Nutrient ppm from Fertiliser Weight

This is the arithmetic behind every hydroponic feed chart, and doing it yourself is what makes a nutrient program legible rather than a matter of following the bottle. The concentration of a nutrient element is the fertiliser weight times the element's share of that fertiliser, divided by the water volume.

One convenient fact makes the mental version easy: because a litre of water weighs a kilogram, one gram in one hundred litres is ten ppm. So 10 g of calcium nitrate at 15.5% nitrogen into 100 L gives 1.55 g of N in 100 L, which is 15.5 ppm.

The trap is the same one that catches field growers, and it is worth stating plainly: fertiliser labels report phosphorus and potassium as oxides, not elements. The P in an N-P-K analysis is P₂O₅ and the K is K₂O. To get elemental phosphorus multiply by 0.436; for elemental potassium multiply by 0.830. Hydroponic recipes are almost always written in elemental ppm, so feeding to a label's P number without converting overfeeds phosphorus by a factor of 2.3.

Reference targets for a general vegetative feed, in elemental ppm: N 150–200, P 50, K 200, Ca 150–200, Mg 50, S 60, with iron near 3 and the other micronutrients below 1. Those numbers are why calcium and magnesium deserve their own attention — they are needed at concentrations comparable to phosphorus, and a base nutrient designed for soft water will not supply them.