Here's another perspective to consider: asking whether a business idea is good is one thing; asking whether starting a specific business is a good idea for you is another. Let's assume for a moment that sustainable agriculture is a market that is growing, and will generate lots of profits over the coming decade. Within that market, there are going to be a lot of great businesses that succeed, and a lot of mediocre businesses that will fail. That's normal. Whether your company will be one of those in the winning group is notoriously difficult to predict. It's certainly possible, but it can't be guaranteed. I'd suggest you read a few books on entrepreneurship to explore that idea further. (Actually, asking for great books on entrepreneurship might be a good question to ask on CareerVIllage).
So if we assume that nobody can tell you for sure whether you'll be a part of the winning group or the losing group, then the question of whether starting a business in this market is good for you is actually a question of whether you can handle the uncertainty. I can tell you for sure that being an entrepreneur is a wonderful experience, but I can also tell you that it's a difficult thing to do and that there will always be that risk that the odds don't go in your favor. To do it, you'll have to have a lot of grit, a lot of determination, and a willingness to do whatever it takes (emotionally and financially) to give it your best shot. What do you think?