You can add 0.1 M HCl to the soil samples, after several hours, and measure the TOC of solutions. Additionally, you could also use the potassium dichromate volumetric method (the name is from a paper, in Section 2.1).

Thank you very much!

I will try both! Maybe you can advice me, what proportion should I use for 0.1 M HCl? Will 1:5 work?

I did not find the ratio (soil: HCl) in the Internet. So I think you'd better read some relevant papers. You are welcome.

take 3 gm soil sample digest with nitric acid & perchloric acid for 150 C  & dilute 100 ml sample and filter the sample .& run on toc.

Thank you! I think, I will wait for solid combustion unit - we should obtain one soon. It is not really adventorous - but still should be more effective than other metods.

loss on ignition could be the easiest mean to measure SOM, and it is also reliable.