I have not tried this particular soda bread recipe, but I have just tried one from River Cottage website which I made with just 500g of white bread flour, 2
tsp of bicarbonate of soda, 1 teaspoon of fine sea salt, and 400 ml of soured milk by adding 2 1/2 tbs of pure lemon juice to 400 mil of milk, whisking in and standing for 20 min.
As there is no yeast to raise the loaf the production of carbon dioxide from the chemical
reaction of bicarbonate of soda and acid (citric acid from lemon juice, lactic acid from buttermilk or yoghurt or tartaric acid as cream of soda, or even a teaspoonful or two of vinegar) will have the same effect.
Made these tonight, using 1/4 of a teaspoon
of bicarbonate of soda and adding 1 teaspoon of protein power.