Limiting the Effect of the Adjustment layer
To limit the portion of the image to which the adjustment is applied by the adjustment layer, you can paint on the layer mask. To do this, click the Layer Mask thumbnail and notice that the foreground/background color palette switches to black and white. Select a brush to use and paint with black on the image to remove the effect of the adjustment layer on that part of the image (see Figure 5). Paint with white to replace the adjustment layer effect and paint with gray to create an area in which the adjustment layer only partially affects the layer or layers below it.
Figure 5 With the Layer mask thumbnail selected, you can paint with black to remove the affect of the adjustment layer on the layers below.
This layer mask is fully editable at any time simply by clicking on it.