Historically, the Yoruba were primarily farmers, growing cocoa and yams as cash crops. These are planted in a three-year rotational system, alternating with cassava and a year of diverse crops including maize, peanuts, cotton, and beans. At the end of this three-year cycle the land is left fallow, sometimes for seven years. It is estimated that at one time nearly 70 percent of people participated in agriculture and ten percent each working as craftspeople and traders within the towns. Yorubaland is characterized by numerous densely populated urban centres with surrounding fields for farming. The centralization of wealth within cities allowed for the development of a complex market economy which encouraged extensive patronage of the arts.