the buildup of salts occurs due to high evaporation rates... a natural process that geologically resulted in things like the Dead Sea and Utah's Salt Lake. It's how "sea salt" is precipitated from sea water... by fresh water contains minerals and irrigated land is also treated with fertilizers... all these concentrate in the soil if not enough rainfall to flush them out.

It's one reason why the fertile crescent is not so fertile any longer.. and plowing the soil increases the rate of evaporation.

So how do we cultivate enough land to feed folks without depleting the fertility of the soil? The strategy adpated by early people included pastoral grazing of their herds to find fresh pasture and then move on before the land is damaged... a practice still followed in Mongolia and sub Saharan Africa.