I am not expert on farming, but some of my friends grew up on farms..
Seems like farmers leasing land to solar outfits is the free market at work. Also, sometimes farmers will fallow fields; leaving them unplanted or planted with cover crops. For PAM, JJ and Mn; this is good for soil health along with pest and weed control.
Or, sometimes, instead of growing corn or beans, they will grow grasses and / or alfalfa to harvest hay. This makes good livestock freed and is good for the soil.
As murph points out, a solar farm is a farm that grows and harvests electricity. They do so while allowing the land to lay fallow.