"KH and Alkalinity are the same thing, both measure water alkalinity." Almost true — and that "almost" breaks your calculations. KH and Alkalinity measure similar things but in different units and with different precision. The bigger problem: even knowing alkalinity, most growers still adjust pH "by eye", adding acid until they hit the target number — instead of calculating exactly how much acid is needed to neutralize the buffer once and for all in a single mixing cycle.
Quick glossary: Alkalinity — total water alkalinity, the total concentration of all anions that neutralize acid (carbonates, bicarbonates, hydroxides); measured in mg/L CaCO₃ or meq/L. KH (carbonate hardness) — a related parameter but measures only carbonates and bicarbonates; for typical tap water KH ≈ Alkalinity, but they can differ in well water or after softeners. pH — solution acidity. Acid demand — the amount of acid in meq/L or mL needed to fully neutralize the buffer in a specific volume of water.
How KH Differs from Alkalinity and When It Matters
In most tap water, KH and Alkalinity are nearly equal — both reflect bicarbonate (HCO₃⁻) content. In practice, growers often use these terms interchangeably, and that is almost always correct.
The difference becomes important in two situations. First — well water, where significant concentrations of hydroxides (OH⁻) or silicates may be present; in that case Alkalinity can be substantially higher than KH, and calculating acid dose from KH alone will be insufficient. Second — water after an ion-exchange softener: Na⁺ has replaced Ca²⁺ and Mg²⁺ but carbonates remain — KH and Alkalinity may both be high, but the ion composition is completely different.
For practical purposes in most situations: if you have standard tap water or RO with remineralization — KH and Alkalinity are interchangeable. If you have well water or a non-standard source — run a full analysis.
How to Calculate Acid Demand
Acid demand is how much acid is needed to neutralize the entire carbonate buffer in a specific volume of water. Without this calculation you add acid "until pH drops" — and get a different result every mix.
Formula for phosphoric acid (85%):
Acid demand (mL) = Alkalinity (mg/L CaCO₃) × water volume (L) × 0.00074
Example: water with Alkalinity 180 mg/L, volume 100 L: 180 × 100 × 0.00074 = 13.3 mL of 85% phosphoric acid for full buffer neutralization.
The coefficient 0.00074 is for 85% phosphoric acid. For 60% nitric acid — 0.00065. For practical use: calculate the estimated amount, add 80% of it, check pH — then add the remainder if pH is still above 5.0. This is safer than adding everything at once and ending up at pH 4.0.
Practical Mixing Protocol
A step-by-step process that eliminates pH drift once and for all for your specific water source:
Step 1 — measure Alkalinity (with a KH test kit or drop-based alkalinity test) and EC of the source water.
Step 2 — calculate acid demand using the formula above. This is the amount of acid needed to neutralize the buffer — add it to clean water before adding nutrients.
Step 3 — add the acid, stir, measure pH. If the buffer is neutralized — pH should be below 5.0 and stable (not "bouncing" back up within 5–10 minutes).
Step 4 — add nutrients according to your recipe, stir.
Step 5 — fine-tune pH to the working range of 5.8–6.2 with a small amount of acid or base. After buffer neutralization this adjustment is minimal — hundredths of a mL per liter.
Once you find the right acid dose for your source and volume — record it and use it as your standard. When the season or source changes — recheck KH.
Three Mistakes That Cost the Most
Calculating acid by pH instead of alkalinity. "I add until pH hits 6.0" — every time a different amount depending on how much buffer remains. The solution is unstable, pH drifts. Correct approach: neutralize the buffer with a calculated dose, then fine-tune pH.
Ignoring Alkalinity when switching water sources or during seasonal changes. The same acid volume that worked perfectly in winter may be insufficient in summer when the supplier changed the source and Alkalinity increased. Checking KH for each new batch or once a month takes 2 minutes.
Neutralizing alkalinity only partially "to avoid over-acidifying." "I added a little acid, pH is 6.5 — looks fine." But residual buffer still pulls pH up after an hour. Full neutralization followed by precise fine-tuning gives more stable results than partial neutralization repeated all day.
How to Know Alkalinity Is Under Control
After neutralization and mixing: pH holds steady in the 5.8–6.2 range for 12–24 hours without plants, with no more than 0.1–0.15 deviation. The acid volume for each mixing session is consistent and predictable. If pH drifts more than 0.2 over 12 hours without plants — either alkalinity has changed, or neutralization was incomplete.
If you want to go deeper: pH in Hydroponics: How Acidity Determines Nutrient Availability — after understanding alkalinity, the next step is understanding how pH affects nutrient uptake and why the working range is 5.5–6.5.