Q. Why should I use a liquid instead of a powder?

A. There is ample evidence to indicate the superiority of the quick dip over the powders.  In current research, a quick dip was as good as or better than the powder at promoting root growth. Reasons for the performance difference between the powder and liquid may be the uniformity of coverage and the more rapid absorption of IBA that the liquid allows.  When the same proportion of IBA is used, it is less effective in the powder than
in the liquid solution.(1)

Reference:
(1) Dirr, Michael and Heuser, Charles. The Reference Manual Of Woody PlantPropagation. Athens, GA: Varsity Press, 1987.