If you want to see incredibly large olive plantations go to Jaen.
If you want to see miles and miles of oranges trees then go to Valencia or Castellón provinces (not to Alicante). Nowadays there are also very large plantations in Murcia and Andalucía but traditionaly Valencia has been the biggest orange producer in Spain, with a very large experience in that bussiness.
At the end of the XIX century Valencian orange exports to Europe made of Valencia one of the most richest regions in Spain at that time.
There exist also a stock market for oranges in Valencia city called
Sociedad rectora del mercado de futuros y opciones sobre cítricos How it works At this moment there is big crisis due to the competition of other nations with much lower producing costs.
Old orange adverts (click on the names at the left side for more pictures)
The Museum of the Orange in Burriana My grandfather was a farmer in the town of Silla. My parents inherited from him a small plantation of oranges near the Albufera lake. I remember the delicious smell of the orange blossom in April and how pleasant for me it was helping my father to water the trees in summer or picking the mature fruits in winter.
Oh! Talking about watering the trees. The Water Wisdom or Water Court has been meeting every thursday since the year 960 at the Valencian Cathedral to resolve the farmer's diputes about irrigation. It is considered one of the oldest law institutions in Europe.
Tribunal de las Aguas