"Lol. How the heck do they thing this would work? We better patent earth fast!"
First you need to understand what a patent is. It is an exclusionary right (meaning you can legally prevent others from using it) given by the government for an "invention". An invention is defined as something industrially applicable (it has to be useful, duh), novel (new - not something that has been done before), exhibit a sufficient "inventive step" (not obvious) and you must provide full disclosure on how it works/how you did it for it to be protected under patent law.
N.B. Patents are not universally accepted, so a patent in America may not be respected in another country eg. China.
Agricultural patents are typically given to companies who produce a method for improving a strain of plant by giving it various beneficial properties. The patent protects the harvesting/sowing of the fruit/seeds but it the patent itself describes how to make that particular plant so special. You can see one of Monsanto's patents here:
http://www.google.com/patents/US4535060 (as I said before, patents must be fully disclosed to get protection, otherwise its just a trade secret)
I'm not supporting agricultural (or for that matter pharmaceutical) patents in any way, but just letting you know that what they are patenting is not so trivial as "I want to patent plums!".