Hydroponic nutrients 101: feeding your plants made simple

What nutrients are, how to mix them, and what happens if you get it wrong. Explained for people who have never done this before.

Three unlabelled nutrient concentrate bottles beside a measuring jug of pale amber solution, a measuring syringe and a beaker of clean water on a white worktop next to a reservoir.
Measure into water, never the other way round, and add one part at a time.

In soil, plant food arrives slowly as organic matter breaks down. In hydroponics you supply it directly, already dissolved and ready to absorb. That is the whole difference, and it is why a bottle of hydroponic nutrient is not interchangeable with a box of garden fertiliser.

What is actually in the bottle

A complete hydroponic nutrient contains seventeen elements. You do not need to memorise them, but it helps to know the shape of the list.

GroupElementsWhat they do
Primary macrosNitrogen (N), phosphorus (P), potassium (K)The three numbers on the label. Leaves, roots and flowers, and overall vigour.
Secondary macrosCalcium, magnesium, sulfurCell walls, chlorophyll, and protein building. Often short in soft or filtered water.
MicronutrientsIron, manganese, zinc, copper, boron, molybdenum, chlorineNeeded in tiny amounts, but a shortage of any one stalls the plant completely.

Why garden fertiliser will not do

One-part, two-part, or three-part?

  • One-part. Everything in a single bottle. Simplest, and completely adequate for greens and herbs. Start here.
  • Two-part (A and B). Splits the ingredients that would react and precipitate if concentrated together. A small amount of extra work for better stability.
  • Three-part (grow, micro, bloom). Lets you shift the ratio as the plant moves from leafy growth to flowering. Worth it once you grow tomatoes, peppers, or strawberries.

How to mix, in the right order

  1. Start with water, never concentrate

    Fill the reservoir first. Adding water to concentrate rather than concentrate to water is how you get precipitate.

  2. Add one part at a time

    With multi-part nutrients, add the first part, stir until fully dispersed, then add the next. Never pour two concentrates together. Calcium and phosphate will bind into a white sludge your plants cannot use.

  3. Mix, then measure pH

    Nutrients change the pH of water, so measuring before you add them tells you nothing useful. Mix first, then test, then correct.

  4. Use half strength for seedlings

    For the first two weeks after transplanting, mix at half the label rate. Young roots burn easily, and a burnt root system is much harder to recover from than a slightly hungry plant.

How often to change the reservoir

Plants do not drink the solution evenly. They take up some elements faster than others, so the balance drifts even while the level and the EC reading look reasonable. In any recirculating system, replace the whole reservoir every two to three weeks. Between changes, top up with plain water only.

Working out what half the label rate actually means for your container is the fiddly part. Our nutrient dosing calculator converts any label dose into the exact amount for a jar, bottle or bucket.

Kratky systems are the exception. There you mix once and never change it, because the crop finishes before the drift matters.

Next: pH and PPM for beginners