Carbonate hardness (KH, from the German Karbonathärte) measures the bicarbonate and carbonate in water. Despite the name, it is really a measure of alkalinity, not of calcium or magnesium. It is also called temporary hardness because boiling drives off CO2 and precipitates part of it as limescale. Aquarium test kits report it in degrees of carbonate hardness (dKH).
In practice
KH is what makes hard tap water resist pH Down. A grower with water at 8 dKH (about 143 mg/L as CaCO3) needs considerably more acid per litre than one at 2 dKH (about 36 mg/L), even if both waters start at pH 7.8. An aquarium KH drop-test kit is a cheap way to know your water. Above about 6 dKH, plan for regular acid dosing or blending with RO water.
Units and formula
- 1 dKH = 17.86 mg/L as CaCO3 ≈ 0.357 meq/L
- 1 mg/L as CaCO3 = 0.056 dKH
Don't confuse with
- General hardness (GH): calcium and magnesium content.
- Alkalinity: KH is a way of expressing it.
- Potassium (K): the "K" in KH is German, not the element.
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.CO2 enrichment raises carbon dioxide in a sealed grow space from about 420 ppm to roughly 800–1,500 ppm, letting plants use higher light intensities productively.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.pH Down is any acid used to lower solution pH, commonly phosphoric, nitric or sulfuric acid; each adds a nutrient, and all need gloves and eye protection.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.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.General hardness (GH) is the calcium and magnesium dissolved in water, expressed as mg/L CaCO3 or German degrees (dGH); hard water supplies some Ca and Mg.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.
