HELEN CZERSKI
Why lazy gardeners should start watering their plants
POTTED PLANTS CAN be ungrateful things. I’ve usually got a bit too much on the go, and the nice thing about plants is that you can get on with being busy for a while and they’ll probably still be alive when you come back.
But when you bounce along with your watering can after a period of (let’s be honest) neglect, full of good intentions to make it up to them, they often reject your offerings. This week, I noticed the dry soil around the herbs on my balcony, and went out with water to help. But the water sat in jaunty droplets on top of the soil, and as I added more, the droplets joined together and flowed out to the sides, leaving seemingly untouched dry soil in their wake. It’s a weird thing to watch, and it only happens when the soil is dry to start with. I’ve seen this happening for years (I’ve neglected a lot of plants in my time), but it was only this week that it really occurred to me to wonder what was going on.
With my physicist hat on, it seems that this is what you’d expect from a hydrophobic material (one that repels water). But soil can’t be hydrophobic – one of the main reasons that plants can grow in it is that it’s fantastic at transporting and holding water. Soil is made up of specks of rock, organic material and microbes mixed together with gaps in between, and water seeps into all those tiny holes and tunnels really easily. Those gaps act like the pores in a kitchen towel soaking up a spill, and water will creep along the surfaces, filling in the gaps as it goes. Any plants parked in the soil will have access to whatever water is nearby, because the soil acts like a sponge and holds the water in.
I didn’t give up on watering the parsley, because I know how to get around the problem. You make a little dip in the soil and fill that with water so that it can’t run off immediately. And after a few seconds, it seeps in, wets the soil, and then when you pour more water on it’s taken up immediately. But this sounds like Catch-22 – if the soil is only hydrophobic when it’s dry, how do you get it wet enough to make the problem go away?
After a bit of investigation, I found that the problem occurs when the amount of water in the soil falls below approximately a quarter of the
“The molecules are also assembling
a reprimand to lazy plant-carers like me. Get that watering can out
more often!”
total volume. At this point, the soil particles become coated with lipids and wax that are produced by either the plant or the microbes around it. Oily and fatty substances repel water, so the water droplets just sit on top of the dry and waxy soil. If you get those coatings wet for long enough, they change and the water can touch the minerals or organic matter directly. The uncovered surface can then stick to the water molecules, and the water can begin creeping into the pores and down into the soil. The details of these coatings are still a bit of a mystery, even though it’s an important problem for farmers in hot countries. If you have dry, water-repellent soil, the rains may come, but the water will simply run over the top into the nearest stream and be wasted.
It’s weird to think of tiny organic molecules shuffling around the soil as it dries out, building up a waxy wall between the soil particles and the air. But as well as constructing a coating, the molecules are also assembling a reprimand to lazy plant-carers like me. Get that watering can out more often!