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Monday 25 March 2013

Crocks (broken clay and terracotta) at bottom of pots Daily Telegraph 23rd March 2013

'Fill the bottom of the pot with extra drainage material, such as polystyrene pieces or crocks,” says the RHS book How to Garden. Most people seem to agree: the BBC Gardeners’ World website has instructions on planting up scores of different kinds of pots, but all begin with some variation on: “Place a layer of crocks in the bottom of the pot to improve drainage.” So there you are, summer is just around the corner and you’re happily chucking broken crocks into the bottom of a plant container before adding some compost. While you’re doing this, “textural discontinuities”, “capillary barriers” and “funnelled flow” are probably not uppermost in your mind. But maybe they should be. Soil scientists, hydrologists and environmental engineers have long known that peculiar things happen at the junction between two layers of soil with different textures, and especially when a fine layer sits on top of a coarse layer. For example, scientists trying to track the movement of fertilisers, pesticides or other contaminants down soil profiles sometimes find that if the stuff they’re following encounters such a discontinuity (especially if it’s not perfectly level), it can stop heading downwards and zip off sideways, ending up a long way from where they expected to find it. Fair enough, you may think, but what has that got to do with me, and can I go back to planting up my pots? Yes, in a minute, but first here’s another funny thing. Because it resists compaction and provides good drainage, sand is the basis of most modern golf course putting greens. But the downside of sand is that it holds little water, dries out rapidly and needs a lot of watering. The most popular solution to this problem is around 300mm of sand over a 100mm layer of gravel. Capillary forces within the sand mean that water is unwilling to cross from the (relatively fine) sand to the (much coarser) gravel, creating what hydrologists and geologists call a “perched” water table, essentially one that is higher up than it should be, and above the “real” water table. Maybe you’re now starting to see the parallel between the sand and gravel beneath a putting green and the compost and crocks in your plant pot. Both are a fine layer over a coarse layer. But the former is designed to reduce water loss from the fine layer and keep it wetter than it would otherwise be, while the latter, if we believe the gardening books, is to improve drainage and keep the fine layer drier. They can’t both be right, although in a sense they are. During heavy rain, the putting-green sand layer eventually becomes saturated, gravity overcomes capillary forces and the water has nowhere else to go but into the gravel, where it drains away rapidly. So the sand/gravel sandwich is well-drained. But once the surplus water has drained away, the sand remains wetter than it would be if it were just sitting on more sand. Exactly the same happens in your plant pot. When you pour enough water in the top of the pot to saturate the compost, gravity overcomes the capillary barrier at the compost/crocks boundary and it drains away through the crocks and out of the drainage hole. But it would do exactly the same if the crocks weren’t there, and when you stop watering, you’re left with a perched water table in either case, crocks or no crocks. The only difference is that if there’s a layer of crocks, the water table is perched at the compost/crocks boundary, and if there isn’t, it’s at the bottom of the pot. So there’s no harm in continuing to bung crocks in the bottom of containers if you feel you ought to, or because your mother did, but be aware that their only practical effect is to reduce the volume of compost available for plant roots. More at:

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