In answer to your question, incentives can be varied, and I think you have to look at specific examples.
Briefly, to try to provide some framework for going forward, the incentive has to somehow at least match the cost incurred. In our case we have seed, equipment, time, labour, and fuel costs associated with establishing these fields. So the incentive has to at least somewhat balance that.
Where we can't cover all the costs of the farmer who's incurring this cost on his land, then maybe there has to be some other accrued benefit. In this case, it's the benefits to soil fertility that drive the farmer forward.