cybersix said:
@sooby, many thanks for taking the time to write your useful post. I read on an italian forum that horse manure is good for alkaline and clay soil, because it helps organisms and earthworms so in a certain way it brings back soil to life, then the things living down there make a better soil. But I'll take the time to understand things better!
You're welcome, it was basically just cut and paste from a document I'd prepared a few years ago. Horse manure will certainly improve a clay soil, but your problem with chlorosis may not be fixed by horse manure because it will not do much if anything to lower the soil pH. In some cases horse manure can actually make a soil more alkaline, especially if the stables have used lime as a disinfectant.
I had problems with interveinal chlorosis in my daylilies years ago and I had amended the soil (natural pH here is about 6.8 so lower than yours) with horse manure - we had horses at the time. I had the daylily leaves tested and the problem was manganese deficiency due to the pH being too high, and since then I've found quite a few people have this problem with certain daylilies.
To see how important the pH is to nutrient availability, check out this chart from Michigan State University. You can see how the availability of the micronutrients tails off as the pH gets higher (except for molybdenum).
http://msue.anr.msu.edu/upload...
I certainly wouldn't discourage you from improving your clay with composted horse manure (definitely don't use poultry manure), but it won't necessarily fix the chlorosis problem.
