Why would the soninke people trade gold for salt?

"Gold was not useful for making tools or weapons. A Diverse Capital. Koumbi, the capital city of Ghana,was the wealthiest city in West Africa because of the gold/salt trade. Many of the traders who went to Koumbi were Muslims. The local Soninke people kept their culture in tact by separating themselves from the Muslims."

The soninke people were the first powerful West Africa Kingdom because of the control of gold and salt trades.  The gold came from a forest region between the Niger and Senegal rivers.  But large amounts of the salt came from the Sahara Desert.  Other traders wanted salt over gold for the different flavors it gave their crops and to buy different goods.  They could supply salt to the sub-Saharan economies that gold was abundant.  Salt was very sufficient in sub-Saharan Africa it was promoted in the trades too.