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  • From: Sojourner <sojournr AT missouri.org>
  • To: market farming <market-farming AT franklin.oit.unc.edu>
  • Subject: Green manure and mulch, was Re: Horse manure/sawdust
  • Date: Mon, 31 Jan 2000 15:28:12 -0600


Alex McGregor wrote:
>
> Sojourner wrote:
>
> > The belief that sawdust will "tie up the nitrogen" in your soil is a
> > myth based on studies done with sawdust that had been "worked in" to the
> > soil - e.g., was decomposing (or rather not) in anaerobic conditions.
> >
> > Spreading sawdust on TOP of your soil will not cause a nitrogen
> > deficiency. Aerobic decomposition, taking place in the presence of
> > oxygen (which is what goes on when you surface spread) is a whole
> > 'nother animal than anaerobic.
>
> It is true that adding any dead plant material to the soil
> will "tie up" the nitrogen. This means that the nitrogen
> is used by the population explosion of the soil life that
> happens when you add a lot of "food" for them. The nitrogen
> is still there- just in the bodies of the living microbes and
> they can out compete the plants for N.

I THINK you're agreeing with me? Under these circumstances the microbes
are utilizing nitrogen from the air, and not from the soil. So they're
not "tying up" SOIL nitrogen, which is what most people worry about.
And since our atmosphere is, what, 70%? Nitrogen, they're not really
outcompeting plant life for airborne Nitrogen, either.

Then, as the little soil buggers die off and their tiny little bodies
rot, extra nitrogen that would otherwise have remained unfixed
(free-floating in the atmosphere) is added to the soil.

Your plants will thank you for this, really they will, by giving you
better yields of healthier looking veggies. This is an ongoing,
sustainable process - doesn't happen overnight, like with chemical
fertilizers. But unlike chemical fertilizers (which are about the
equivalent of a nuclear bomb as far as soil life is concerned) it not
only increases your soil fertility but also improves tilth and soil
composition as well.

> Adding material to the surface as mulch slows down the
> decomposition and most of the "Feeding frenzy" is going on
> at the soil surface. This results in N in the soil still
> being available to plants.

Right.

> As far as aerobic vs. anaerobic- plant material or manure
> buried too deep is where anaerobic bacterial breakdown
> happens. The byproducts of this process are plant toxins
> (formaldehyde being a major one).

Right. 20 and 30 years ago it was common for garden books to recommend
digging down a foot or two, moving the top layer of soil from the next
trench to the bottom of the first trench so you were completely turning
upside down your garden area soil. This would bury all organic matter
from the top layers at the bottom of these trenches. In my Ohio clay
soil, the stuff would never rot - I'd come along the following year and
start digging again, and there it was, all the good stuff I'd buried
last year, still unrotted and smelly to boot. Must have been the
formaldehyde.

> Burying it in the top of the soil where gas exchange happens
> produces aerobic decomposition. The byproducts of this are
> plant nutrients.
>
> So, the moral of the story is to bury cover crops, etc. as
> shallow as possible and use all the mulch you want.

Right. I actually don't "till in" any more, I only till where I have to
plant and that basically so I can get at the dirt. I have used
buckwheat to improve really bad heavy clay soil (there are better cover
crops but none that I know of that will do as well as buckwheat will in
heavy compacted clay). Typically I plant in early spring, let it go to
seed, and mow it where it stands. It reseeds itself this way. You can
do this for at least 2 crops in Ohio, I think I occasionally got 3 this
way. I can definitely get at least 3 down here in MO (although my soil
here is so much better that I don't really need the buckwheat). The
trick is to mow your LAST buckwheat crop before it sets seed if you
don't want it to self-seed again, say if you want to put in winter rye
as a winter cover crop. If you cultivate at all, I wouldn't cultivate
more than the top 2" or 3" of soil. Anything more than that and you're
just wasting all that nice humus the buckwheat grew for you.

I haven't decided how I'm going to handle things now that I'm moving
from a small garden (my last garden was around 20x40 with half fallow
every year in cover crops) first to a mid-sized garden (50x60 or so) and
then hopefully to a market garden.

In my home garden I've always used raised beds in permanent enclosures,
but that's a bit much for a larger garden. This year I'll just sow
white dutch clover, plant in blocks, and mulch heavily with sawdust, and
see how it goes.

--
Holly ;-D
Contrary Peasant
sojournr AT missouri.org



  • Green manure and mulch, was Re: Horse manure/sawdust, Sojourner, 01/31/2000

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