Here are some contrary views from science based sources:

Linda Chalker-Scott, WSU extension horticulturalist:

"A commonly held assumption states that coffee grounds are acidic, but this does not hold true experimentally. While two studies on coffee ground composting reported mildly acidic pHs of 4.6 and 5.26, others have measured neutral (7.7) to somewhat alkaline (8.4) pH levels. One researcher found that the pH of soil treated with coffee compost increased after 14 to 21 days of incubation, gradually decreasing thereafter. Obviously the pH of decomposing coffee grounds is not stable and one shouldn't assume that it will always, or ever, be acidic."

See also this Science Daily article sourced to an OSU compost researcher:

http://www.sciencedaily.com/releases/2008/07/080707171641.htm

Again, we cannot be sure of any direct relation between the pH of a substance tested in the kitchen and its effect over time on the pH of the soil. Generally the long term effect of any of the common organic materials seems minimal, except for mushroom compost that has had a lot of (inorganic) lime added to it. Decomposed organic matter seems to have a buffering or stabilizing effect on soil pH.

I apply a few cups of coffee grounds per year per rose with no discernible effect on pH.

Here is a link that might be useful: Chalker-Scott

hi, Strawberry Hill,

I like your excitement about your plants and finding natural solutions to your garden and soil problems. I do have some questions, though about your conclusions.

Rhodos are acid-loving plants and prefer a pH of 4.5 to 5.5. Could a small amount of expresso grounds drive your soil underneath the rhodo below that amount?

If rhodos are planted in soil that has too acid of a pH what happens? My reading tells me that a very low pH causes the rhodos not to uptake nutrients and the leaves turn chlorotic, or yellowish.

Some rhododendrons have foliage that turn reddish in autumn. Is there a possibility that is what caused the "pink" foliage rather than a small amount of espresso grounds?

I'll be sure to mix the alfalfa meal in with compost. Thanks for the info

Ingrid

Hi Harborrose: My rhododendrons are 9 years old. They are right in front of my door - look the same every year until now, the leaves have pink streaks when I put espresso grounds (we have lots of rain). I don't know what's in the espresso that put pink stripes to the leaves - I tasted the espresso, extremely bitter compared to my hubby's ground. For the past month all my roses are great - dark green leaves with hubby's coffee ground from Sam's club.

Hi Michael: Thank you very much for your info. I tested the acidity of both espresso and coffee ground twice: with red cabbage boiled in alkaline hard water, the second time with red cabbage boiled in distilled water. What happened puzzled me until you clarified - many thanks:

Red cabbage indicator boiled in alkaline tap water: espresso ground and coffee ground both turned pinkish (acidic) within 10 minutes. After 4 hours the reddish water changed to CLEAR in the coffee ground solution (you are right about its instability, I also read that coffee ground is a buffer, rather than acidic). The solution above the espresso ground retained its pink color.

I repeated the experiment again today, with red cabbage boiled in distilled water. Both coffee ground and espresso turned reddish brown. After 1 hour, I was surprised to see a CLEAR SOLUTION above the coffee ground -the espresso ground solution was still pink.

I also tested red cabbage boiled in distilled water with at least 10 other samples. Here are the results:

Purple indicator turns BRIGHT pink with a few drops of vinegar (reported pH 2.4) - Espresso ground is next in pink (my estimate pH 3 to 4) - Coffee ground is slight pink but turned CLEAR after 1 hour.

The control red cabbage color is the SAME as indicator solution in Epsom salt (pH is neutral as expected). Previous wrong alkaline result with Epsom salt was from boiling red cabbage in hard alkaline water. There is no change in color with calcium citrate (it's just more whitish with calcium dust).

Baking soda is bluish-green (reported pH of 9). Magnesium supplement (magnesium oxide) is bright green, and the most alkaline.

Peatmoss is the same color as alfalfa meal within the first 5 minutes - but after 1 hour settling down the peatmoss is clearly darker pink. Reported pH of peatmoss is 4, my estimate of alfalfa is 5.

MiracleGro potting soil is slightly pink (reported pH of 6.5). My amended topsoil is slightly bluish-purple (reported pH of 7.7, tested professionally by EarthCo.) - The bottom yellow clay is more blue, my estimate pH of 8, same as tap water tested with fish-tank litmus paper.

Here's my big disappointment with HomeDepo's EarthGro bagged topsoil (the white/red bag). It's greenish blue, at least 8.5 pH. Horse manure is alkaline as expected, my estimate at 8.7 (others report a higher value). The Moo Manure (sold at HomeDepo) is the most alkaline, the same color as the baking soda solution (reported pH of 9).

The most prolific bloomer in my garden is Radio Times, planted in Scott's Premium topsoil with peatmoss - I did not test this, but expect it to be neutral, or slightly acidic with peatmoss. Other roses amended with HomeDepo's Earthgro topsoil or Moo-manure don't bloom as much, with more yellowish leaves. Now I know why.

"Bagged topsoil" is not topsoil. It is usually some undetermined mixture of waste organic matter. Could contain municipal compost, sewage sludge, composted paper or wood, spent mushroom compost, etc.

Actual topsoil is mineral soil with moderate organic content. It has been near the soil surface for a while and has developed structure through natural processes.