Re: [compost_tea] Re: Alaska compost tea

From: soilfoodweb@aol.com
Date: Sun Jan 26 2003 - 03:13:17 EST


Let's go through the various ways of measuring oxygen:

Total dissolved gases:
N2 gas makes up most of the atmosphere, about 75%.
Oxygen maximum is about 22%. CO2 is typically about 3%. But there's usually
a balance between these two gases. As oxygen is used up by respiring
organisms, O2 drops and CO2 increases. When O2 drops below 15 to 16%, or CO2
increases to 9 to 10%, things are getting anaerobic. As CO2 continues to
increase, the material just gets more and more anaerobic.

Dissolved Oxygen/CO2
Here the two "important" gases sum to 100. So, oxygen usually is in the 95
to 99.9% range. CO2 is then between 0.1 and 5%. As oxygen is used up, O2
drops and CO2 rises. When oxygen drops to the 70 to 75% range, things are
getting anaerobic,

Oxygen concentration
The concentration of oxygen in water can only be 16.1 mg O2/L MAXIMUM, which
occurs at sea level, at freezing temperatures. As temperature increases,
oxygen concentration drops. So at room temperature, at sea level, oxygen max
will be around 9.5 (I'm doing this from memory, so I could be off a touch,
check the USGS tables on their web site). As temp rises, oxygen
concentration drops even more, so near 100 degrees, at sea level, oxygen will
drop into the anaerobic range for most aerobic organisms.

As you go up in elevation, oxygen concentration will also decrease. So, at
2000 ft above sea level, at room temp, MAXIMUM oxygen concentration in water
will
be about 8.8 mg O2/L.

So, when you say that 0.5 mg O2/L is about 100% oxygen saturation, that is
correct, IF you are using the dissolved oxygen/CO2 scale.

Elaine
President, Soil Foodweb Inc.
www.soilfoodweb.com

In a message dated 1/24/2003 10:19:55 AM Pacific Standard Time,
elprune@yahoo.com writes:


> The measure of oxygen you want to use is oxygen concentration, not
> >percent dissolve oxygen. You need above 6 ppm, or 6 mg O2 per L.
>
> Just so we are on the same page:
>
> Dissolved oxygen concentrations are most often reported in units of
> milligrams of gas per liter of water - mg/L. (The unit mg/L is
> equivalent to parts per million = ppm).
>
> >At sea level, at 72 F, the typical oxygen concentration is around 9
> >to 9.5 mg oxygen per L.
>
> That looks like around 100% Oxygen saturation
>
> >If you are reading dissolved oxygen versus dissolved carbon dioxide,
> >you should have around 80 to 95% DO.
>
> Streams with 90% Oxygen saturation or above are considered healthy.
>
> Based on that, you would need around 8 ppm, or 8 mg O2 per L. at a
> temperature of 72 F. to achieve 90% Oxygen saturation.
>
>




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