David Bockman wrote:

You can simply top-dress with any good organic material, and over time it
will leach downwards and really improve the soil structure. Scratching the
hardpan surface first will help start the process, as would doing something
like core aeration (as is done with turf). In very compacted conditions, I
suppose it would be possible for thickly laid top dressing to wash away in
a hard rainstorm (as rainwater sheets across the hard surface), in which
case you would need to rethink how you're going about soil improvement.

As to how quickly organic material is leached into and incorporated into
soil structure, there are so many unknown factors it would be foolish to
predict.

The only substances that will leach into the soil are those that
will dissolve. Organic materials containing any residue of
cellulose will not dissolve or leach.

They will gradually be incorporated into the soil by earthworms and
other non-microscopic fauna. They will also be incorporated by
tilling and the digging done during planting.

If you merely top-dress with compost, manure, peat, or other
organic matter without tilling, you will create a distinct layer.
Water might have trouble penetrating the interface between the
layers. The roots of small plants started in the organic layer
might not grow past that interface, leaving them shallow-rooted
(vulnerable to drying and even cooking in the summer). The result
could be worse than not top-dressing at all.

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening pages at URL:http://www.rossde.com/garden/