Why does Okonkwo threaten to break Nwoye's jaw?

Nwoye had split a seed yam which should not have been split because it was too small. Also, Okonkwo is trying to encourage both Nwoye and Ikemefuna to be great yam farmers, and thus great men. Okonkwo wishes Nwoye not to be lazy.

Seed yams need to be a particular size to grow properly. The bigger ones can be split to produce two seed yams. However, if a yam that is too small is split, it will not grow properly and is only useful for eating.