Soil pH mystery

Hi, I'm new on this forum.
We recently bought a house in east London. The garden has enviabl soil -- sandy loam, well enriched over the years, that drains fast.
In one patch, I'm making a bog garden and have been testing the soi pH. Overall, the garden reads 7 in places and nearly 8 in others. Th patch I'm interested has been slowly dipping below 7 as I add sulphu and saturate the ground.
But if you push the probe below the surface, you can hit 'hotspots where the meter spikes out and goes above 10. It's not calibrated abov ten, but it looks like a 12.
For those who don't already know, this is incredibly alkaline. pH 1 is 1,000 times more alkaline than neutral soil and pH 12 is 100,00 times more alkaline.
I know that the previous owners limed their compost, but these reading are bonkers. Could the fact that I recently used cement in this are have anything to do with this?
Any other explanations
-- Homer Sparrow
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Even over-liming would only give you a pH of 12 if there was a bag of the stuff buried. I wonder what has been going on in the past as overall pH 7 to 8 is high unless you are in a salt pan, on bedrock of limestone or something similar. Most natural soils, especially with a high organic content, tend to be slightly acid.
Have you tried digging down at these spots to see what is there?
Are you sure your meter is accurate? The cheaper metal probe type can be confused by variations in moisture and dissolved salts. Perhaps you can callibrate it against a dye indicator system.
David
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IMHO that's most likely cause of problems. Correct procedure of soil pH measurements calls for glass (or MOSFET) pH electrode (see procedure outlined here: http://www.ph-meter.info/pH-measurements-other-procedures ). Calibration of metal type probe against indicator won't help, as the probe result will change with the soil humidity level, while idndicator results will not.
Mr.pH -- http://www.ph-meter.info http://www.chembuddy.com
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Those metal probe pH meters are pretty much of a joke.
The only way to measure pH reliably is to remove a sample and mix it with water and measure it that way with a real meter or pH testing kit.
You can try the metal probe in the wet sample but your mileage may still vary a lot.
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