So we’re working on getting the methods ready for the Real Food Campaign lab, and one of those methods is mineralizable carbon. We’re following a method similar to the Solvita “Burst” type method (24 hour incubation after adding water) but instead using a CO2 sensor rather than a solvita type test (cheaper, easier).
Here’s the problem - you get a different ‘burst’ based on the soil type and amount of water you add. Ideally, you want to add 50% of the water holding capacity of the soil (abbreviated AWC). Problem is, different soil types and amounts of organic matter impact the water holding capacity.
So, we have a few options:
- Do what solvita does and just add a fixed amount of water, and to hell with the error.
- If we know the soil type (and better yet the organic matter), we can use this formula http://www.weap21.org/WebHelp/Mabia_Alg_AWC.htm (scroll to the bottom) to estimate the AWC and therefore the amount of water to add. We will be measuring Total Organic Carbon, which probably relates to organic matter but isn’t it.
- We could manually measure AWC on each sample that comes in.
- We could check the range of possible water additions (maybe 3g - 9g max) from all soil types (or at least 99% of them), and choose one which is >50% for all soil types but less than 100% for all soil types… does that exist? What is the biological impact of 25% AWC versus 90% AWC versus 50% AWC? My guess is here is a reasonable cutoff there where activity peters off.
- Perhaps we just use the 3 day? That way the activity has hopefully topped out and isn’t going up any more, at which point the moisture level is less relevant?
… Given that we’re trying to keep costs down and keep methods simple, (3) above seems like a tough sell. (1) is ok but I’m not sure how much error it’ll produce. I love (2) but we’d need to know the GPS location with some accuracy (not impossible) or we’d need to determine soil type in the lab quickly.
My starting points for next steps -->
- Perhaps we can simplify things by putting values in rough boxes… like “sandy”, “sandy-loamy”, “loamy”, “loamy-clay” and “clay” is good enough? We could probably do that in the lab. That may get a big quality improvement without a lot of effort.
- Also, could we guess at or are there standard calculations to get organic matter from total organic carbon? My guess that would improve our accuracy, and since we’re doing total organic carbon on each sample, that’s a data point we’ll have anyway.