When you’re working on a layer mask, you can soften the selection edges by using the Feather slider in the Properties panel (Window>Properties). But, what if you want to soften only part of the edge and not the entire mask? All you need to do is go a little old school (like we did before Adobe added the Feather mask option). Make a selection around the area you want to feather, and click on the layer mask thumbnail in the Layers panel to make it active. Go to Filter>Blur>Gaussian Blur, and add the amount of Radius that you need to feather the edge of the mask.
This tip previously appeared in Colin Smith’s Photoshop Tips column, in the May, 2019 issue of Photoshop User magazine.