UK query: Top Dressing

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
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BrucUK


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The clay barrier will still be there. The organic material will decompose and disappear, and you'll be back to where you started.
Now if you could incorporate this organic material, and a few inches of sand into the top 6" to 12" of soil (perhaps you could hire someone to do it, or do it yourself in stages), you would provide soil microbes with an environment where they could create even more (deeper) soil. You would need to use organic fertilizers, like fish emulsion, to occassionally give the soil community a boost in nitrogen. The ecology of your soil will provide nitrogen from the life cycles of its microbes (bacteria, fungi, nematodes, and worms (not to mention a large number of arthropods). This nitrogen is locked into the soil. This community of soil flora and fauna will encroach on the clay beneath it, creating even more soil.
Chemical fertilizers kill large portions of this interdependent, soil community. Moreover, chemical fertilizers are water soluble and are easily flushed out of the soil with water to pollute waterways and local drinking water sources. All studies on the effects of chemical fertilizers on top soil, show loss of top soil.
I believe the above is a viable long term solution.
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- Billy

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On 7/26/2009 9:23 AM, BrucUK wrote:

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.
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David E. Ross
Climate: California Mediterranean
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This sounds like the best plan except that I would grow a deep rooted cover crop for a couple months first and then proceed with the above.
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On 7/27/2009 3:22 PM, J.R. wrote:

That would work even better if the cover crop were planted after the initial gypsum treatment and the first tilling. The roots of the cover crop would then more likely penetrate the clay. Afterwards, a second gypsum treatment would be appropriate.
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If water puddles on your clayey lawn area now simply adding a top dressing won't help, whenever it rains you'd just have more volume of soupy mud... you need to get down below the clay so it can perc or devise some way to drain the water away. My first cause of action would be to attempt a deep rototilling in hopes of getting below the clay. Next attempt would be to create drainage away from that area, perhaps with a series of trenches that hold perforated pvc pipe and covered with gravel... essentially creating a leaching field. Then add a good thick layer of rich soil. But odds are that clay runs deep rather than shallow. To me it seems like you may want to do something different with that area rather than planting a lawn, perhaps an attractive flagstone patio built on a porous base. Good luck.
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BrucUK wrote:

An area that floods over a clay base is not going to be productive for roses or veges so you need to address the drainage issue. You could:
- divert ground water away from the area and/or - build raised beds and/or - add drainage lines in filled trenches.
David
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> An area that floods over a clay base is not going to be productive for > roses

Thanks people - useful replies, may need a rethink! Bruce
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Prolly best to ask this question in uk.rec.gardening so I am replying to your post and adding a crosspost to that group too.
The has been a problem in the UK with horse manure being contaminated with a strong residual herbicide. You'll need to know about this so you can source good quality horse poo and the UK newsgroups has a lot of keen gardeners who give good advice.
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