DIY soil carbon test?

Hi,

I want to test the organic contents of my soil to show that my cultivated soil stores more carbon from the atmosphere than comparable non-cultivated soil in my region.

Judging from the plant growth, the color and the texture of the soil, I'm pretty sure that my cultivated soil contains more humus, ie. carbon, than comparable non-cultivated soil in my region; however, it would be good to have an actual figure to know by how much exactly the carbon content has increased.

There are no soil labs in my remote region and since this is a private initiative I pay for out of my own pocket, I don't want to go into a lot of expenses. I seem to remember that there is a method of burning off the organic contents in a soil sample at high heat in an oven and then calculate the amount of carbon from the organic content which is the difference of the soil weight before and after heating.

Does anybody know how it is done exactly? Is there another method?

Thanks,
Dieter

Here are some ideas;
How do you calculate carbon content in soil?
Image result for calculate the amount of carbon from the organic content which is the difference of the soil weight before and after heating.
The most accurate standard laboratory test for soil carbon is dry combustion using an elemental analyser. These instruments heat a small sample (usually a fraction of a gram) of dry pulverized soil to around 900 degrees C and measure the carbon dioxide gas that is a combustion product.

How do you calculate organic carbon content in soil?
Image result for calculate the amount of carbon from the organic content which is the difference of the soil weight before and after heating.
Soil organic carbon is usually reported as a percentage of your topsoil (0 – 10) cm. This value can be converted to a meaningful volume for a paddock. For example: i.e. 10,000 m2 in one hectare x 0.1 m soil depth x 1.4 g/cm3 bulk density x 1.2 % = 16.8 t/ha.
managingsoilorganicmatter-part-2.

good luck

Dieter,
I am wondering if you had any success measuring your soil carbon? This spring I would like to do much more analysis on my soil to try and quantify the improvements I make and soil carbon is obviously one I hope to improve vastly!
Simplistically I'm thinking the test would be: Take a sample of soil and dry it at c. 120 degrees Celsius for a few hours until no more weight change - that gives a dry sample. Note the weight of the dry sample. Put it in the firebox of the stove for a few hours until no more weight change - and weigh again. The difference between the two will give the soil carbon.
Difficulties I can foresee:
1) Measuring the weight accurately enough.
If soil carbon % is typically between 2 and 10% then you would be looking for a max 10g change in a dry soil sample of 100g. That's a pretty large sample, but to see an increase of 10% in the carbon in your soil, you would would be looking for a difference of 1g. That's pretty much the accuracy of a home scales. To get better resolution you would either need to have a bigger sample, or better scales. I'm thinking maybe I could get some balance scales to measure to 0.1g by using water in a measuring cylinder as the counter weight. Taking bigger samples I think could get silly; 500g is quite a big sample, and you’d probably want to take more than one to get some idea of consistency.
2) Not losing the sample in the fire.
You’d need a crucible that would stand the fire temperatures without losing mass itself. I’m pretty sure I’ve got some old chemistry crucibles somewhere that would be ideal, although maybe a little small. It’s possible that an old coffee mug would be OK. It would need to be wedged in such a way that as the fire burns it stays upright. The fire would need to be burning pretty hot for a while to burn off any organic matter.
It would be pretty slow to do the test as well. For ease of handling and accuracy (due to convection effects) the samples would need to be cooled near room temperature before weighing. It would take some experimenting to get a feel for how long the samples would need to be burnt for, to be confident that no more water or carbon remains.
I’d also be interested if anyone else has had a go at this and how they got on?

If you follow Nancy's instructions, which is effectively what's called a loss on ignition protocol, you may need to stir the sample a few times during the heating cycle. This is to make sure that all the stuff that can oxidise gets exposed, otherwise you may have some carbon left behind that throws off your measurement.

Be careful not to A) burn yourself(!), B) spill any material, and C) knock any ashes or embers into the crucible. In laboratories they use a muffle furnace for this operation.

I've done this a number of times, both to get an idea of soil carbon content, and also to figure out the ash fraction of various samples of biochar. It's a little tedious and best results come from doing multiple samples and averaging the values.

Phil Stevens wrote:
I've done this a number of times, both to get an idea of soil carbon content, and also to figure out the ash fraction of various samples of biochar. It's a little tedious and best results come from doing multiple samples and averaging the values.

Brilliant! It's nice to know I'm not completely barking trying to do this myself. Yes, stirring the sample during burning off the organic matter is a good idea, thank you.
What sort of sample weight did you find worked best and how many samples do you think are necessary. Or does the latter depend on the soil itself, and how variable the carbon turns out to be?

I usually do samples of 50-100 grams at a time and usually just do 2-4 samples because (honestly) there are only so many hours in a day and I can only do two at a time. I learned early on that bigger samples, although easier to weigh, needed too much stirring. Now I have a digital scale that's supposedly accurate to 0.1 grams, so the smaller sizes are practical.

I use the stainless steel pans that I make biochar in during fire season, giving them a good scrub beforehand.

Hear about how soil color can help you analyze things like organic matter content, anaerobic condition, soil fertility, and soil nutrient composition in the new Garden Master course excerpt now available. It may be of interest: