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Why Soil pH Testing is Misunderstood

BY W.F. BRINTON, Ph.D.

Recently, the centennial of the discovery of pH as a measurement tool relevant to biological systems has come and gone quietly across the world. Displaced from its early historical roots, the importance of testing the acidity of soil as pH, is virtually taken for granted today in farming and gardening circles. It’s a good time to take a refresher course on the topic, asking what is pH exactly and how does it apply as a management tool in biological farming?


Firstly, pH is a chemistry concept and clearly appears to be used in the conventional and organic farming movements to almost an equal extent. It has attained universal acceptance and applicability. And limestone, the “antidote” for soil acidity, considered a “natural” soil amendment, is used without any reservations in organic farming.

We are all very familiar with the popular farming and gardening charts seen in virtually all the literature that tell us that plants are sensitive to pH values. Also, quite popular are nutrient tables showing how much the solubility and availability of plant nutrients is affected by soil pH reaction. This information is used to make recommendations for corrective actions, including the addition of limestone, the choice of crops, the adjustment of soil mineral balances, and more. It is hard to imagine ignoring these precautions.

Sorensen’s Platinum-Hydrogen Electrode, from 1912, enabling the very first measurements of
the extremely tiny quantities of free hydrogen present as acidity in living systems.

Yet, a review of the origin of pH and how it became applied to soils, raises some challenging if not disturbing questions about all these popular assumptions. If we consider this along with the organic and ecological premise of fostering soil/plant self-regulation and improving native soil biodiversity, the concept of needing to adjust pH according to some abstract principle of long-forgotten origin deepens the mystery.

The author’s curiosity about soil pH increased recently due to a farm soil study in New England that compared variability of soil chemistry tests on 18 differing dairy farms. We chose dairy farms since they mostly re-use all their manures and possess to a high degree a nutrient sustainability groundwork, even if not organic. The comparison of differing soil tests concluded that pH appeared to be the least variable soil variable between fields and across all farms in 3 states, compared to nutrients like Ca, Mg, and especially phosphorus, which were the most variable. Curiously, pH stood out as showing very little variability, which looked not only improbable but distinctly requiring more investigation. This was especially the case as Ohio State University has recently promoted the idea that pH testing is so reliable that it should be “the gold standard,” which in fact it is not.

HISTORY OF pH

A long time ago, around 1909, the Danish physiologist and mathematician Søren Peter Lauritz Sorensen first developed the concept of pH, basing it on extensive studies on how much our bodily enzymes are affected by soluble hydrogen ions, the cause of acidity. Living organisms possess an extraordinary pH control system, especially the blood, and this metabolic process bathes all the supported interconnected organs in a remarkably buffered stream that resists change. Only a tiny drop — a ¼ pH unit in the blood, for example — can mean death.

For Sorensen, the problem was that the concentration of dissolved hydrogen was so extremely low and varied so much over such a vast range, that he proposed compressing it to report it logarithmically, such as by 10-6, 10-7 and so forth. This could be compared to the common practice in counting soil bacteria and fungi and reporting them in log terms like 2.5 x 106 instead of real numeric terms such as “2,500,000 cells.”

As a result of this variance, Sorensen came up with the concept of “pH”, literally meaning “potenz Hydrogen.” Next, he created a standardized scale for it, where only coincidentally, the mid-point meant a solution that was neither acid nor alkaline, or “neutral.” To popularize this pH scale, he dropped the base (10) notations altogether and instead just used the mantissa portion of the logarithm. This was still too complicated, so to further simplify, he also removed the negative exponent sign! The result was that now an acidity concentration of 10-7 ions (0.0000001 H+) became simply “pH 7.0.” At one point, Sorensen became concerned that people misunderstood that the acidity scale “goes in reverse,” meaning that higher numbers mean lower acidity. Today, this does not seem to concern us as we have been taught that low pH means more acidity.

