Confused by Soil pH? A Detectorist Explains All ("An Essential Bit of Kit")

  Soil pH map UKSO 2024   

Ooo needsta go to Skool to make an impresshun in merry tekkie-land? A notorious English tekkie old-timer on his arkie-bashing blog drags up, as a tip "for the less experienced", some half forgotten tekkie-lore from the 1970s and writes about "An essential bit of kit" (John Howland, 29th December 2023). 

Back in the late 70s and early 80s it was discovered (by a detectorist) that high pH (alkaline) readings in soil strongly indicated evidence of past human presence. As we know, where there’s been human presence or habitation… it’s highly likely there’ll be tangible evidence of such in the form of coins and relics. Unsurprisingly, there was a mad rush to get one of these pH gizmos from local gardening centres. They were back then, and still are today, available at nickel and dime prices. These pH meters are invaluable little gadgets for determining human habitation/activity in a suspected area. Using a pH meter increases your finds potential [...] – it will give you a definite edge and is probably the most useful bit of kit in your armoury after your metal detector.

So, you wander across the landscape and take measurements with a soil pH meter and these pH readings, he says, will guide you to the artefact productive sites. The results should be interpreted as follows:

"Acidic (Human habitation free)
acidic….3.5
Extremely acidic….3.5 – 4.4
Very strongly acidic….4.5 – 5.0
Strongly acidic….5.1 – 5.5
Moderately acidic….5.6 – 6.0
Slightly acidic….6.1 – 6.5
Neutral….6.6 – 7.3
Alkaline Readings
Slightly alkaline….7.4 – 7.8: Possible human occupation
Moderately alkaline….7.9 – 8.4 Probable human occupation
Strong alkaline….8.5 – 9.0 Likely human occupation site
High alkaline….9.0 JACKPOT!

Take several readings across the area your intended search area so as to ascertain its acidity or alkalinity, or to ‘outline’ the habitation area. Readings above ‘7’ are indicative of human existence. The higher the pH reading, then the higher the probability of extended human habitation in the form of settlements, turnpikes, and the like, all of which transmits to…FINDS. In effect, your pH meter suggests human occupation and your metal detector proves it by what it locates".

Simple, eh? And very "scientific", imagine how impressed yer farmer's going to be.... 

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This is a very good example why I am very sceptical of my Bitish archaeological colleagues and their wishy-washy ideas that "metal detecting" (artefact hunting) is done by "good ol' boys who just want to learn something", who want to find out something... and we should be encouraging (and even promoting) this. This is what they say (though I can guess after hearing such things how many of them have ever [like me] spent any time on a metal detctorists' forum to check out their impressions of who and what it is we are dealing with).

Let's skip this guy's faulty use of his own language (the word "potential"), and move onto those soil pHs... 

I think there are probably very few farmers in the UK who do not know about (or could talk about) pH. It is a pretty fundamental soil characteristic. It affects microbial activity and chemical activity, which in turn affects availability of elements, microelements to the growing plants (thus crop yield). Soil pH needs to be managed if it starts to shift outside a certain range. I once was shown a photo of what I was told was John Howland's house, he'd ripped up a large portion of the green bits and gravelled the lot, apparently not keen on the gardening. This might explain his total ignorance of soil pH (BBC 2020). Any gardener would know that a soil with a ph of 9 is not a "jackpot", it is a desert.  Duh. That takes an enquiring mind about 12 seconds mouse-work to find out. Howland apparently did not even have an inkling (what did they teach them about pH in his school?) that there was anything more to find out. 

A desert (Msimbira and Smith 2020)

How many tekkie-supporting archaeologists knew that? So, what is the mechanism by which, allegedly, "high soil pH strongly indicates evidence of past human presence"? Howland does not say, again, it seems not to be something he even thought of investigating. In fact there is quite a lot of literature on geochemical changes in soils caused by human activity, in the past (and present, understanding this is a vital part of soil chemistry). Actually, in most of that done on archaeological sites pH is not one of the main subjects of investigation. Without going into it in any detail, suffice to note that the chemical processes occurring in soils are very complex and there is no simple equation between soil pH and human activity. How could there be?

    Soil pH. Cranfield 2018   

But what beats me is the apparent total lack of awareness that soil pH is variable across the UK. See the vignette at the top - it has no key, but reference to the one of the one on the  right will show that according to Mr Howland's tekkie lore  rule-of-thumb, huge areas of the country were devoid of any kind of human activity because the pH is too low...  and the concentration of human activity was on the basic soild of the chalklands and limestone regions. That of course goes very well with a 1950s "Labybird Book of Prehistoric Britain" vision of the past - does not really tally with what we actually know about past and present settlement of the British Isles.  Also what Howland seems unaware of is that (as the Cranfield research that produced this map shows), soil pH changes with time due to a variety of factors. 

The issue here is not that some British tekkie has unreflexively posted something stupid. The issue is with the model that British archaeologists can lazily shrug their shoulders at the depradation of 40000 people just like Mr Howland emptying the archaeological record of diagnostic and other metal artefacts all over the country, day after day under the shallow pretence that they are doing some kind of "citizen archaeology" and "learning about the past". Time and time again, what the metal detecting milieu actually produce show that the majority of them are incapable of actiually learning anything. In the same way, we cannot rely on their "observations" of the context (or lack of it) of the objects they bring to us as some kind of ersatz archaeological information. The majority of people that go out with metal detectors are not equipped to produce it.

Fact.

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References:

BBC 'Soil pH level explained', Gardeners' World Magazine, 10 January 2020

Cranfield University 2018, "Modelling national-scale changes in soils".

Msimbira, Levini A. and Donald L. Smith 2020. “The Roles of Plant Growth Promoting Microbes in Enhancing Plant Tolerance to Acidity and Alkalinity Stresses.” Frontiers in Sustainable Food Systems. 

UKSO 2024, Countryside survey of topsoil in Great Britain UK Soil Observatory.