Why the number is logarithmic
Scale: pH runs from 0 (acid) through 7 (neutral) to 14 (alkaline).
Tenfold steps: Each full point is a ten-times change, so pH 5 is ten times more acidic than pH 6.
Hundredfold gap: pH 4 is one hundred times more acidic than pH 6.
Solubility window: Most nutrients stay dissolved near 5.5 to 6.5 in mineral soil and 5.6 to 6.4 in hydro media.
What lockout looks like
Precipitation
Outside the window, dissolved nutrients fall out of solution and roots cannot take them up
Acid extreme
Plants in a very low-pH root zone stay tiny, sometimes only a few inches after months
Alkaline extreme
High pH plants look pale, sickly and stunted
Medium matters
The same water can behave differently in soil, soilless mix or hydro, so always read the fertilizer label
Rosenthal's working ranges
Adjust band
Bring irrigation water to about 5.8 to 6.3 if it sits outside that range
Hydro target
Acceptable 5.6 to 6.4, with 5.8 to 6.2 as the preferred hydro band
Soil target
Acceptable 5.5 to 6.5, with 6.0 to 6.3 as the preferred soil band, or 5.8 to 6.5 in a later soil note
After nutrients
Always measure pH after fertilizer goes in, because salts shift the balance
How often to test
Tap water
Recheck often. Utility flushes change pH without warning
Soil or buffered mix
Weekly checks are enough when the medium holds pH well
Hydro
Check every day or two, because roots and microbes move the reservoir faster
Tools
A meter is easier than a reagent kit, but either beats guessing
How to move pH
Small doses
Add adjuster in tiny increments until you know your water's usual dose
Garden products
Measured pH Up and pH Down from hydro shops are the convenient option
Raise pH
Potassium bicarbonate or wood ash can neutralize water that is too acidic
Lower pH
Nitric acid, sulfuric acid, citric acid, kombucha or vinegar can pull alkaline water down