That's why education is key as well. People who are well-educated have less children, on average. Look at the US, for example. So educate them, and enable them to become wealthy. Right now, they can't.

But that doesn't mean that a well educated farmer will have less children. He can be as educated as you like but unless he can increase productivity in some way (access to loans for equipment say, like through Fairtrade) then he still has as much work to do and still needs to provide the labour.

Suppose the education increase yields, unless we're on top of the trading practices then the value falls - everyone's yield has increased, if this means the market saturates he might make less money (perhaps none). If the market hasn't saturated he still needs more labour to gather the increased yield but gets the same income to share amongst the labourers - increasing the pressure to provide cheap labour by expanding his family.

In short the problem is more complex and IMO is not due to lack of education. What you're saying is that a poor agricultural labourer should sacrifice there own ability to earn more in order to stave off over-population - why is it down to them.

I'll go with the converse though. If you ensure that workers get a fair wage (Fairtrade again), one that can pay basic healthcare and education costs and ensure they have healthy food intake and a reasonable dwelling, then there is less pressure to reproduce (but possibly more opportunity to choose to? again it's complex).