard.