The soil here is hard-packed clay. I think I'd need dynamite to plant
anything. I know that if I add organic material it will help a lot,
but if I add inches of manure or compost, doesn't that change the
grade? We had problems with water in our crawl space in the last house
we had, and I don't want to raise the grade so that rain runs back into
the foundation. If I add the organic material, does it break down over
the summer so that the slope of my yard doesn't really change? I hope
this makes sense - I don't know if I am making myself clear. I want
better soil - but I don't want a wet crawl space.
Where are you planning on putting the beds? I don't think you need to
be concerned about the grade unless your lot slopes down into the
house. If so, I'd look first into regrading the lot, then into any
Just breaking up the soil to create a bed changes the grade as you are
unpacking a tightly packed bundle. Think of it as opening up
something fluffy that was vacuum-sealed - it occupies a lot more
space once open! If left alone, it will slowly pack back down, but
that may take a few years.
I started with hard-packed clay, too. What I found is that the best
time to dig a new bed is in the spring - the ground is at it's softest
from all the winter precip. This does require some long-range
planning, but saves having to using a pick-ax to break open the soil
(been there, done that).
I only amend the soil if planting tender annuals. Bulbs, day lilies,
irises go straight into unamended soil that has been completely broken
up down to 8" deep. If I need to amend the soil, I just use regular
top soil purchased from Lowe's or Home Depot and mix it into the clay.
I use a ratio of 1" top soil to 8" clay. The only place I used peat
moss instead of top soil was in the rose beds.
My soil is adobe clay. To break it up, I scatter a generous amount
of gypsum over it, enough to coat the soil solid white but only
about 0.1 inch or less. Then, I lightly sprinkle with water to
start dissolving the gypsum into the soil without washing it away.
After that, I allow the garden sprinklers continue leaching the
gypsum into the soil. After about 2-3 weeks, I can dig without
having to scrape the sticky, pasty clay off the spade.
However you do it, once the soil is broken up and organic
amendments turned in, water will soak into the soil rather than
running off. Unless you get a downpour (or your soil freezes in
winter), you should not have to worry about water flowing towards
or under your house.
If you are still concerned, you might want to create low landscape
berms near enough to the house to be partially under your eaves.
Not only would these divert water, but they also are good for
plants that are sensitive to poor drainage.
David E. Ross
Climate: California Mediterranean
Hard-packed clay is actually a good thing next to the foundation,
provided the grade slopes away from the house. Watch carefully after
a rainstorm to see where the water puddles. I love gardening, but the
house foundation has a high priority. You can leave a couple feet
untouched next to the foundation and work your planting beds from
there. I do not allow any plant to touch the house and the space
against the foundation allows for a quick and easy monthly termite
inspection by the home owner.
If you add three inches of manure, it will change the soil level by
approximately half an inch after earthworms incorporate it into the
The other posters gave good advice, but one other thing you can do if
you want to break the soil further in is to plant fava beans for the
season (after having spread the compost). If they cannot break it, no
plant can. There are other plants, all with strong taproots, that will
do the same.
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
David J. Bockman, Fairfax, VA (USDA Hardiness Zone 7)
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
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