Organic manure is used in agriculture around the globe. I am interested in how does organic matter affect the salinity of soil? Does it increases, increases or remains unaffected? and is there a study to support the argument?
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.
First of all the whole idea by using fertilizer is to add different types of salts to the soil,Fertilizers from animals or plants will contain most of the types of salts the animals or plants have absorbed.
I guess you are asking about NaCl and yes manure will over time acumulate this type of salt in the soil,But we are talking many years or decades before this is a problem and other types of salts will build up too.
The main way the animals get rid of exsess salt is in the urine and if one use this for fertilizer the build up of NaCl will be greater in the soil.
A lot of the salts will be removed by rain and irrigation so the build up of salts is mostly a problem in dry areas of the world,And crop rotation might help keeping the salt level down(if it is done right).