A couple things you can do. First I'm assuming this is not your primary income.
1. Your homeowners policy more than likely includes coverage for Other Structures (Coverage B ). These are generally defined as other structures on the insured premises not attached to your primary residence. You should be able to increase the Coverage B on your HO policy to cover your barn and possibly contents. Generally HO policies allow for incidental business coverage (as long as it's not your primary income). For your tractors and equipment you can get an Inland Marine Policy to cover these items. The inland marine policy will be one policy with everything listed you want to insure (tractor, mower, baler, etc).
2. Check with your local FB or independent agent, they often have policies available that will cover home, barn and equipment in one policy. Note however that this coverage may or may not be written on a FB policy. There are other insurance carriers that specialize in this type of coverage so the FB agent may write it on another company's paper. There is probably an agent in your area who specializes in writing policies similar to what you're looking for; ask around, he/she shouldn't be hard to find. For a comprehensive policy, don't be surprised if you see large differences in premium between carriers. These carriers are usually what are defined as "surplus lines" carriers which means their rates are not as heavily regulated as "admitted" carries (admitted carriers would be State Farm, Allstate, FB, etc). There's nothing wrong with surplus lines carriers, they just represent such a small percentage of the market that their regulatory requirements are just not as burdensome as admitted carriers.
A good agent should be willing to research and quote you both options above. I would also recommend that you get the two scenarios above from several different agents. Quotes for these items can vary a lot between insurance companies. Lastly, check your HO policy and make sure you're carrying the highest liability limit they offer. Insurance companies usually quote policies with a $100k or $300k liability limit but offer higher liability limits but don't actively advertise the higher limits. Case in point, I'm insured with USAA and had a $300k liability limit on my HO policy. A few years back I noticed that they offer a $1M liability limit; I was able to increase my $300k liability limit to $1M for $13 annually. Needless to say if someone sues me, USAA is going to send a plane load of blood sucking lawyers to defend me. This coverage would extend to incidental business coverage as well on my particular HO policy. If you decide to carry an umbrella policy as well, a $1M liability limit on the underlying HO policy will decrease the premium on the umbrella policy. The umbrella policy attaches after the liability limit on the underlying HO policy.
I'm an actuary for a personal lines insurance company in Florida but I believe the items above would be applicable in Maryland as well. Don't hesitate to shoot me a PM if you have any more questions.