What is the significance of the yam as a symbolic crop among the Igbo people?

The yam is a central feature of the Igbo creation myth. The god Chukwu gives one to the first human family he creates and supplies them with seeds to grow more to feed themselves. As the world at the time is covered entirely by water, Chukwu instructs the first man to kill his eldest son and daughter and to plant the seeds upon their graves. The yam, like Chukwu, is a demanding master. Chinua Achebe calls it “the king of crops, a man’s crop.” It takes considerable toil to tend to yam crops, and men who do it successfully must continue to make sacrifices to ensure a good harvest. Their sacrifices are not usually as severe as the ones Chukwu demanded, but sometimes things fall apart.
Because yams are so difficult to grow, a man’s wealth can be measured in them. A bountiful harvest indicates a long, difficult, and ultimately successful season of hard work. It reflects well on the man’s character. If he is weak or lazy, he will have nothing to show for himself. He will not be able to support a family or take part in the upper levels of Igbo society. Leadership roles are reserved for men. Men who cannot toil, in war or in agriculture, are not considered to be men at all.
In Things Fall Apart, Okonkwo’s father isn’t able to support his family and has nothing to give his son as he enters adulthood. Okonkwo makes a deal with a wealthy community member and plants his first seed-yams as a sharecropper during a season of unprecedented weather, including flooding. He succeeds where more experienced farmers fail because, he feels, his will is uncommonly strong. His new beginning recalls the myth of Chukwu’s first man creating a new life out of a flooded world.
http://www.iupui.edu/~womrel/REL%20300%20Spirit/REL%20300_Spirit/Igbo%20Culture%20and%20History.pdf