Can anyone help me with information on growing yams?

I get quite a good crop, but they are mostly very small. Yet in the shops
one can buy yams that are 100-150mms long.

Should I mound up the rows like potatoes?
What fertilizers should I use (apart from sheep pellets and general garden
fertilizer)?

--
Murray and Judy Campbell
15 Mill Street
NELSON
Ph/Fax (03) 548-1170
E-mail