pH is the negative base-10 logarithm of the hydrogen ion activity in a solution. Low pH is acidic, high pH is alkaline, and 7 is neutral at 25 °C. In the root zone, pH controls nutrient solubility and uptake: iron, manganese and zinc become less available as pH rises, while calcium, magnesium and molybdenum are less available when pH falls too low.
In practice
Accepted ranges, which GenetixBay treats as settled practice:
| Medium |
pH |
| Soil |
about 6.0–7.0 |
| Coco, rockwool and hydro |
about 5.5–6.5 |
Drift within the range is normal and even helpful: a solution that wanders from 5.6 to 6.2 over a few days exposes roots to the best uptake window for different elements. Correct only when it leaves the range. Established.
Measure pH of your input solution after adding nutrients, because fertilisers shift it. In soil, the medium buffers pH strongly, so the input matters less than in hydro. Check runoff occasionally to see what the root zone is doing.
pH = −log₁₀[H⁺]. A change of 1 unit means a 10-fold change in hydrogen ion concentration.
Don't confuse with
- Alkalinity: the water's capacity to resist pH change, not the pH itself.
- EC: total dissolved salts; unrelated to acidity.
Myth check
"pH must be held at exactly 5.8" is a Myth. A range is the target; see pH and nutrient availability.
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.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.Molybdenum (Mo) is the micronutrient needed in the smallest amount; plants need it to convert nitrate into usable nitrogen.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.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.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline) with 7 neutral; it is logarithmic, so each whole unit is a tenfold change in acidity.pH drift is the gradual rise or fall of a nutrient solution's pH between adjustments, driven mainly by which ions the roots take up.Nutrient lockout is when plants show deficiency symptoms although nutrients are present, because pH, salt build-up, ion antagonism or damaged roots stop uptake.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.