More revealing is the implication of the logarithmic compression. What does this mean? Some scientists call it “normalization” when you take highly variable numbers and compress them to remove the uncertainty. With what Sorensen did, in effect, a pH of 7 was 10-times less acid than pH 6 – i.e. the difference between 10-7 vs 10-6. Looking more closely, a pH of 6.0 is twice as acid as 6.25. This indicates the hugeness of this compression. In other words, small differences in reported pH represent large differences in actual acidity. In fact, in view of this huge compression in pH values, it is difficult to comprehend how the popular literature deduces that nutrients and plants are “pH sensitive” — clearly nearly the opposite is the case.

Popular representation of dependency of soil nutrients on pH. The information is not practical.

The use of pH concepts for soils, does not appear in Sorensen’s time at all, but only in the late 1930s, and really only after World War II. In fact, Sorensen, in spite of and perhaps because of his brilliance, did not anticipate that pH applied to soil at all. When asked what other systems on earth the concept and measurement of pH would be relevant to, he commented that it would be the oceans. This is an extraordinary and insightful remark. It is now known that the oceans buffer pH change almost in the way blood does, and all the sea organisms that are bathed in it, depend on this.

The adaption of pH to soils posed a new challenge. Soils are not liquid and thus testing them violated Sorensen’s basic rule that acidity must be soluble to be accurately reported. This problem was first tackled in the post WWII years, with the expansion of mining of soil minerals and limestone and applications to commercial needs.

The basic soil explanation is often expressed as this: Hydrogen ions (the source of acidity) carry a positive charge (this is why it can be measured electrometrically). Soil particles, especially clays, carry negative charges on their surfaces and so positively charged hydrogen ions adhere to soil particles to a degree. Water, the most common solute for soil pH testing, has a poor ability to release the acidity present, since it carries no particular charge.

Soil scientists in Europe have long been aware of this problem and prefer extraction methods such as very dilute calcium chloride (CaCl2) or potassium chloride (KCl) solutions to test soil pH, since these natural compounds displace the hydrogen ions from their attachment to soil particles fairly completely, so that they are actually being measured in the soil solution, Sorensen’s basic requirement. We can know exactly the pH of a physiologic fluid such as blood, since it is in solution, but in soils the real pH cannot be so easily known using only water and extracting only a portion of the real amount. In fact, the actual pH of soil is now known to be ½ – to 1 log digit lower than measured in water; i.e. the acidity that plants are actually experiencing is 7-10 times stronger than common plant and nutrient charts suppose, making these pH-sensitivity tables even more problematic.

Early cautionary notes by scientists about this dilemma in attempting to measure soil pH properly indicated that pH measurements must be carried out using an electrolyte solution of known composition, otherwise soil tests cannot be compared. In most respects, this issue was never resolved pro or con, and instead, the debate just lessened over time. This follows the principle in science and chemistry, namely that errors that are not life threatening and persist for long enough become accepted, requiring a huge effort later to overthrow

What this means really in the soil pH world is that in order to obtain comparable results from different soils, the exact method needs to be published. For a variety of industry and scientific reasons, as above suggested, this also never happened, and discussion and debate around this theme simply was dropped.

If there was any resistance to changing this requirement, namely that methods be published and measurement be based on the real, complete release of active hydrogen in soil, it may have been due to the fact that modern charts and tables for “pH preferences” became in time based on the simpler, but much less accurate soil-water method of testing acidity.

If we take these facts alone, it should make it obvious that from a biology perspective, plants (and nutrients) are actually remarkably pH-insensitive. In fact, unlike our knowledge of acidity in bodily fluids, we really don’t know what the real plant and nutrient thresholds are. They exist across a wide range of conditions. There is in fact no proper soil-water solution and therefore there is no ideal soil pH to be obtained, and in nature, no ecological pH-balancing system in soils exists comparable to ocean-buffering or physiologic fluid homeostasis. To understand soil pH, you have to examine the geologic legacy of a soil and its plant communities.

The fact as we have established that plants are fairly insensitive to a wide range of soil acidity should not be a surprise, and is ecologically of huge significance. We have only to consider the wide range of environments within which plants have evolved and adapted-to over millennia. Furthermore, as more becomes known, it turns out plant species diversity such as at the pasture level, is nearly inversely proportional to pH as acidity — i.e. if you manipulate it upwards, plant diversity, if not accustomed to higher pH values, will decline markedly.

This linkage of native soil mineral-balance to what we will call “natural pH” and to native diversity and animal health in situ, was substantiated in the famously comprehensive studies conducted out of Linz, Austria in the late 1960s. These examined soil mineral-plant-animal conditions in four agricultural regions of the country and measured dairy-herd health against biotic and mineral diversity. Ultimately, the healthiest farms were the ones that had not altered to any extent the native soil mineral composition and especially not applied lime to significantly alter pH balances outside of what was geologically indicated by the local soil origin. Yet these studies were largely ignored outside Austria (today Austria has the largest percentage of arable land area in organic farming (24%), undoubtedly partly because they figured this out early and put it into extension work, which helped make organic soil practices so much more cost effective with herd health improving drastically.

An increasing amount of ecological studies, to the extent they can penetrate the severely chemistry-dominated world of agronomy, are anticipated to show that plant biodiversity leading to forage quality and good nutrition will generally depend on not significantly manipulating soil pH (and other nutrient factors). The author grew up in Eastern Pennsylvania around arable soils that graded from the richer Lancaster-type Alfisols to piedmont soils of Ultisol taxonomy, and management requirements were drastically different. On the Ultisols, adding lime significantly and rapidly reduces trace element uptake in forage, but not so 30-miles west on the geologically distinct Alfisols, which are more resilient.

In any event, the continued emphasis in popular guides on the pH sensitivity of plants and nutrient availability based on pH, and the need to manipulate based on generalized concepts, is arbitrary if not misleading, particularly so for organic and eco farming.

GOING FORWARD

There are several conclusions that can be drawn. Popular charts on pH preferences for plants and minerals provide virtually meaningless information for any specific, local situation and should rarely be used.

An early “lesson” received was when the author interviewed Scott Nearing around 1973, in reference to growing blueberries organically. The author asked if Scott controlled soil pH, since all the popular literature tells us that blueberries “require low soil pH.” He responded that this was “totally unnecessary” and that he didn’t know why, attributing it, partly correctly, to the “power” of compost additions to soil. Why would that be? Curiously, we don’t have a good chemistry theory yet as to why organic and biodynamic farms evolve to become virtually pH insensitive, but this exercise may suggest the answer. Living systems exercise pH management internally and “communicate” it with surrounding environments. Nature possesses unique skills at overcoming pH restrictions. In organic management, it is hardly desirable to recommend manipulation, unless in the rarer cases of counteracting elements that in some weathered soils — prevalent in New England — can lead to phytotoxic effects, such as aluminum ion. Aluminum can interact harshly with phosphorus, for example, but only does so on certain soil taxonomic groups, and not at all in others. Similarly, soils with magnesite in parent material won’t ever likely need magnesium for animal nutrition, nor need it to be re-balanced for pH reasons; in these cases, the care should be to make sure plants don’t indicate any calcium deficiencies. The point is, not to control the pH, but where appropriate seek to reduce the aluminum activity, which may be the cause of phosphorus deficiency and to some extent calcium deficiencies, and vice versa, in limestone derived soils in which manures and fertilizers that possess free acidity, can improve phosphorus uptake.

The good news for ecological farming, especially at the farm scale, is that soil pH adjustment may be less important than previously believed, but also more intrinsic for biology than truly recognized. After all, the blood-based living kingdoms and the oceans depend on a tight acidity-control system. In many ways, the very early work and insights of Sorensen, and his hesitancy to see pH as relevant to soils, has cast a long, questioning shadow over a century, from which we are now recovering.

W.F. Brinton, Ph.D. is the owner and founder of Woods End Laboratories, Inc in Mount Vernon, Maine. This article modified from “The Case of soil pH” in Biodynamics (2019).