On 02/10/2016 20:04, Chris Hogg wrote:
On Sun, 2 Oct 2016 18:44:06 +0100, Stuart Noble
wrote:

tephra which has since weathered to rich soils

I gather tephra is just Greek for ash. So how does ash weather into
rich soil? I must be missing something, like where the nutrients come
from. Nothing much will grow in ash surely.


It's a slow process, and AIUI two things happen in parallel. Simple
organisms like algae first colonise the ash, followed by increasingly
more complex plant forms. At the same time, weathering breaks down the
ash, releasing nutrients such as phosphate and potash, together with
trace elements, and changing the mineral composition of the ash, often
to the clay mineral montmorillonite, found in many weathered volcanic
ash deposits. Montmorillonite has the ability to absorb cations (pron.
cat-ions) such as calcium, sodium, potassium, magnesium etc. and
release them slowly over time or as conditions change. The combined
effect of these two processes is very slowly to make a humus-rich and
very fertile soil. The weathering can take centuries. The island of
Surtsea, off Iceland, which appeared between 1963 and 1967, showed
signs of plant life in 1965, and is now mostly covered in mosses and
lichens, although higher plant forms are now beginning to appear. As
they die and rot down, their compost will contribute to the soil and
assist other plants to grow.

At least, that's what I understand.

See also http://tinyurl.com/hqyq36c, http://tinyurl.com/hhf2z5h and
http://tinyurl.com/jeq7uk7

Many thanks, Chris. That explains everything! Fascinating stuff,
particularly Surtsea, which we're able to observe in real time. 2-5 new
species a year. Impressive!

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