A soft edge can be made by painting wet in wet. This can be done by painting into a wet wash or, if the area you want the soft edge is dry, by first wetting it with clean water. If you are painting a soft edge into a wet wash, make sure there is more pigment in the color you are applying than is in the underlying wash or obvious blooms will be created. If painting a soft edge onto an already dry wash, be sure the underlying wash is thoroughly dry.
Here the soft tree shapes on the left were all applied wet in wet then, once dried, contrasting fine rigger lines were added to tie this softer area with the focal area. The reflections in the water were painted onto a dry under wash and then quickly softened on either side with a clean, damp brush. The same damp brush (a rough ½-inch bristle) was then dragged horizontally through the reflections in a couple of places.