What nitrogen, phosphorus and potassium do
Nitrogen is part of proteins and other compounds needed for growth. Phosphorus participates in energy transfer and metabolism. Potassium helps regulate enzyme activity and water balance. Plants use all three throughout their lives; “nitrogen for leaves, phosphorus for flowers” is too crude a rule for choosing a recipe.
Demand changes with growth, but raising one nutrient does not guarantee a particular result. More feed will not provide the energy missing under weak light. Damaged roots can struggle to use a correctly mixed solution. Keep those limits in mind before treating every disappointing crop as a fertilizer problem.
Read the label before comparing formulas
In common NPK labelling, the first number reports nitrogen as N, while phosphorus and potassium are expressed as P₂O₅ and K₂O equivalents. These are reporting conventions, not a claim that the bottle necessarily contains those oxides. Check the product's labelling convention and explanatory notes.
That means a label such as 10-5-15 cannot automatically be read as percentages of elemental N, P and K on the same basis. With conventional oxide reporting, multiply P₂O₅ by about 0.436 to obtain elemental phosphorus and K₂O by about 0.830 to obtain elemental potassium. If you formulate by elements, a mistake here carries into the entire recipe.
For liquids, establish whether percentages are by mass and what density applies. Millilitres and grams are not automatically equivalent. If you use a ready-made feeding schedule in ml/litre, follow it and measure the result instead of building a new formula from the three large numbers on the bottle.
Check the rest of the formula
Look for calcium, magnesium and sulfur, plus micronutrients including iron, manganese, boron, zinc, copper and molybdenum. Water may supply some nutrients, but you need to know its composition. “Hard water” without analysis does not tell you how much useful calcium or unwanted salt it contains.
A fertilizer intended for soil may assume nutrients from the soil, different solubility or another application method. Look for explicit suitability for your growing method and a schedule that covers the crop cycle.
Why concentrates come in separate parts
Some salts are incompatible at high concentration. Calcium can precipitate when it contacts concentrated phosphates or sulfates, for example. Manufacturers therefore keep parts such as A and B separate until each has been diluted in a larger volume of water.
Part names differ between brands. The letter A does not define a universal composition or mixing order. Follow the instructions for your set and avoid substituting a similar bottle from another line without a compatible recipe. Two products with the same NPK label can differ in nitrogen forms, micronutrients and concentration.
Mix a fresh reservoir step by step
- Establish the actual water volume. Measure the source water's EC and pH and prepare clean measuring tools.
- Choose the manufacturer's dose for your crop and stage. Check that the schedule refers to the complete product set.
- Add the first component to water and mix thoroughly. Add the next only after that, using the specified order.
- Check for sediment or unexpected cloudiness. Measure EC after mixing.
- Once concentration matches the working range, check and adjust pH. Record the recipe and final readings.
Do not pour A and B into the same measuring cup before dilution. Avoid pouring concentrate directly onto roots. Adding acid to dissolve an unidentified precipitate leaves you with an uncertain composition. After a mixing error, preparing fresh water and following the correct order gives you a recipe you can account for.
A dosing example
Suppose a label specifies 2 ml of A and 2 ml of B per litre, and the reservoir actually contains 12 litres of water. This illustrative recipe calls for 24 ml of A and 24 ml of B, added separately to the water. These figures are not directions for a particular brand.
If you later add 3 litres of water to an old reservoir, that does not automatically call for another 6 ml of each part. Salts remain in the tank. Restore the working volume, mix, measure EC and consider the feeding history first. An old solution with unknown composition is harder to calculate than a fresh batch.
Record the dose of each component separately. “Added 48 ml of fertilizer” loses the A-to-B ratio and does not let you repeat the example. Writing the reservoir volume beside the doses also helps distinguish a recipe mistake from an error in measuring water.
Investigate symptoms before adding supplements
Yellowing older leaves can suggest nitrogen shortage. Yellowing between veins can lead you to investigate magnesium or iron depending on leaf age. Similar symptoms can also result from unsuitable pH, poor root condition and other problems. Leaf appearance helps you choose a check; it does not establish the diagnosis.
An excess of one nutrient can interfere with another. Adding calcium or magnesium without reviewing the whole recipe can therefore worsen an imbalance. Tissue testing can help confirm a deficiency when the symptoms are uncertain.
Inspect roots, check meters and verify water access first. Then compare the solution with the recipe and review source water and pH. Look at new growth to assess a correction. A damaged area on an existing leaf may remain unchanged even when the problem has been addressed.
Change feeding with the growth stage
Use the transitions specified in your fertilizer program. A fruiting plant still needs nitrogen; the appearance of flowers is not a reason to remove it entirely. Do not raise EC solely because the plant is larger. Consider its condition and the growing environment as well.
If you want to compare feeding regimes, keep the crop, water and light the same and record the change. Switching brand, dose and lamp together makes the result hard to explain. For a recurring problem, checking water composition and the recipe is more useful than adding another supplement without a diagnosis.