Question
QUESTION: Hi,

I have a question regarding soil amendment and hope you could help me with this one.

I would like to decrease the pH soil of some potted plants from 7.0 to 5.0 using H2SO4 (density 1.27 kg/ltr).
How much H2SO4 in ml should be mixed per 1 ltr of destilled water ? How many times of watering with this acid ingredient approximately to reach the target pH ?

The reason behind was that I need more acidic soil for some plants such as rose and hydrangea (blue). Have a bottle of H2SO4 (liquid one litre), d=1.27 kg/ltr, used normally for car battery. Instead of buying Sulphur Powder, I'm thinking about using this liquid H2SO4. If the concentration still too high, the plants will die but if too low then pH will never or change very slow.

Rgds,

Ari

ANSWER: Technically, you are right -- and to put this very simply, you could lower the pH with the method you describe.  Electrolytes in lead acid battery cells contain 33.5 percent Sulfuric Acid.  It also has a pH of around 0.5 to 1.0.  A product we could loosely call Chemical Fertilizer is produced from a 62.18 percent concentration of Sulfuric Acid.

And technically, you can indeed calculate the amount of H2SO4 --Sulfuric Acid -- for a given Soil sample.

And technically, Sulfur is a secondary macronutrient (along with Calcium and Magnesium).

Now for the fine print.

FYI: The Gold-celebrating Russians at Chemindustry.Ru point out: 'As a strong dehydrating agent, Sulfuric Acid reacts destructively with some organic compounds.  Thus, starch or sugars, being treated with it, become charred.'  Here's their website for your reading pleasure:

www.chemindustry.ru/Sulfuric_Acid_extra_pure.php

Fact is, Sulfuric Acid will merely acidify the Soil before leaching out.  Slow, enduring Soil corrections may require patience, but they also buffer your Soil.  The pH of the Soil depends not on the pH of the liquid you use, but many properties of the Soil itself.  Sort of like the difference between jumping off the roof of a building and taking the elevator.  You are not going to like the results of that jump.

BTW are you aware that the Blue in your Hydrangea depends not on pH of the Soil, but on the Aluminum that is freed up under acidic conditions?

Bottom line: If you just have to play with the Battery Acid, find a pH buffer solution with a pH of 5.0.  Battery Acid works in theory, but it will dissolve minerals present in the Soil and leave lumps of Salt deposits as the Water evaporates.  Yes, you will still be confronting some precipitation if you use a buffer -- but because it's a buffer, pH won't fluctuate wildly.

THE LONG ISLAND GARDENER

---------- FOLLOW-UP ----------

QUESTION: Hi,

yes, I'm aware about Blue Hydrangea depends on Al under acidic condition. I used (expired) stomach ache medicine consists of Al & Mg.

So far I'm only using 2 ml H2SO4 in 1-2 liter destilled water to water the rose plants or hydrangea sometimes. Oh yes, last time I put approx. 1 TBSP coffee (powder) on rose & hydrangea soil. I think this (coffee) is another alternative to lower the pH, right ?

Rgds,

Ari

Answer
'Coffee powder' sounds like instant coffee.  I don't know what the pH of instant coffee is, but I suspect you could get that easily, given your facile ways w/ chemical compounds.  My guess is that it is slightly acidic, not nearly what you are looking for.  Used Coffee Grounds are a little more moderate, although people assume wrongly that they are highly acidic.  Lemon juice pH is quite low, something like 2 or 3.  Peat is one of the more acidic albeit sterile growing mediums.  

There was an interesting study done a few years ago at Ohio State, 'Composts in the Landscape: Effects on Soil pH and Plant Growth':

ohioline.osu.edu/sc157/sc157_14.html

The researchers found that the pH of their Calcareous soil actually ROSE significantly in plots amended with Leaf Compost, peaking at 7.8.  However, they noted, the 'pH in Peatmoss amended plots was decreased by one full unit, and this effect lasted at least for the entire growing season.'

Sulfur was also used, applied at a rate of 3 lb/100 sq ft, and as expected it lowered the pH progressively over several months.  Then the researchers stopped using Sulfur.  When they tested again 2 years later, they found that pH had risen again.  The pH dropped one full unit in Peatmoss plots, but they also found that pH dropped very little in Compost-plus-Sulfur plots.  Their theory: 'It appeared that all of the Composts buffered the pH against changes due to Sulfur.'

Bottom line: 'Peatmoss plus Sulfur proved to be the best treatment for establishing azaleas in a calcareous Soil.  However, this work also shows that the pH effect is transitory.  Future work is needed to learn how to maintain a favorable Soil environment over time.'

Asked by a consumer how to lower Soil pH from 7 to 4.6, The Berryman recommend 'Sulfur with leaves, sawdust, pine needles, or wood chips...'  Here's their website, complete with pH-lowering Planting Guide:

www.asktheberryman.com/planting_guide/index.html

Your best shot at all this however comes from University of Minnesota's Extension Service.  The section on 'Soil Acidification' posts instructions for lowering pH with Sulfur by one unit by area and volume:

www.extension.umn.edu/distribution/horticulture/components/1731-3-soilpH.pdf

Sulfur is much stronger than Coffee.

THE LONG ISLAND GARDENER