In watercolor, I believe the hardest thing to do is to control the pigment and make it do what you want it to. Creating a soft edge is exactly that, learning to create a soft edge line and take control when painting in watercolor.
The whole thing about Floating-Your-Pigment is exactly what controlling a soft edge is all about.
Watch the video below and I demonstrate how to get a soft edge by controlling the pigment when applying it into water.
Happy New Year!
David