The old wiki is spazzing but you can find all the farming values for fertilizer here under the "Fertilizer and Field Values" section: http://kralith.eu/index.php?title=Agriculture
Coal does indeed lower your plenty. There are a couple that add plenty as well as speed, just remember that each fertilizer used reduces the effectiveness of the next so layer it in the order of what you most want (plenty, followed by speed, etc). (As someone noted earlier, JC recently nerfed dross, should read 125 instead of 200).