British Columbia: Best acidic compost for Rhododendrons?

I have older (10+ years) rhodies and a couple of styrax trees growing in the two concrete planting wells of my small Vancouver townhouse complex (wells are: 5 ft x 20 ft x 6 ft deep). They've been neglected for some time, and I believe they could use some acidic compost mulch. I've heard mushroom compost is not acidic enough. A mulch of bark chips was applied a year ago. (I know that the concrete wells are not good for acid-loving plants, but that's what I have to deal with!)

Can you tell me what is the best compost to use and who might supply it? Should I rake the bark mulch aside before putting the compost down? And then add some slow release acidic fertilizer?

All the leaves are definitely on the yellowish side. The rhodies are blooming now, the styrax is just beginning to leaf out.

Thanks!

 

The best starting point would be a soil test, to try and find out why they are yellowish. It could be nitrogen deficiency, for instance, and not the pH.

 

Thanks, Ron. I did a home test a few months ago and it showed it to to be around 7. I guess my real concern is that the plants are in concrete wells and have not had organic material (other than wood chips) added in many years. I agree that a nitrogen deficiency could be an issue.

What about using something like Sea Soil as a light mulch, along with a light dose of slow release rhododendron fertilizer?

Many thanks.

 

That is fairly high, in which case you would probably want to start working on raising the pH. Acid soil fertilizers might eventually have some effect on this. Otherwise, mulching is a good general practice and is best done with clean woody material such as arborist wood chips (without weeds and debris).

 

You'll probably want to lower the ph:) Try some sulfur... http://www.improve-your-garden-soil.com/use-of-sulfur-to-lower-soil-ph.html

 

Don't do anything drastic, it's detrimental to soil microlife. A change in pH of even on point is a huge step. Expect to be gradually increasing the acidity over a period of years.

 

Thanks for your replies! Last year it was mulched with arborists wood chips. I guess my concern is that there might not be much real life in the soil since it's contained in those concrete planting wells. I thought adding organic material would help, and using something on the acidic side would be best.

Ron... when you say "acid soil fertilizer" do you mean what is sold as rhododendron fertilizer?

Many thanks.

 

If you want to boost soil life, add some humic acid. In my experience it works wonders. I added it to my orchard soil last year and my earthworm/bio population exploded.

http://www.blackearth.com/homeContent.asp

You can buy it locally in 50 pound bags for cheap. 1 bag would be all you'd need.

 

Note that wood chips are known to use up nitrogen as they rot. You definitely need an acidic source of nitrogen to compensate. Ammonium sulfate is one non-organic source used by blueberry farmers.

 

Concern: Wood chip mulches will
tie up nitrogen and cause deficiencies in
plants.
Evidence: Actually, many studies
have demonstrated that woody
mulch materials increase nutrient
levels in soils and/or associated plant
foliage. My hypothesis is that a zone
of nitrogen deficiency exists at the
mulch/soil interface, inhibiting weed seed
germination while having no influence
upon established plant roots below
the soil surface. For this reason, it is
inadvisable to use high C:N mulches in
annual beds or vegetable gardens where
the plants of interest do not have deep,
extensive root systems

http://www.puyallup.wsu.edu/~linda ...Myths_files/Myths/magazine pdfs/Woodchips.pdf

 

In regards to nitrogen being locked up in the decomposing chips, as long as you don't mix the mulch in with the soil you'll be ok.

Some acidic fertilizer will help as well, as it sounds like the soil is nutrient deficient anyways.

http://www.store.tlhort.com/p-21670-gardenpro-rhodo-and-azalea.aspx (scroll down)

Get a soil test if you really want to know your soil conditions.