Question
Hi,
I am writing from France.
We moved into a new home June last year. As the garden is on a downhill slope and we were afraid of erosion from autumn rain, we seeded in hot summer and thanks to substantial watering made it grow. However, in autumn/winter, the soggy soil and ongoing rain killed and washed away lots of the grass. A word about the soil: according to the analysis of the builders engineer we have what they call here "sables limoneuses" which probably means "silt and some clay". Further they stated that they found randomly floating water in various depth on the ground. Probably some layers block water and it has to find a way.....
During construction the guys basically moved the topsoil down and the deeper soil up to the surface.

My cure by now: 1) 1cm of compost in mid-April (I screened in total 11 tons to separate coarse material from fine stuff) allover the existing lawn.
2) Alfalfa pellets spread on the lawn (neighbors think I am crazy I bet)
3) mowing high (6,7 - 8,0 cm)and I leave the clippings
We have birds, we have toads.

BUT ---> after some days of rain I can see my lawn going yellow on some spots, some leafs have red color. If I pass a verti-cut rake, I got many "dead leafs" on it. I guess its a fungus caused by humidity!? Or all "food" for the grass is washed out?! What to do ?

Overseeding: What磗 the trick? In early May I cut an area fully of empty spots really short and put seeds down. However, the seeds never really germinated. I fear that the faster growing existing plants simply cut the sun and take away the water ? Any advise on overseeding for me?

Wherever the lawn is not dense enough there is clover or other non-grass-plants. How to get rid off without chemicals.

Re the soggy lawn/ running-off water problem,,,,,do you think I should install a french drain in fishbone pattern to decrease the fungi problem ?

Thanks for advise !
Dirk

Answer
'sables-limoneuses' - 'mud and sand' says my Babelfish translator.  Sacre bleu!

The mud -- This is CLAY mud?

Here's what I'm going to tell you.  You're not going to like it.

Monsieur, GET A SOIL TEST!

You're a progressive Gardener -- you have birds, toads, clover.

You're sophisticated.  Not only do you know how to use Alfalfa pellets, but you know WHERE TO BUY THEM!  in FRANCE?

Tell me -- you did this Soil Test already, perhaps?

You also know that Fungus is triggered by moisture.  No soil test?  Out of character here.  I think you got one.

But if you had any soil test results, you wouldn't be asking me soil questions, no?

The Gold Standard of soil testing is conducted by Soil Foodweb, Inc.:

http://www.soilfoodweb.com/

an international group of soil biologists with members in the US, UK, South Africa, New Zealand, Australia and more recently Canada.  The UK lab has a website with instructions for submitting soil samples for analysis:

http://www.laverstokepark.co.uk/html/home.html

Please consider this seriously.  Let's move on.

Clay and Sand is the recipe parfait for HARDPAN!  You know what that is?  That's the stuff you can DRIVE on.  It makes a perfect parking lot.  Probably stage coaches liked it.  But it's not the kind of stuff you can farm in.  A hint about this hardpan 'sables limoneuses' may be that you did not anywhere mention 'Earthworms' -- 'Vers de terre'. What's up with that?  What ARE those Birds and Toads eating for Breakfast?  You don't sound like the kind of Gardener who dabbles in chemical warfare with Nature.  No Scotts for you.  Earthworms are not just a sign of a chemical-free Garden.  They're a sign of HEALTHY soil, the kind you need to grow world class Grass.

The sables-limoneuses contributes to any trouble your Grass is having.  Your addition of Compost -- perfecto! but for the possibility that you did not apply any of this to the slope where it is desperately needed due to the rains.  Plus I am guessing you did not core-aerate as part of this plan.  But more important, what kind of Grass did you Grow on the sables-limoneuses slope?

