You can merge an adjustment or fill layer several ways: with the layer below it, with the layers in its own grouped layer, with other selected layers, and with all other visible layers. You cannot, however, use an adjustment layer or fill layer as the target layer for a merge. When you merge an adjustment layer or fill layer with the layer below it, the adjustments are rasterized and become permanently applied within the merged layer. You can also rasterize a fill layer without merging it. (See Rasterize layers.)

Adjustment layers and fill layers whose masks contain only white values do not add significantly to the file size, so it is not necessary to merge these adjustment layers to conserve file space.