What nutrients do hydroponic plants need, and how do I manage pH and EC/PPM?

Hydroponic plants need 17 essential elements, the same set every plant uses, but three of them, carbon, hydrogen, and oxygen, come free from air and water, which leaves 14 mineral nutrients that you have to deliver in the solution.[1][2] In soil, clay and organic matter hold a reserve of those minerals and release them slowly, but a soilless system has no such buffer, which is why the makeup of the nutrient solution and steady monitoring matter more here, and why a problem can appear in the leaves within days rather than weeks.[1]

The 14 mineral nutrients split into two groups by how much the plant uses. The six macronutrients, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, are taken up in the largest amounts and build the bulk of the plant's tissue and machinery.[1][2] The eight micronutrients, iron, manganese, zinc, copper, boron, molybdenum, chlorine, and nickel, are needed only in trace amounts but are just as essential, since a shortage of any one stalls growth.[1][2] A complete hydroponic fertilizer already contains all of them in balance, so day-to-day management is less about choosing nutrients and more about keeping the water in the range where roots can absorb them.

That management runs on two independent measurements. pH governs chemistry: it decides which of the nutrients already in the water stay dissolved and available for uptake, and for most crops the working window sits between about 5.5 and 6.5.[3][8] Electrical conductivity (EC) governs strength: it reads the total concentration of dissolved salts, which tells you how rich the feed is overall but nothing about the ratio of one nutrient to another.[6][4] Parts per million (PPM) is a second way of expressing EC, and it carries a catch, because two meters can convert the same water into very different PPM readings.[7]

Each nutrient has a specific job, and the job tells you what a shortage will look like and where on the plant it shows up first.[1][2] The macronutrients build most of the plant's structure, while the micronutrients act mostly as helpers inside enzymes and in photosynthesis.[1]

Nitrogen, phosphorus, and potassium

These three drive the bulk of growth. Nitrogen is a building block of amino acids, proteins, and nucleic acids, so it powers leaf and stem development and is usually the first nutrient a fast-growing crop draws down.[1] Phosphorus sits at the center of energy transfer through ATP and forms part of cell membranes and DNA, which makes it central to rooting and flowering.[1] Potassium activates enzymes and regulates the turgor and water balance that keep cells firm and stomata working.[1] All three move freely within the plant, so a shortage pulls them from older leaves to feed new growth and the yellowing appears on the lower foliage first.[1]

Calcium and magnesium

Calcium and magnesium cause outsized trouble when they are off. Calcium builds and stabilizes cell walls, and it travels only in the transpiration stream, so it cannot be moved back out of old tissue to rescue young growth.[2] That one fact explains tipburn in lettuce and blossom-end rot in tomato, where the solution is full of calcium yet the youngest tissue still goes short because water is not reaching it fast enough.[6][2] Magnesium is the atom at the center of every chlorophyll molecule, so a shortage reads as interveinal yellowing, and because it is mobile that yellowing starts on the older, lower leaves.[1][2]

Sulfur and the micronutrients

Sulfur is part of several amino acids and proteins, so it sits in the structural group even though plants use less of it, and in a complete recipe it rarely runs short.[1] Among the micronutrients, iron causes the most day-to-day trouble, because it drives chlorophyll formation and its availability collapses as pH rises; the tell is interveinal yellowing on the newest leaves, usually from a high pH locking the iron away rather than a missing dose.[2] Manganese, zinc, and copper work as enzyme activators in amounts measured in fractions of a part per million, where the practical risk is buildup and toxicity rather than shortage.[1] Boron supports cell walls and new growth, so like calcium its deficiency appears on the newest tissue, while molybdenum, chlorine, and nickel are needed in such small amounts that a standard mix covers them without separate attention.[1]

Keep the nutrient solution's pH between about 5.5 and 6.5, the band where the widest range of nutrients stays dissolved and available to roots.[3][8] pH does not feed the plant; it governs chemistry, deciding which of the nutrients already in the water the roots can take up, so outside the window elements drop out of solution or bind into forms the plant cannot use even while a lab test still shows them present.[6]

Why the range matters

Each nutrient has its own availability curve across pH, and 5.5 to 6.5 is the overlap where most of them are reachable at once.[3] Nitrogen is most available from about 6.0 upward, while iron and the other metal micronutrients get harder to absorb as pH climbs, which is why a high pH shows first as iron-deficiency yellowing on new leaves.[1][2] Push below roughly 5.0 and the balance tips toward possible toxicities.[2] Some growers run a tighter target, such as 5.6 to 6.0, to favor phosphorus uptake.[5]

