Effect of organic matter on salinity
Organic matter affects the pH and makes the soil more acidic. The best example of this is in peat bogs, where only acid tolerant plants can live and the peat is so acidic that micro-organisms don't flourish, thus preventing decay. There is the famous Tollund Man, found in a peat bog in Denmark if I remember rightly. The body is in an almost perfect state of preservation, yet is about 3,000 years old! Tollund Man was apparently executed by hanging in ancient times and his body thrown into the bog. The principle is the same as the jars of eggs, onions, gherkins and so on which are pickled in vinegar.