How do yams contribute to the meaning of Things Fall Apart?

In the Ibo society, yams mean everything.  The amount of yams that a man can produce determines his worth.  Their entire year revolves around the planting, caring for, and harvesting of this most important crop.  The planting season starts with the "week of peace," which is to pay homage to the earth goddess Ani in order that she may bless them with fertile ground and agreeable weather.  Once the tireless work of the harvest has been completed, there is large festival to celebrate the blessings of their hard work and to, again, thank the goddess of the earth.

Everyone needs to eat.  Yams are the staple food of the Ibo diet and without them, they would surely all parish.  That's why a bad harvest is compared to a "sad funeral."

The Ibo are, initially, an isolated people that are completely self sufficient.  They produce everything that they need to live within their system of villages.  This includes yams, which are the most important thing because they sustain their very ability to live and, more importantly, defend themselves.

Therefore, yams become the currency in this society.  The more yams a man can produce, the more he is worth to the village.  Similarly, the more yams a VILLAGE can produce determines their place in the intertribal pecking order.  Clearly, Umuofia is a very successful tribe in the beginning if the novel with the ability to produce yams AND warriors.

This brings us to status.  In Ibo society, in order to advance up the tribal class system, it is necessary to throw lavish feasts for the entire village.  In order to be able to so this, a man has to have proven himself as a great farmer that can produce the amount of yams necessary to be able to put on such a festival.  This shows us that in order to sustain themselves as a people, they need leaders that are able to produce the food that they need to survive.  So, such men are honored.

Okonkwo, the protagonist of the story, comes from a lazy father that never earned any status and was a careless farmer.  However, through hard work and determination, he rose to a high status in the society.  He is able to support three wives and still advance in rank.  The reason that he is successful is because of the HUGE importance of yam farming.