How to move pH and hold it

You raise or lower pH by adding a base or an acid to the reservoir, not by changing the fertilizer.[6] To bring pH down, growers add a food-safe acid such as phosphoric, nitric, or sulfuric a little at a time, since a large dose can drop the pH sharply and stress the roots; a potassium hydroxide base brings it back up.[5] pH also drifts on its own as plants feed and as the water warms, so check it often, calibrate the meter about once a week against pH 4 and pH 7 reference fluids, and correct in small steps.[8]

Set electrical conductivity (EC) to match the crop and its growth stage, then hold it steady as the reservoir changes.[4][6] EC measures the total concentration of dissolved salts in the solution, which tells you how strong the feed is overall but nothing about the ratio of one nutrient to another, so treat it as a strength gauge, not a balance check.[6][4]

What EC to target

Freshly mixed solutions usually land between about 1.5 and 3.5 dS/m, written interchangeably as mS/cm since the two units are numerically equal.[4] Start a young crop at the low end and raise the EC as it matures and its demand grows.[4] Leafy greens sit near the bottom of the band and fruiting crops near the top, since tomatoes handle higher salt levels than cucumbers, peppers, or lettuce.[4]

Managing EC drift

EC does not stay put. As plants pull nutrients out of the water the EC falls, and as water evaporates or transpires away the salts concentrate and the EC rises.[6] The day-to-day fix is to top up with plain water when EC climbs and to add fertilizer concentrate when it drops.[6] In a media-based system, a useful check is the runoff: the leachate draining from the slab should stay within about 1.0 dS/m of the solution you applied, and a wider gap signals salts building up in the root zone.[4] Because small errors add up, most growers fully replace the reservoir every few weeks rather than topping up indefinitely.[9]

A PPM reading only means something once you know which conversion scale the meter used, because no meter measures parts per million directly.[7][6] Every TDS or PPM meter reads electrical conductivity and then multiplies it by a fixed factor to estimate the dissolved solids, and two common factors are in use.[7]

The 500 and 700 scales

The 500 scale, sometimes labeled NaCl or TDS, multiplies EC in mS/cm by 500, so an EC of 2.0 reads as 1000 PPM.[7] The 700 scale, sometimes labeled KCl or 442, multiplies the same EC by 700, so that identical solution reads as 1400 PPM.[7] Neither is measuring sodium chloride or potassium chloride specifically; each is just a different assumption baked into the same conductivity reading.[7] Bluelab's own example makes the gap concrete: a solution at 2.4 EC shows as 1200 PPM on the 500 scale and 1680 PPM on the 700 scale, a difference of 480 PPM for water that has not changed at all.[7]

How to avoid the trap

Match the scale on your meter to the scale in whatever recipe or chart you follow, or the numbers will not line up.[7] The cleaner habit is to work in EC directly, in mS/cm or the numerically identical dS/m, which removes the ambiguity because EC is the actual measurement rather than a converted estimate.[7][4] When a grower quotes a bare PPM target with no scale attached, convert it back to EC before comparing it against your own system.[7]

The hardest part of hydroponic nutrition is what the meters do not tell you, so a few limits are worth holding in mind alongside the target numbers.

EC hides the balance

EC reports total salts, which means a solution can read exactly on target while a single nutrient is depleted or oversupplied.[6][4] If plants show a deficiency the meters call normal, the answer is usually a fresh reservoir or a lab test of the solution, not another dose against the same EC number, and the position of the symptom is the clue: old lower leaves point to a mobile nutrient like nitrogen or magnesium, while the newest leaves point to an immobile one like calcium, iron, or boron.[1][2] Working from a complete, balanced fertilizer keeps the ratios sensible between those checks.[4]

Tipburn is a water problem, not only a feeding problem

Lettuce tipburn is a calcium shortage in the youngest leaves, but it often happens when calcium is plentiful in the solution, because heat and high humidity slow transpiration and calcium moves only with water flow.[6] Trials found that increasing airflow with vertical fans, or easing the EC down in hot weather, controlled tipburn better than adding more calcium to the recipe.[6]

Source water and local conditions

Test the water you begin with, since calcium, magnesium, iron, and manganese already in the supply count toward your totals; useful ceilings are roughly 150 ppm calcium, 75 ppm magnesium, and 1 ppm each of iron and manganese.[8] Climate matters too, so warm-region growers are advised to run more dilute solutions and irrigate more often than the stronger recipes written for cooler northern greenhouses.[4]