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4hv.org :: Forums :: Chemistry
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Soil pH measurement and amendment

Author Post
Mon Jun 12 2017, 07:01PM Print View
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1501
Our Dawn Redwood tree (Metasequoia glyptostroboides) has outgrown its 1-gallon plastic pot. We aren't sure where it belongs in the ground, so it's about to move into a nominal 35-cm plastic pot. It's known as an acid-loving plant (pH 5.0 would not be too low), and soil dug up around here has measured pH around 7.0 or 7.5.

Who here is good with soil pH testing conventions? My kit from the hardware store comes with a clear plastic cuvette. You put in soil up to the line at about 1 cm^3. Is that packed or loose? Then you add some powder (entire contents of one gel cap), and water up to the line at about 5 cm^3. Mix, let the particulates settle, and read the color. Not sure if the powder has anything besides indicator dyes and inert filler.

I saved the juice after cooking some red cabbage (Oma called it Rotkohl) for dinner. Counted drops of vinegar and ammonia to toggle its color; then left the rest in fridge until mold grew on it.

Later, at the science store, I got a small bottle of Methyl Red and a pack of three-bar pH test strips. The color-matching card for ph 7 to 10 is on the other side of the box.

For those I've been using a glass test tube, with soil/water ratio similar to that in the Luster Leaf kit.
I know how the concentrations of H+ and OH- work (like electron and hole densities in solid silicon). But it's been many decades since I knew how to be quantitative about strong and weak acids, buffers, etc.

The amending of soil for agriculture has been studied since prehistoric times. It's been quantitative, in pH terms, since the 19th century. For acidification, some suggested additives are elemental sulfur, aluminum sulfate, and ammonium sulfate. Quantities are often expressed as equivalent weights of straight S. Which, apparently, slowly changes to sulfate through microbial action.

To avoid having to wait, or to pick a cation, I have used sulfuric acid. For example: 15 ml of drain opening fluid in 200 to 400 ml of water, used to dampen 2 coffee cans full of coarsely sifted soil, is not quite enough. Anybody got practical wisdom to offer here?
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Tue Jun 13 2017, 12:38AM
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Posts: 432
Peat and fresh compost can be quite acidic. I'd use one or both of those as the basis for an acid loving container plant. Ongoing breakdown of organic matter produces humic acid, which will help maintain pH. Mineral additives like sulfuric acid get washed out of a pot fairly quickly.
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Tue Jun 13 2017, 06:58PM
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 208
usually soil from under pine trees is very acidic
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