Most problems in hydroponics — unstable pH, phantom deficiencies, a plant that "won't grow" despite a correct recipe — share one common root: an unknown source water composition. Tap water is not neutral. It already contains salts, minerals, and buffers that actively affect your system — and until you measure them, any adjustment is working blind.
Quick glossary: EC (Electrical Conductivity) — shows the concentration of dissolved salts in water. KH — carbonate hardness, water alkalinity; determines how strongly the water pulls pH upward and returns it after correction. GH — general hardness, the calcium and magnesium content of the water. pH — the acidity of the solution; plants in hydroponics require the range 5.5–6.5.
What Water Does to Your System Before the First Nutrient
Tap water or well water already has its own EC — meaning it contains dissolved salts before you add any fertiliser. If source water EC is 0.4–0.6 and your recipe is calculated for "blank" water at EC 0.0 — the plant is receiving more salts than you think, and more than it needs.
At the same time, KH determines pH behaviour. Water with high alkalinity will constantly pull pH upward even after correction — not because the pH meter is broken or you used too little acid, but because the carbonate buffer in the water physically drags the solution back into the alkaline zone. Without knowing KH you will adjust pH every day and never understand why it won't hold.
And GH — the calcium and magnesium content of the water. If there is already a lot of both, and the recipe adds them in full — excess calcium blocks magnesium and potassium uptake. Symptoms look like deficiencies even though the recipe is correct.
What to Measure and How
Basic analysis requires three measurements: EC, pH, and KH. EC and pH are measured with portable meters — every grower has them. KH is measured with a drop-test kit (sold in aquarium shops, inexpensive). GH can be measured with the same kit or obtained from the water supplier.
This takes ten minutes and gives a complete picture of what you are working with.
If the water comes from a well or the composition is unknown — it is worth running a full laboratory analysis once a year. It will show iron, sodium, heavy metals, and other parameters that drop tests cannot measure. Tap water composition changes seasonally — a test stays relevant for roughly one year.
What to Do with the Results
Once you have the numbers, you fall into one of four situations:
EC < 0.4, KH low, pH stable — the simplest case. Water is suitable with minimal preparation; the recipe is adjusted for background Ca and Mg.
KH high (> 5–6°dH), pH constantly drifts upward — alkalinity problem. Solved by acidification or reverse osmosis.
GH high (Ca > 120 mg/L) — adapt the recipe for background calcium and magnesium, or replace part of the water with RO water.
Well water or unknown composition — full lab analysis, and most likely reverse osmosis with remineralisation.
Without analysis you do not know which situation you are in — and any decision about water treatment is guesswork.
Three Mistakes That Cost the Most
Assuming "normal water" needs no analysis. "Normal" means drinkable — not suitable for hydroponics. Drinking water can have KH 8–10 and EC 0.6–0.8, which already significantly complicates system management.
Running one test and treating it as valid for years. Tap water composition changes between seasons — in summer and winter suppliers use different sources and different treatment processes. Well water composition changes after heavy rain and flooding.
Buying filtration equipment before testing. A carbon filter for a hardness problem, a softener for an alkalinity problem — money spent, problem remains. A water test costs less than incorrectly selected equipment.
How to Know the Water Is Under Control
After measuring the EC, KH, GH, and pH of source water and selecting the appropriate treatment: the working solution's pH after mixing sits stably in the range 5.8–6.3 and does not shift more than 0.2 over 12 hours without plants. If it drifts — either the alkalinity is not neutralised, or remineralisation after RO is not calibrated correctly.
For deeper understanding: Water Preparation: From Analysis to Working Solution — the next step once you know your numbers.