Usually "organic" means that no synthetic pesticides or genetic modification were used. You are allowed to use pesticides from natural sources. In practice it means that organic farmers are using less effective pesticides and must use more of it to reach the same effectiveness.

http://en.wikipedia.org/wiki/Organic_certification

It's a classic Appeal to nature tactic. http://en.wikipedia.org/wiki/Appeal_to_nature

It depends quite considerably on which country you're in. In the UK 'organic' is heavily regulated and means pretty much that, in the U.S it's more lax. But my guess is that the poster is saying that because organic farmers have a much smaller range of less potent sprays available, they have to spray more by volume.