Hydroponic nutrients explained: EC, pH, how to measure and when to change the solution

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In soil, the ground buffers mistakes: it holds nutrients, releases them slowly and evens out swings in acidity. In hydroponics there is no such cushion. The solution is everything the plant gets, so two numbers matter more than anything else: how strong the solution is (EC) and how acidic it is (pH). Learn to read these two and most problems become easy to diagnose.

What goes into the solution

Use a nutrient made for hydroponics. Soil fertilisers often rely on the soil to supply some elements and to break down organic matter, so they can leave a hydroponic plant short. Hydroponic nutrients come as one-part or multi-part concentrates. Multi-part products split ingredients that would react with each other if stored together, so always add each part to the water separately and stir in between. Never mix concentrates directly together. Follow the label for dosing, and start at the lower end for seedlings.

EC: how strong the solution is

Electrical conductivity (EC) measures how well the solution conducts electricity, which rises with the amount of dissolved salts, including your nutrients. It is a measure of total strength, not of which nutrients are present.

It is usually given in millisiemens per centimetre (mS/cm). Some meters show TDS or ppm instead, which is EC converted using a factor that varies between meter makers. Stick to one unit, and if you follow someone else's numbers, check which scale they use.
Leafy greens generally want a lower EC than fruiting crops. Lettuce is often grown at around 1.2 to 1.6 mS/cm, with seedlings lower; too strong a solution can cause tip burn on lettuce leaves.
Measure your tap water first. Hard water already has a reading before you add anything, and that counts towards the total.

Reading the change in EC tells you something:

EC rising while the water level drops: plants are drinking water faster than they take up nutrients. Top up with plain water.
EC falling steadily: plants are taking up nutrients faster than water. Top up with a mild nutrient solution.

pH: how acidic it is

pH affects whether plants can actually absorb the nutrients in the water. Outside the right range, some elements become unavailable even when they are present, and the plant shows deficiency signs.

Most hydroponic crops do well somewhere between pH 5.5 and 6.5. For lettuce, research has found around 5.5 to 6.0 to work well.
Adjust with pH up and pH down solutions sold for hydroponics. Add a few drops at a time, stir, wait a minute and test again; it is easy to overshoot.
pH drifts naturally as plants feed. Check it every day or two while you learn how your system behaves.
Adjust EC first, then pH, since adding nutrient shifts the pH.

Measuring

Pen-style EC and pH meters are the most common choice. pH meters need calibrating regularly with buffer solutions, and the probe must be kept moist in storage solution, never dry.
Liquid pH test kits with colour charts are cheaper and fine for a start, though less precise.
Measure at a consistent time, after stirring, and keep a log.

Changing the solution

Topping up never fully resets the solution. Over time, nutrients the plant uses little of build up and others run short, and EC alone cannot show that imbalance.

A common practice is to replace the whole solution every one to three weeks, more often for large or fast-growing plants and in small reservoirs.
Change it sooner if it smells, looks cloudy or green, or the plants show problems you cannot explain.
Rinse the reservoir when you change it.
Dispose of old solution sensibly: diluted, it can water garden plants or houseplants. Do not tip it into ponds or watercourses.

Water temperature

Cool water holds more dissolved oxygen than warm water. A reservoir that sits warm for long periods is more prone to root problems, so keep it out of direct sun and away from heaters, and use an opaque container. Post your EC and pH readings and what your plants look like in the comments, and others can help you read them.