The Plantball is the first stage of a process to create Biofuel Cells. It is created using tree Saplings, sugar canes, Wheat or leaves (rubber tree leaves won't work). The Plantball is placed in a Compressor, resulting in Compressed Plants.
The cheapest way to craft Plantballs (EMC-wise) is by using leaves, as they only cost 1 EMC. Note that leaves are more difficult to obtain than, say, Sugar Cane or Wheat, as they require shears to harvest.