I could see, SOC% = TC% - 0.12 * CaCO3, but it doesn't help because I don't have CaCO3 measured.
Any suggestions would be highly appreciated.
You could just check your samples with HCl to confirm there is no CaCO3. But if there is CaCO3 in your samples you can hardly avoid measuring CaCO3. You could as well measure SOC directly by wet oxidation method like Walkley &Black
As already said by Dr Shaaban, you can calculate SOC through SOM. If Walkley &Black methode is too long you can go through weight loss of organic matter by combustion at 550°C for 8 hours as substraction of organic matter from initial weight and residual ash. Then you can apply OM=OC*1.724. I should precise that this method is used only for soils with TOM is higher than 3%. For soils with less than 3% TOM, Walkley &Black method is more suitable and precise.
I am sorry if my question was unclear but the concern here is NOT the Organic matter but is Total Carbon which includes both Organic and Inorganic fraction of Soil carbon.
Then it's clear, you can use the soil sample sublimation at 550°C for 8 hours and calculate the difference of weight (soil sample weight=ash content+organic matter). The TOM is transformed into CO2, and also total carbonate. Then you divide TOM by 1.724 to have TOC. For more precision, total weight loss is used for soils with 1
When you only have one and need the other there is never going to be a "good answer" unless you want to do more lab work or make some assumptions. There are some standard conversions, you could measure carbonate content of a second sample, you could burn off C at different temps to get a better grip on how much of the carbon is coming from where.
Not all soils contain inorganic carbon e.g. carbonates. As mentioned before, take your soil sample and add HCl. If it fizzles, you have carbonates. If you are interested in total carbon (TC), then analyze the sample as is and it will give you TC. If you are interested in SOC then take a subsample of the soil and remove the carbonates using 0.5M HCl, and analyze this, which will give you SOC. Then you can do a subtraction to determine the quantity of inorganic carbon ((IC) in the sample:
TC-SOC = IC
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