The pH scale expresses acidity and alkalinity of water-based solutions, usually from 0 to 14 at 25 °C. Values below 7 are acidic, 7 is neutral and above 7 is alkaline. The scale is logarithmic: pH 5 has ten times the hydrogen ion activity of pH 6, and a hundred times that of pH 7.
In practice
The logarithm is why small numbers matter. Moving a hydro reservoir from 6.0 to 7.0 is not a small nudge; it is a tenfold drop in hydrogen ions, enough to make iron and manganese markedly less available. It is also why "a little" pH down can overshoot in unbuffered water: RO water has almost nothing to resist the change.
Reference points: lemon juice about 2, rainwater about 5.6, pure water at 25 °C 7.0, many tap waters 7–8.5, baking soda solution about 8.3, household ammonia about 11. Cannabis root zones are kept at roughly 5.5–7.0 depending on medium.
pH = −log₁₀[H⁺]; [H⁺] = 10^(−pH) mol/L.
Don't confuse with
- pH: the measured value; the scale is the system.
- Buffering capacity: how hard it is to move along the scale.
pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Alkalinity is water's capacity to neutralise acid, mainly from dissolved bicarbonate and carbonate, reported in mg/L as CaCO3; it decides how much pH Down you need.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Buffering capacity is how strongly a water, solution or medium resists pH change when acid or base is added; high capacity means stable but hard-to-move pH.A pH meter measures solution pH with a glass electrode whose voltage changes with hydrogen ion activity; it needs regular calibration and a moist probe.A pH buffer is a solution containing a weak acid and its conjugate base that resists pH change; in growing it means calibration buffers or water chemistry that stabilises pH.
