fungus gnats and seed starting

Rodger Whitlock
Wed, 09 Nov 2005 12:05:54 PST
On  9 Nov 05 at 11:58, Linda Foulis wrote:

> ...I garden indoors a lot. I started off using a mix of
> 1-1-1, peat-vermiculite-perlite.  Over the years I've
> reduced it to half and  half of peat and vermiculite. 
> Needless to say, I'm still looking for the ideal mix...

> I have gnat problems every year and I'm convinced that they
> comes in with the peat.

I'm pretty sure I'm repeating myself, but evidently we have at 
least one newcomer who might benefit, so please, everyone, be 
tolerant of reading this all again.

Twenty-five or more years ago, I heard a talk by a local herb
grower, Capt. Bob Chicken, on his methods. Among the topics he
discussed was the choice of soil mix. He objected to soilless
mixes (variously mixed from peat, perlite, and vermiculite)
because they lacked any clay fraction and therefore had little
or no adsorptive capacity for nutrients. [N.B: *ad*sorptive,
not *ab*sorptive]

He opined that the intended use of these soilless formulas was
in sunny California where dilute liquid feedings could be
applied regularly. Here in the cloudy Pacific Northwest, if
you fed these mixes adequately, you ended up with a soaking
wet soil all the time, uncongenial to many plants.

His solution was to switch to a soil-based mix, the classic 
John Innes formula. Because almost all soils have a significant 
clay fraction, soil-based mixes adsorb nutrients easily 
releasing them to the plants slowly.

Fungus gnats seem to be related to the use of very peaty mixes 
and mixes with high organic content, and a JI-type formula in 
my experience doesn't support them as well.

These days, I use a simplified soil mix formula combining soil
bought in bags from Canadian Tire, perlite, fertilizer, trace
elements, and lime. The soil comes from an old lake bed,
contains a surprising amount of twiggage and such (you must
screen it), and includes both organic materials and silt, the
latter providing that all-important clay fraction, the former 
the moisture retentiveness we associate with peat.

But note: no added peat.

Moral: consider switching to a soil-based mix. You may find 
your fungus gnats dwindling away once they're deprived of the 
gooey, peaty soils they seem to love.

It's unlikely that CT carries the exact same soil (Island's 
Finest "Topsoil") in your area, but they or a local 
garden center probably have something similar. 
Rodger Whitlock
Victoria, British Columbia, Canada
Maritime Zone 8, a cool Mediterranean climate

on beautiful Vancouver Island

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