Some threads never die
On page 48 of the 1983 edition of Exotic Tropical Fishes, Herbert
"European aquarists sometimes use peat, loam or earth beneath the
gravel or in pockets of rock to nourish the plants. General
experience is, however, that such materials sooner or later cause
trouble; either they get stirred up and make a mess of the water or
they turn bad and have to be removed. If one is interested primarily
in growing fine plants it is possible to take care and use these
substances, but they are hardly to be recommended for the amateur.
The plant growth that results is from the material supplied beneath
the gravel, and the plants may not, therefore, be performing their
essential function of removing much of the waste products of the
fishes as well as they otherwise would. This is a point which hasn't
been clarified, and further research and observation are needed."
This discussion sounds vaguely familiar...