Yellowing leaves can be an indication of anything from soil pH to nutrient deficiency, water issues. Magnesium deficiency can be a problem on them. If this is a problem, usually the yellowing occurs between the veins of the leaves. You did not indicate the time span between applying the magnesium and now. It takes a while for some of the elements to be fully absorbed into the plant and correct the problem so it may just be a matter of waiting. Nitrogen and manganese deficiencies can also cause yellowing. Your best approach might be to use a balanced fertilizer to supply the major nutrients as well as micronutrients. I am not sure what they use in Croatia, but here in the US a fertilizer with an analysis of 13-13-13 is quite common. The rate would be 1/2 pound of this type fertilizer per mature plant (not a newly planted one). There may be something else going on with the plant rather than nutritionally. As mentioned above, yellowing leaves can mean many things.