home made fertilizer

Toby Hemenway hemenway at jeffnet.org
Tue Mar 20 13:00:44 EST 2001

on 3/19/01 10:51 PM, Evan Davies at evangardener at hotmail.com wrote:

> The reason there is no smell using the sawdust method to store urine, is
> that sawdust, or wood shavings(sawdust will absorb the liquid a lot
> quicker)is a large proportion carbon

For those who are interested, that's one of several reasons. BTW, I prefer
using fine shavings unless I can get coarse sawdust, because the sawdust
from all but the big mill saws is too fine, and often packs down into a
gooey, anaerobic lump that gets really foul in the center and is hard to
spread out evenly in a compost pile. Which leads me to additional reason 1:
Using shavings or sawdust, which have lots of  air between the particles,
results in aerobic fermentation of the urine, while a bucket of liquid will
quickly become anaerobic. Anaerobic processes are far more stinky than
aerobic ones. Aerobic fermentation will yield more nitrate (NO3), not a gas,
while anaerobes, lacking those oxygen molecules, are forced to make gaseous
ammonia, NH3. Bye-bye nitrogen.

 Also, shavings and sawdust have lots of surface area, allowing quick
colonization by huge numbers of microbes, which will metabolize the ammonia
and other volatiles in decomposing urine before much of it can outgas. Any
gases from the puddle in the bottom of the bucket must rise up through this
fluffy bacterial filter before they can escape, and get absorbed. And third,
wood has an extremely high C/N ratio, 400-800 to 1, so any nitrogen in the
urine will be quickly gobbled up by microbes to be bonded to the
overwhelming amount of carbon they get from the sawdust. The bugs are
building molecules of roughly 12:1 C/N, so they'll lock up nitrogen quickly
to do that (which is the opposite situation we hope to have when we're
fertilizing, where we want to release nitrogen--slowly--rather than tie it

I like to build my compost piles on the woody side, more like 40:1 C/N.
Nitrogen is lost rapidly even in compost piles that are built to the ideal
25:1 ratio. In a fast, hot pile of 25:1, the rapid metabolisms of the
microbes often lurch into anaerobic processes and give off a lot of ammonia.
You can catch that ammonia by adding gypsum to the pile (the CaSO4 of gypsum
reacts with ammonia to give non-volatile ammonium sulfate) or, if there
aren't lots of seeds to sterilize in the pile, I just make a woodier 40:1
pile and let it moulder more slowly. This cooler pile seems to leave more
nitrogen in the compost. At least the resulting soil tests higher for
nitrogen, so I assume that's what's happening.

Hope this isn't too tedious. Thanks, Evan, for giving me an excuse to
blather about soil chemistry, one of my pet subjects.


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