I have used Algamin kelp meal:
http://www.4hydroponics.com/nutrients/algaminkelp.asp
And PVFS (they had this in a gallon last year, but I only see
the 5-gallon there now):
http://www.groworganic.com/item_F133..._5_Gallon.html
Both worked. The liquid stuff is better if you are going to
spray it, inject it into a drip system, etc.
Since this is an extract (1/4-1/2 tsp per gallon) rather than
ground kelp, apparently the same stuff that Maxicrop
packages in their own 10.7oz plastic container for about
the same price, this might be a good buy:
http://www.groworganic.com/item_F136...Extract_Lb.htm