At first glance, you might think that vacant land doesn't need to be insured. If it's truly vacant, there are no buildings, you don't have any property on it and there aren't any people living there. That eliminates most of what you'd buy insurance for, but it doesn't eliminate everything.
Insuring Vacant Land
When you own vacant land, you don't just own the dirt. You also own the responsibility for what happens on it. If lightning strikes dry grass on your land and starts a fire, which spreads to the adjacent parcel and burns down a house, you can be held liable. You can also be held responsible if someone gets injured on your land even if you didn't give him permission to be there. Vacant land insurance covers your liability for what happens on your dirt.
Your Homeowners Policy
If you have a house and carry homeowners insurance, you might already be covered. Most standard homeowners policies include coverage for vacant land under the liability section of the policy. However, if you engage in any farming on the land, you wouldn't be covered. Many policies define vacant land ambiguously, so it's wise to confirm with your insurance agent that your vacant land truly is vacant. If it isn't, it won't be covered under your homeowner's policy unless you specifically add it. You can usually extend your homeowners insurance policy to cover vacant land, even if it isn't located in the same city or state as the home you have insured.
Vacant Land Insurance
Vacant land insurance covers your vacant land separately from your homeowner's policy. These policies are frequently liability-only policies that will pay to defend you if you're sued by anyone that is injured on your land and will pay if you lose the suit. They won't pay for any damage that gets done to your land, though.
Hunting Land Insurance
If you use your vacant property for hunting, you'll need to purchase insurance specific to that. The use of firearms ups your risk. If you have no plans to hunt on your property but you give someone else permission to hunt on your land, your insurer may require you to have them sign a waiver that you aren't responsible. Although you could still be sued, this will give you an additional layer of protection.
If you have other liability coverage needs, an umbrella policy can also cover your land. This type of liability insurance policy covers issues above and beyond your existing insurance. For instance, an umbrella would pay liability costs from a car accident after you've exhausted your auto insurance's limit. Your umbrella insurer may let you add your land to your policy at little or no cost.