I believe silt is predominantly quartz crystal with a ‘Moss’ defect and I guess feldspar that has been ground to a size, by definition, between sand and clay in size. SpringerLink - Journal Article
, Silt - Wikipedia, the free encyclopedia
The reason for collecting silt from a delta is silt tends to deposit in areas that experience flooding. Silt and clay principally account for water turbidity; silt I think is a eutectic or a near eutectic mixture. I can’t, off hand, find a citation.
Silt in and of itself should have no organic nutritive value. Clean silt really should not smell (or rot).
I do not pretend to know Tom Barrs’ mind in recommending silt; I have yet to read that, I assume (always dangerous) that it relates to the high cation exchange capacity, CEC, coupled with silt’s easy availability and low cost (effort) to obtain.