Sørensen's pH Scale and Agriculture
Søren Sørensen and the pH Scale
Measuring the hidden chemistry that governs life
As agricultural science moved deeper into chemistry, one persistent problem remained:
Soil reactions mattered—but they were difficult to describe with precision.
Farmers knew some soils were “sweet.” Others were “sour.” Certain crops thrived in one field and failed in another, even when nutrients appeared similar.
What was missing was a common language for acidity and alkalinity.
That language arrived in 1909 through the work of Søren Sørensen.
Why acidity mattered
Long before the pH scale existed, growers understood its effects.
They observed that:
- legumes struggled in sour (acidic) soils, while crops favoring sweet (alkaline) conditions performed better
- lime improved structure and crop performance
- nutrients behaved differently from field to field
But these observations lacked precision.
Without a way to quantify soil reaction, recommendations remained general and inconsistent.
Chemistry needed a ruler.
The birth of the pH scale
Sørensen, working in biochemical research, introduced the pH scale as a way to measure the concentration of hydrogen ions in solution.
The scale:
- ranges from acidic to alkaline
- is logarithmic rather than linear
- reflects exponential change with each whole unit
Here’s the simplified breakdown:
- More hydrogen ions → lower pH → acidic
- Fewer hydrogen ions → higher pH → alkaline (basic)
pH Value — What it Means
- 0–6: Acidic (lots of hydrogen ions)
- 7: Neutral (balanced)
- 8–14: Alkaline (fewer hydrogen ions)
This exponential point is critical.
A shift of one pH unit represents a tenfold change in acidity.
Suddenly, subtle differences could be expressed clearly—and compared reliably.
From laboratory to soil
Although developed for biochemical applications, the pH scale quickly proved invaluable to agriculture.
Soil reaction was no longer subjective. It could be measured, tracked, and adjusted.
This transformed soil management.
pH emerged as a master variable because it influences bulk processes that early 20th‑century scientists could observe and measure:
- Nitrification — the conversion of ammonium to nitrate slows sharply in acidic soils
- Organic matter decomposition — acidity alters decay rates and nutrient release
- Root function — acidic conditions increase root injury and restrict elongation
- Calcium availability — low pH reduces calcium presence on exchange sites
- Toxicity — acidity increases the solubility of elements like aluminum and manganese, which can damage roots
- Chemical reactions at the soil interface — governing whether nutrients remain available or become locked away
In many cases, nutrients were present—but unavailable due to pH constraints.
Why pH governs nutrient access
Each nutrient operates within a preferred pH range.
Outside that range:
- phosphorus can bind tightly to other elements
- micronutrients may become insoluble or toxic
- microbial processes slow or shift
This explains a common frustration:
Why does a soil test show nutrients that plants cannot use?
pH determines whether chemistry is allowed to function.
The garden lesson: chemistry before correction
Gardeners often rush to add nutrients when plants struggle.
Sørensen’s contribution reminds us to pause.
If pH is out of range:
- fertilizers may be wasted
- amendments may underperform
- biological activity may stall
Correcting pH does not feed plants directly.
It creates the conditions for feeding to work.
pH as a bridge concept
The pH scale quietly unites chemistry and biology.
It is a chemical measurement. But its consequences are biological.
Roots sense pH. Microbes respond to it. Minerals behave differently because of it.
Sørensen did not study soil specifically. Yet his work gave agriculture one of its most powerful interpretive tools.
Why Sørensen belongs among the pioneers
Sørensen did not tell farmers what to add.
He gave them a way to understand why additions succeed or fail.
The pH scale allowed agriculture to:
- diagnose before prescribing
- compare soils meaningfully
- refine mineral and biological management
It also prepared the ground—quite literally—for deeper insights into balance, cofactors, and biological mediation.
Setting the stage forward
With pH, agriculture gained an exponential lens.
Small changes mattered. Thresholds became visible.
This concept will return when we revisit CHNOPS, cofactors, and biological exponents.
Because in living systems, scale is never linear.
Next, we continue forward—toward how pH, minerals, and biology intersect to govern the chemistry of life in soil.