I think the first thing to do is to take soil samples from varies areas of the field, mix them together and get a soil test done ASAP.
As mentioned above, this will give you the soil pH which is important, but also the general fertility level.
After soil testing you will know what the feild needs as far as lime and/or fertilizer.
I don't think there is anything wrong with seeding more clover into it as long as in the process you do not destroy what is growing there naturally. I like to seed clover with a no till drill with a small seed attachment. You can usually rent one. Good, persistent, innoculated (don't forget to buy innoculant for clover seed if not preinnoculated) clover seed is expensive. You might as well do it right.
Good luck. Jim