Re: It's all in the 'title'
You can certify documents if you are on a list of regulated professionals. Farmers are not part of a regulated profession and therefore farmers cannot certify documents.
The reason for the existing situation is basically twofold.
1) Being on a list of certified professionals you can check that a certified professional is actually a certified professional, and you can also pull their contact details from the database of their regulating body and drop them a message saying "did you certify the documents for X?"
2) Being on a list of certified professionals means you can be tracked and basically struck off and made unemployable if you make a mistake. The idea is that the prospect of total ruin will inspire some level of caution in what you are certifying.