The secret to soil revealed!

Originally Posted by Nina

However, the sharp edges do NOT -- repeat, do NOT -- cause roots to divide.

True. If you look at roots, they do not have a dichotomous branching pattern- that is, they don't split in half, and in fourths, and in eighths, etc., the way they would if the tip were constantly splitting.

The tip elongates, and the way it manages to push its way through soil or sand or clay no matter how hard is by forming a slimy root cap that lubricates its way through cracks and crevices. Farther up along the older root, lateral roots form from a layer deep in the root. These are the roots that make a root system bushy. Partly the trait of bushiness is under genetic control, and partly it depends on the soil type- compact or loose, waterlogged or dry. But "sharpness" has nothing to do with it.

Nina, whose Ph.D. was on root anatomy.

Originally Posted by malhomme

However, regarding particle sharpness and it's role in forcing roots to branch, you may be perpetuating a myth.

Originally Posted by Will

Bonsai also require the formation of fine feeder roots to assure optimal conditions in the confines of a pot and to create good nebari. If the saying that the top mirrors the bottom is true in bonsai, the fine roots are indispensable for aesthetic reasons as well. For this reason one may choose the glass shards over the round smooth marbles or the teeth over the rounded gravel

Originally Posted by rockm

We discussed why Terra cotta WOULD NOT make a good soil ingredient--depending on where you live--a while back. Frost and freezes will turn it into mush pretty quickly. Small grade crushed brick mulch sold at Home Depot or the like might be a better alternative...