Roots need air

If you remember nothing else from this module, remember this: roots need air.

It sounds wrong. Roots are underground; we picture them drinking water all day like a straw in a glass. That isn't what's happening.

Roots are living tissue, and living tissue needs oxygen. Roots respire, meaning they burn sugar with oxygen to release energy, the same thing happening in your own cells right now. They do it through their cells the same way leaves do, and they get most of their oxygen from the air pockets between the particles of substrate, the potting mix a container plant grows in. Not from the mix itself. From the gaps between its particles. A good container mix holds roughly 20 to 25 percent of its volume as air space once it has drained (Nemali, 2018).

When you overwater, the trouble isn't the quantity of water. It's that water fills those pockets and pushes the air out. Roots deprived of oxygen stop taking up water within hours, and if the substrate stays saturated they begin to break down (Pedersen et al., 2021; Morard & Silvestre, 1996). A plant surrounded by water then wilts as though it were in a desert. (This is why an overwatered plant often looks exactly like an underwatered one: same symptom, opposite cause.)

This single fact sits upstream of about half the decisions you'll ever make:

• Substrate matters because some hold more air than others. (Module IV)
• Drainage holes matter because without them, water pools at the bottom and the roots sitting in it run out of oxygen. (Module V)
• Pot size matters because too big means more substrate, more water held longer, less air for longer. (Module V)
• Drying between waterings matters because that's when the air comes back. (Module III)
• Repotting matters because old substrate breaks down, the air pockets close, and the roots run out of oxygen. (Module VIII)

Half the curriculum, one insight.

Fig. 2. Roots draw oxygen from the air pockets between substrate particles. Fill those pockets with water and that oxygen supply is cut off.