Roots need air

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

It sounds wrong. It's easy to picture roots underground drinking water all day like a straw in a glass. That isn't what's happening.

Roots are living tissue, and they need oxygen. They respire, meaning they burn sugar with oxygen to release energy, the same thing happening in your own cells right now. In a pot, the air that supplies that oxygen sits in the pockets between the particles of substrate, the potting mix a container plant grows in. Not in the mix itself: in the gaps between its particles. A good container mix should have roughly 20 to 25 percent air space once it has drained (Nemali, 2018).

When you overwater, the harm comes through the air: water fills those pockets and pushes the air out. Roots short of oxygen can struggle to take in water: when aspen seedlings growing in nutrient solution had the oxygen around their roots cut to a low level and left there for a week, water moved through the roots far less easily (Tan et al., 2021). In hydroponic tobacco, a week of low root oxygen made no difference to how readily water moved through the roots, and the authors note that other studies have found that flow reduced or unchanged depending on species and setup (Tan & Zwiazek, 2019). Leave the substrate waterlogged and the fine roots are damaged; the main sign is wilting or yellowing of the lower, inner leaves, even though the pot is wet (University of Maryland Extension, 2023). 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 one fact sits behind several of the decisions you'll make:

• Substrate matters because some hold more air than others. (Module IV)
• Drainage matters because even in a pot with drainage holes, a layer of mix at the bottom stays saturated and holds almost no air. (Module V)
• Pot size matters because a pot that's much too big gives the roots extra mix, and the extra mix holds extra water and stays too wet. (Module V)
• How you water matters because once the extra water drains out after a watering, air moves back into the larger pores. (Module III)
• Repotting matters because over time a mix breaks down and gets packed tighter, and packing cuts its air space. (Module VIII)

Four later modules, one insight.

Fig. 2. Roots draw oxygen from the air pockets between substrate particles. Fill those pockets with water and far less oxygen reaches the roots.