—orig. developed through MOICANNA
– originally developed through MOICANNA
—orig. developed through MOICANNA
– originally developed through MOICANNA
HERBARIUM
A grower sees numbers.
pH 5.8… pH 6.4… EC 1.2 mS/cm… EC 2.1 mS/cm….
The plant does not see numbers. The plant experiences chemistry.
That difference is the beginning of serious cultivation.
A pH or EC meter is not a judge, a doctor or a recipe.
It measures a defined property of the sample touching the probe and turns that measurement into a number the grower can interpret.
The danger begins when the reading
becomes a ritual instead of a question.
A meter does not explain the system.
It gives the grower
one piece of evidence about it.
The pH scale looks simple because the numbers are small.
5.0… 6.0… 7.0…
But the scale is logarithmic.
The pH concept introduced by Søren Peter Lauritz Sørensen in 1909 gave chemistry a logarithmic language for hydrogen ions. The modern definition is based on hydrogen-ion activity.
A difference of one pH unit corresponds to a tenfold difference in hydrogen-ion activity.
The display looks linear.
The relationship is not.
That is the first hidden lesson.
A shift from pH 6 to pH 5 is not merely one equal step on a linear scale. It represents approximately ten times greater hydrogen-ion activity.
A shift from pH 7 to pH 5 represents approximately one hundred times greater hydrogen-ion activity.
This does not mean every 0.2 drift is a disaster. Substrates and solutions differ in buffering behaviour, and the significance of a drift depends on the crop, medium, alkalinity, nutrient formulation, sampling method and duration.
A decimal change is evidence to interpret
—not an automatic emergency.
The grower who treats pH as a casual decimal
has not understood the scale.
pH does not tell the grower
how much nutrition is present.
It describes one chemical condition that can influence nutrient solubility, speciation, adsorption, precipitation and biological activity.
Iron, manganese, phosphorus, calcium, magnesium and other elements do not respond identically to changing pH. Their behaviour also depends on the medium, ion concentrations, exchange surfaces, water chemistry, microbial processes and root activity.
Nutrients may be present while part of the supply remains poorly soluble, chemically bound, unevenly distributed or difficult for the plant to acquire.
The question is therefore not only:
“What did I add?”
It is:
“What forms may now exist in this root zone,
and what evidence shows that the plant can acquire them?”
The plate can be full
while part of the meal remains out of reach.
EC feels more direct than pH.
Within the same water and nutrient formulation:
Useful—but incomplete.
Electrical conductivity measures the ability of a solution to conduct electrical current. That conductivity is influenced by the concentration, charge and mobility of dissolved ions, as well as temperature.
In cultivation, EC is therefore used as a practical indicator of the combined conductive effect of dissolved ions in irrigation water, nutrient solution, drainage or a defined substrate extract.
It does not count ions, identify them or measure every dissolved substance.
It is not a complete nutritional analysis.
EC can help monitor changes in soluble salts across irrigation water, nutrient solutions and consistently collected root-zone samples.
But the reading remains non-specific: it cannot reveal the individual nutrient profile behind the conductivity.
EC does not tell you which ions are present.
The meter sees conductivity.
It does not see intention.
This matters in cultivation because the plant does not want “EC”. It wants a balanced set of usable ions at the right concentration for its stage, environment and water movement.
EC can reveal that the conductivity of a solution has changed. Consistent measurements may help identify trends in feed preparation, water quality or root-zone salt concentration.
But EC cannot tell the grower:
EC is honest, but narrow.
It measures conductivity,
not nutritional balance.
EC meters measure electrical conductivity.
Many instruments can also display a number labelled ppm or TDS, but that value is usually calculated from the measured conductivity through an assumed conversion factor.
The conversion factor is not universal.
On a nominal 500 scale:
1.0 mS/cm may appear as approximately 500 ppm.
On a nominal 700 scale:
the same 1.0 mS/cm may appear as approximately 700 ppm.
The solution did not change.
The display language did.
This becomes dangerous when a feeding recommendation gives only a ppm target without identifying the conversion scale.
“Feed at 1,000 ppm” is incomplete information.
It may represent approximately 2.0 mS/cm on a 500 scale or approximately 1.43 mS/cm on a 700 scale.
EC avoids this particular ambiguity because it reports the conductivity measurement rather than a derived concentration estimate based on an assumed conversion factor. Other conversion factors also exist, which is why the scale must always be declared.
A ppm or TDS display can still be used when the conversion convention is clearly stated and all comparisons use the same scale.
The rule
ppm without a declared scale
is a number without a common language.
EC does not stop working because a system is described as organic.
It continues to measure dissolved charged species present in the sample at the moment of measurement.
Some organic fertilisers already contain substantial concentrations of soluble mineral ions and may raise EC immediately. Other materials contain nutrients in molecules or particles that must first be decomposed, mineralised or chemically transformed before those nutrients appear as free ions.
EC cannot measure that future release.
It also cannot distinguish between useful nutrient ions and unwanted salts.
The lesson is not:
“EC does not work with organics.”
The lesson is:
EC measures present conductivity,
not total fertility or future mineralisation.
