I am aware of several opportunities for travelers to volunteer at farms in Nicaragua. Most of these are reasonably organic. I'm sure there are some others. There aren't as many as in Costa Rica, or as many travelers looking for volunteer opportunities.
Nicaragua is truly for the hard-core cultural explorer, interested in immersion in Spanish and immersion in poverty.
Abundance Farm, a small semi-organic farm where you can volunteer in Nicaragua, is a small family farm with a 2-room guesthouse for volunteers. It is run by my ex-girlfriend Yasmina, her parents, and her brothers. My daughter Gloria also lives there. The guesthouse has solid walls and solid beds. Room and board costs $10 per day for the first three weeks, and there is no work requirement. After that, there is the possibility of negotiating a lower daily rate for working volunteers. There is also a program for volunteers to work at their local school. The farm is located right next to a huge waterfall and has a distant view of the Pacific Ocean. Reservations are usually not required for individuals or couples, but arrival information is appreciated. I actually handle the emails myself at present, because there is no email at the farm. The email address is contact@abundancefarm.com. This farm is run by Nicaraguans, and you will be experiencing a rural experience of subsistence-level existence among poor people without much formal education. Children are welcome. A few other external reviews of abundance farm in nicaragua and also this review of abundance farm.