Soft filter effect
  1. Open document flowers.jpg
  2. Go to Layers > Duplicate Layer...
  3. Go to Filters > Blur > Gaussian Blur..., and set Radius between 13 -15
  4. Adjust the Opacity to 30 - 50%
Soften the edge of a selection
  1. File > Revert
  2. Draw a 2" square with Rectangular Marquee
  3. Copy and paste into (500 pixels Width and 500 Height) new document
  4. Go back to your old selection
  5. Choose Select > Feather with Radius 5 and copy and paste into (500 pixels Width and 500 Height) new document
  6. Compare results
For a very subtle softening, you can type a value less than one pixel (for example, .5).
The larger the value, the larger the feathered or blurred transition.

You can also soften the edges of a selection by switching to the Quick Mask mode and using a soft-edged brush to adjust the edges.