Last year on a scottish gardening programme called beechgrove they did an experiment, half if the tomato plants were fed with comfrey tea, half were fed with tomorite (most popular uk chemical tomato liquid feed)
They were surprised to find the comfrey tea fed plants out performed the tomorite plants
edit, this is the factsheet from the programme with the results
http://www.beechgrove.co.uk/module_u...sheet_20_2.pdf