That's a rather broad question. I think the best way to use it to help farmers is to create a crowdfunding platform to help them buy supplies for their farms and make BTC a payment option.

There are already some platforms doing this, like Cropital, so you'd already have an example to follow and then simply allow cryptos to be used, no need to make your own blockchain/alt. This would be more a charity since it wouldn't always be certain that there'll be a return on investments.