In biologically active substrates, EC should therefore be interpreted with the water source, extraction method, substrate properties, crop response and time.
Two irrigation waters can show the same pH
and behave very differently.
pH describes the current acid–base condition of the sample.
Alkalinity describes its acid-neutralising capacity—commonly arising from bicarbonate and carbonate in irrigation water.
Water with low alkalinity may change pH sharply after a small acid addition. Water with high alkalinity requires more acid to produce the same movement and may push substrate pH upward over repeated irrigations.
This is why correcting irrigation-water pH without knowing alkalinity can become a ritual of temporary numbers.
Reverse-osmosis water often makes this behaviour especially visible.
Effective RO treatment commonly removes much of the source water’s dissolved mineral content and alkalinity. With little acid-neutralising capacity, a very small addition of acid or base may cause a large pH movement.
A fluctuating pH reading in nearly demineralised water does not necessarily mean that the meter has failed.
The sample may have very little resistance to chemical change, while its low ionic strength can also make a stable glass-electrode measurement more difficult.
The number may be chemically easy to move
and technically difficult to stabilise.
In nutrient preparation, the more useful pH is usually the pH of the completed, thoroughly mixed solution—not the unstable number shown by nearly demineralised water before its intended inputs have been added.
Product compatibility and mixing order
should still follow the manufacturer’s instructions.
Do not chase the pH of an unfinished solution.
Hardness mainly describes calcium and magnesium. The two measurements may be associated in some water sources, but they answer different questions.
pH tells you where the water is now.
Alkalinity tells you how much acid
it can neutralise before it moves.
The grower measures a reservoir, an irrigation solution, a drainage sample or a substrate extract.
The root lives in the rhizosphere.
That word matters. The rhizosphere is the narrow zone of soil or substrate directly influenced by roots and their associated microorganisms.
Its chemistry can differ from that of the bulk medium.
Roots alter local pH through proton transport, nutrient uptake, exudation and metabolic activity. Microbial transformations and the buffering properties of the medium modify the same environment.
An imbalance between cation and anion uptake can cause roots to release or consume proton equivalents to maintain charge balance. High ammonium nutrition commonly promotes rhizosphere acidification, while nitrate-dominated nutrition often promotes alkalisation—but the magnitude and even the direction of the response can vary with species, nutrient balance and conditions.
This is one of the most important hidden lessons for growers:
The root is not sitting passively
in the pH you measured.
It is helping to change it.
That does not make measurement useless.
It makes measurement more interesting.
Irrigation solution, drainage, pore water and laboratory substrate extract are not interchangeable samples.
Each answers a different question.
A drainage reading is influenced by irrigation volume, dryback, flow path, timing, substrate structure and the parts of the container through which the collected solution travelled.
A substrate extract depends on the extraction method, dilution ratio, equilibration time and the representativeness of the material sampled.
This means a target developed for one method cannot automatically be applied to a result produced by another.
Before interpreting a number, ask:
A number without a sampling method
is only half a measurement.
Irrigation pH, drainage pH, substrate-extract pH and rhizosphere pH describe related but different chemical locations.
A serious grower does not worship one reading.
A serious grower compares defined measurements across time.
EC is temperature-sensitive.
Measured conductivity changes with solution temperature, so specific-conductance readings are commonly normalised to a reference temperature—usually 25°C.
Automatic temperature compensation can improve comparability, but it relies on an assumed temperature coefficient. That coefficient is not identical for every ionic mixture.
Compensation does not remove the need to record temperature, use consistent procedures and allow the probe to stabilise.
A reading is not merely a number.
It is a number produced
under defined conditions.
pH measurement has a different temperature problem.
Temperature changes the theoretical slope of the glass electrode response. Automatic temperature compensation adjusts the instrument for that electrode behaviour during calibration and measurement.
It does not automatically convert the sample’s chemistry into the pH it would have at 25°C.
The actual pH of buffers and samples can itself change with temperature, while readings may drift until the electrode, reference and temperature probe reach equilibrium.
Temperature also changes the neutral pH of pure water.
At 25°C, neutrality occurs near pH 7. At higher temperatures, the neutral value falls because the ion product of water changes. Pure water near pH 6.1 at 100°C is therefore neutral at that temperature—not acidic in the ordinary acid-versus-base sense.
This is a useful chemical example, but not a direct cultivation target. Irrigation water and nutrient solutions contain ions, buffers and dissolved compounds whose pH–temperature behaviour is more complex.
The lesson is not to chase a moving neutral point.
It is to remember that temperature affects
both the sample and the measurement.
This is where Walther Nernst enters the grow room.
Not as a name on a wall,
but as part of the electrochemistry
that allows the probe to speak.
A pH probe is not a spoon.
It is an electrochemical system whose glass membrane, reference junction, hydration, calibration and temperature all affect the result.
A dry, contaminated, ageing or incorrectly stored electrode may respond slowly, drift or fail calibration.
Calibration is not bureaucracy.
It is a comparison with standards whose values are known.