I had a reader ask an Erosion question not long ago.  I told him to read 'Prevent Soil Erosion by Quickly Establishing A New Lawn', posted on the website of a Farmington Hills, Michigan landscaper:

http://www.angelos-supplies.com/asi2/establish_new_lawns.html

Erosion control blankets and Grass blankets are one technique for handling Erosion problems, along with a specialized seeding technique called Hydroseeding that is catching on here.  Soil Erosion Control Blankets stop your Birds from eating the Grass seed while it's still germinating.  Seed needs at least some contact with the soil plus moisture to grow.  But if your seed was not even germinating, that could be another issue -- Blankets allow light and water to pass through.  They prevent new seed from blowing away or being snacked on.  Also have a look at the Sylva Natives Nursery website:

http://www.sylvanative.com/erosion.htm

Some shrubs are champion foilers of Erosion.   Rugosa Roses are used by landscapers to stabilize beaches and coastline regions subject to Erosion.

Grass Roots cannot penetrate compacted sables-limoneuses; they become shallow roots that need to be watered more often.  When a drought finally occurs, and the plants die, the Roots lose their grip.  Erosion begins.  See Matt Rosenberg's 'Erosion' website on About.com: Geography:

http://geography.about.com/od/physicalgeography/a/erosion.htm

Rosenberg points out: 'The faster water moves in streams, the larger the objects it can pick up and transport. This is known as critical erosion velocity.'  Plants slow water down above ground while gripping the soil, minimizing erosion, when it's raining cats and dogs.

I take it you are not happy about the Clover.  White and Red Clover are desirable species, pouring Nitrogen into your Soil, tipping the odds in favor of Grass.  White Clover prefers moist, rich Soil with excellent drainage, although it will settle for almost any soil if needed.  It has shallow roots and struggles when grown on dry soil.  So you cannot count on it to improve your Erosion problems.

Your Fishbone-Theme French Drain sounds creative and unique.  Worth doing even for someone without a Drainage problem.  You must address your Drainage situation, because bad Drainage will prevent the most important correction in your soil: incorporation of air.  Compost will help you with that.  If there are Earthworms underfoot, they will pull the Compost down, beneath the soil surface, over time as they tunnel.

You describe symptoms on your Grass very much like a possible case of something called 'Red Thread'.  This Fungus usually hits Rye and Fescue in Spring and Fall, but other slow-growing undernourished or stressed cool season varieties low in Nitrogen are also potential targets.  This Fungus thrives in mild, humid conditions with temperatures in the 60s F and 70s F.  See the following photos of afflicted Grasses posted by Cornell University's School of Agriculture:

http://plantclinic.cornell.edu/FactSheets/redthread/redthread.htm

and by the University of Illinois Extension Service:

http://www.ipm.uiuc.edu/diseases/series400/rpd413/

which describes Red Thread's 'conspicuous, pale to bright coral pink, orange, or red mycelial masses on the grass blades and leaf sheaths'.  Red Thread is a problem you find in grass that is deficient in Nitrogen.  Dry weather and Nitrogen go a long way toward eliminating it.

The good news: Red Thread does not usually kill Grass.  So it is not usually held up as a significant threat to Lawns.  Its attacks are worse in Autumn because it thrives in cool, moist weather.  I urge you not to use any Fungicides on this problem -- including ORGANIC kinds such as baking soda -- because you don't want to wipe out your Fungus populations indiscriminately.  Most Fungi are very positive forces in your Soil.  Bad Fungi erupt only when opportunity strikes.  You're best off controlling them by eliminating the opportunity.  If you choose instead to wipe them out, a handful of tough Bad Fungi will endure and seize a chance to bring blight to your flowers.

The 'food' for the Grass would only be washed out if the food was made by Scotts.  Your Organic soil building efforts are glued firmly to your rock solid soil, trapped molecularly, waiting for microbes to return and unlock minerals and vitamins to their hearts' content.

I have tried to address each point you make in your question.  Anything not mentioned, I would pat you on the back for each paragraph if you were standing here.  Vive la Grass!  Merci beaucoups for writing!  Any questions?