If you are talking about natural organic matters that are found in natural environment, they have various functional groups like carboxyl, phonel, amine and thiol groups. They bind with metal ions strongly. So if there is organic matters in your solution, calcium and magnesium cations will be complexed by organic matters. EDTA may not be able to compete with organic matters. So you will need less EDTA to reach the end point because there are less cations that bind with the indicator. Any levels of organic matters could cause interference. It depends on the cation/organic matters ratio. It will not cause any detectable interference if the amount of cations complexed by organic matters is lower than your titration error. It should always change the end point the same way, i.e., underestimate.

Thank you for your answer. However, I fail to describe that this is a soil extract and it is filtrate (with a 6 um filter paper) before titration and therefore only dissolved organic matter will be presented. I was wondering if several different samples would somehow manage to dissolve the exact same amount of dissolved organic matter in such a way that its interference in the EDTA titration would always be the exact same intensity.

Can you oxidize the organic matters using UV and ozone or H2O2? They can destroy the DOM and the excess ozone or H2O2 can be easily degraded and thus will not interfere with your titration.

I was considered doing that but I would rather avoid it if possible not to overcomplete the procedure and oxidize other important parameters to be measured. I guess I'll try it once and see if the changes between soil with organic matter and without are significant in EDTA titration. Thanks for the help!

Follow up question: wouldn't the destruction of organic matter release calcium and magnesium that it's adsorbed by said organic matter and in fact increase their concentration and no longer can be considered as soluble salts?