Hi Mel
I dont know if Germany has a WWOOF site but here in Ireland we are members of the WWOOF organisation, basically you provide food and board for people who volunteer on your property, in turn they help you with work on your farm, you must specify the hours of work, conditions and facilities available and also I would insist on them having insurance (which is available on the WWOOF site). This is our second year being hosts and we find it great, we meet people from all over the world, some stay for a month other stay for two weeks, you can specify on your profile page if you want people to stay longer term, if you need experienced volunteers etc.
Don't know if its what you had in mind but worth a look
Brid