On 7/26/2009 9:23 AM, BrucUK wrote:
I have a lawn with clay underneath that floods every time it rains; it's
also full of tree roots.
I have killed the lawn off, built "edges" and a deck/pier to walk
around on. I plan to top-dress the old (now dead) lawn with homemade
compost and well-rotted horse manure to a depth of about 6 inches,
thereby introducing organic matter, and avoiding having to dig too
much. I have "loosened" the soil where possible.
Any thoughts? Anything wrong with this approach? I will not start
planting until next year (herbaceous shrubs, roses and annual
vegetables).
Is this a sensible strategy? Have I missed anything?
Thanks - Bruce

If you create an area with strata of unlike soils, you will find that
plants will not thrive. Roots of your shrubs and vegetables will not
readily grow through the interface between strata. Water will not
penetrate through the clay, and you will still have some flooding.

I suggest that you broadcast gypsum over the area, creating a 1/4 inch
coating over the soil. Lightly water the gypsum to start it dissolving;
then water it into the soil 2-3 times. This will help break up the clay.

Then rent a power tiller. When the clay is slightly moist (definitely
not wet), till the area to a depth of two feet. Top dress with about 3
inches of your homemade compost and well-rotted horse manure. Till
again to a depth of a foot. Then top dress again with another 3 inches
of the compost and manure. Till one more time to a depth of only 6-9
inches.

If you don't walk on the area when it is truly wet, you won't have to
work the area for several years. You will also find that plant roots
will eventually grow well below the depth that was worked. However, you
might have to spread more gypsum annually to replace what has leached
away; this will keep the clay porous and well-draining.

--
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 diary at http://www.rossde.com/garden/diary