Hi,
This year my lemon tree of about 20 years continues to produce hundreds of fruit well in to summer. Can I take this as an indication of the soil quality and plant other fruits and vegetables?
It is probably just the right plant in the right place. Or, if you live in Florida, your tree may have trestiza. There is a point in the trestiza infection where the tree becomes very prolific with fruit as the tree goes into overdrive trying to reproduce. You get a lot of fruit for a few years and then as the bark peels off the tree, it declines and it is time for the tree to go.