Vermiculite is a mineral; mica, to be exact. When heated, it puffs up, and in this state it absorbs and holds water. However, due to its tendency to compact, it shouldn't make up more than 25 percent of a potting mix.
Perlite is a white volcanic substance that is a derivative of silica. It too is heated so that it puffs up. Yet, perlite does not readily absorb water, making this the main difference between it and vermiculite.
Vermiculite is used primarily in potting mixes (because of its water-holding capacity) and perlite for rooting cuttings. However, because perlite is sterile, the cuttings must be replanted in a potting mix that contains nutrients. Do this once roots begin to form.
You can use both perlite and vermiculite interchangeably. Because perlite and vermiculite are roughly one-tenth the weight of sand, they are ideal additions to potting mixes for folks who garden on terraces or rooftops where weight is an important consideration.
__________________
Mischka
One last word of farewell, Dear Master and Mistress.
Whenever you visit my grave,
say to yourselves with regret
but also with happiness in your hearts
at the remembrance of my long happy life with you:
"Here lies one who loved us and whom we loved."
No matter how deep my sleep I shall hear you,
and not all the power of death
can keep my spirit
from wagging a grateful tail.
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