Since brining is about how much salt/water you use, how do I switch to a soy sauce marinade? If the standard brine for 2lbs of meat is 1/4 Cup of Table Salt and 1 quart of water, then that means that is 28320mg of sodium. For 30 minutes to an 1hr. 1 tbsp of Kikoman Soy Sauce is 920mg. So 28320/920 is 30.7. That means I have to marinate 15 to 30.7 hrs instead of 30 mins to 1 hr? Or increase the amount of soy sauce up to 1 cup?
To use soy sauce as a substitute for salt in a brine, you need to calculate the concentration of salt to water. Since we can accurately calculate grams of salt per volume of soy sauce by using the nutritional information on the label, and 1/4 cup of salt per quart of water is a down and dirty way to make a 5% salt brine, let's simplify things by just using grams of salt/metric volume of liquid.
According to the label, Kikkoman Soy Sauce has 920mg of sodium in 15ml of sauce, so 500mls of soy sauce has 30.7 grams of sodium. NaCl (salt) is 40% sodium by weight, so 500mls of soy sauce has the equivalent of 76.75 grams of salt.
By definition, a 5% salt brine would have 25 grams of salt per 500mls of water.
So, Kikkoman Soy Sauce is almost three times as salty as a 5% salt brine. To roughly approximate a 5% salt brine with soy sauce, mix 1 part soy sauce with 2 parts water.
You can also use a stronger solution for less time, but it will take some tweaking. I recommend that you try it the first time as I recommend for replicating a 5% brine, and then tweak from there. Or use a recipe that calls for a 10% salt brine and use half soy sauce, half water which will cause the chicken (especially the skin) to darken nicely.