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https://www.tomatojunction.com/viewtopic.php?f=10&t=2375&p=44515#p44515•
https://youtu.be/VeEpEnFulF4See the links above. It's remarkable how much the tomato stems thicken. I looked at the product, and it didn't say what was in it besides silica and a little potassium (but there could be other stuff). It's used with hydroponics and container gardening, I believe.
Warm soil seems to thicken stems, too (but I think it does it differently). Phosphorus probably thickens stems, too.