Is zakat obligatory on a plot of land that was purchased with the intention of building a home, then sold after 14 years, given that other plots of land are owned for residential and investment purposes?

Zakat (definition) is not obligatory on land you purchased for dwelling and then intended to sell, because it was not purchased with the intention of trade from the outset. Its price becomes subject to Zakat after a full year has passed on it. Similarly, there is no Zakat on a house you purchased for dwelling, or land you purchased for dwelling and investment through rental. Zakat becomes obligatory on the rental income if it reaches the nisab (definition) and a full year has passed on it. However, if the intention of investment was speculation and selling the land when its price increases, even if you resided in it during this period, Zakat becomes obligatory on it as trade goods.