EC cells also require cleaning and verification with an appropriate conductivity standard. Deposits, trapped air, contamination, temperature and an unsuitable calibration range can all affect the result.
Storage and maintenance procedures should follow the instrument manufacturer because electrode designs differ.
This is one of the most underrated lessons in cultivation:
Before you correct the plant, check the instrument.
Many grower problems begin with a false reading followed by a confident correction.
Then certainty.
Then correction.
The plant pays for a measurement
the grower never verified.
A good meter does not replace
judgement.
It makes particular changes visible.
It can show that a solution differs from its previous condition. It can support comparisons between consistently prepared inputs or consistently collected root-zone samples.
It cannot, by itself, distinguish deficiency from root damage, confirm “lockout”, identify an individual nutrient or prove why a plant changed.
Before acting on a reading, ask:
The best meter still needs
a chemically literate grower.
The instrument gives the reading.
The grower gives it meaning.
Sørensen introduced the pH concept at the Carlsberg Laboratory in 1909, giving chemistry and biochemistry a practical logarithmic language for hydrogen ions.
His original definition used concentration.
Modern pH is expressed through hydrogen-ion activity.
The grow room inherited a number
born inside brewing research.
Arrhenius helped establish the theory of electrolytic dissociation: dissolved electrolytes form charged species capable of carrying electrical current.
He received the 1903 Nobel Prize in Chemistry for his electrolytic theory of dissociation.
Every EC reading belongs to that ionic world.
The reservoir does not contain abstract “food”.
It contains a solution of charged species.
The Nernst relationship connects electrode potential with chemical activity and temperature. It forms part of the electrochemical basis through which a pH electrode’s voltage response becomes a displayed pH value.
The screen looks simple
because the electrochemistry is hidden.
The pH number is not direct sight of hydrogen ions.
It is an electrode response
interpreted through calibration.
Factual Note
pH is a logarithmic expression related to hydrogen-ion activity. A difference of one pH unit corresponds to a tenfold difference in hydrogen-ion activity, but it does not mean that one solution contains exactly ten times as much total acid or requires exactly ten times as much neutralising material. Buffering and alkalinity must be considered separately.
Søren Peter Lauritz Sørensen introduced the pH concept at the Carlsberg Laboratory in 1909. His original expression was based on hydrogen-ion concentration; the modern definition uses hydrogen-ion activity.
Electrical conductivity measures the combined ability of dissolved ions to carry electrical current. It is influenced by ion concentration, charge, mobility and temperature. EC does not identify individual ions, determine their ratios or establish that the solution contains nutritionally appropriate concentrations.
EC values are commonly reported in mS/cm, dS/m or µS/cm. One mS/cm equals one dS/m and 1,000 µS/cm. Many instruments displaying TDS or ppm do not directly measure the total mass of dissolved material; they convert the EC reading through an assumed factor. Under common nominal conventions, 1.0 mS/cm may appear as approximately 500 ppm or 700 ppm, and other conversion factors also exist. A ppm target is therefore incomplete unless the conversion scale is identified.
Irrigation-water pH and alkalinity answer different questions. pH describes the current acid–base condition, while alkalinity describes acid-neutralising capacity and helps predict how repeated irrigation may influence substrate pH. Hardness mainly reflects calcium and magnesium and is not synonymous with alkalinity.
Reverse-osmosis treatment commonly reduces dissolved minerals and alkalinity. Nearly demineralised water may therefore change pH sharply after small acid or base additions. Very low ionic strength can also make conventional glass-electrode pH measurement slow or unstable. In nutrient preparation, the pH of the completed, thoroughly mixed solution is generally more informative than the pH of the unfinished RO water.
The rhizosphere can differ chemically from the bulk substrate. Root ion uptake, proton transport, exudation, microbial activity and buffering all influence local pH. Ammonium-dominated nutrition commonly acidifies the rhizosphere, whereas nitrate-dominated nutrition often raises its pH, but plant species and nutrient balance modify the response.
Irrigation solution, drainage and substrate extracts are different samples. Their pH and EC values depend on timing, irrigation history, dryback, flow path, sampling technique and extraction method. Interpretation limits developed for one method should not automatically be applied to another.
Temperature compensation for EC commonly normalises readings to a reference temperature, but the appropriate coefficient depends on solution composition. In pH measurement, automatic temperature compensation corrects the electrode response; it does not automatically convert the sample to the pH it would possess at another temperature.
The neutral pH of pure water changes with temperature because the ion product of water changes. Neutrality occurs near pH 7.00 at 25°C and near pH 6.1 at 100°C. This pure-water example illustrates temperature-dependent chemistry; it should not be treated as a cultivation target for buffered irrigation water or nutrient solutions.
A pH or EC reading is therefore evidence about a defined sample—not a complete diagnosis of plant nutrition. Reliable interpretation requires a maintained instrument, a documented sampling method, comparable measurements over time and independent evidence from the crop.
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The VADEMECUM is not just a book anymore. It is becoming a living archive of guides, tools, notes and practical plant knowledge.
Free member access. Join early. Keep the archive open.
The VADEMECUM is becoming a living archive of practical plant knowledge.
Free member access.