Lawn irregularities are annoying but will occur as long as helpful earthworms thrive and seasons pass with freezing and thawing.  Such bumps and dips cause few problems.  However, higher spots may interfere with mowing and detract from the appearance of your lawn, and low spots get boggy when it rains.

Minor irregularities are eliminated by top-dressing low spots with a loose mixture of weed-free topsoil and peat or sand.  Apply the soil mixture right onto the grass and smooth it over with the back of a stiff rake.  Repeat this process twice yearly until the surface is level.

Deeper depressions call for more drastic action.  Slice strips of sod about 20 inches wide, cut underneath with a sharp spade or shovel, and roll the strips back completely to expose the low spot.  Fill in the depression with high-quality, weed-free topsoil (ideally, fortify this soil with a little fertilizer rich in phosphorus and potassium ).  Tamp the new soil firmly before rolling sod back into place.  Top-dress the replaced sod with about 1/4 inch of soil, and water deeply.

High spots are tricky to repair and require a heavy roller to do the job right.  First, use a sharp, flat spade to remove thin wedges of turf from the high spot.  Space the slices about ten inches apart, and cut each wedge about six inches deep.  Soak the area thoroughly, and then roll the high spot.  The pressure of the roller should compress the turf and soil-bed enough to seal the spaces left from the removal of soil wedges, thus lowering the hump.  If a slight high spot still exists, wait several months and repeat the process.