ppm of an element

In nutrient recipes, "ppm" of an element means parts per million by mass, which in water equals milligrams of that element per litre (mg/L). "150 ppm N" means 150 mg of nitrogen per litre, whatever salt it came from. Research papers often use mmol/L instead.

In practice

A recipe written in elemental ppm lets growers compare feeds and build their own. To find how much of a salt delivers a target, use its percentage: 1 g/L of a salt supplies (percent × 10) mg/L of each element. Calcium nitrate at 19% Ca gives 190 ppm Ca per g/L, so 150 ppm Ca needs about 0.79 g/L.

Units and formula

• mg/L of element = g/L of salt × element % × 10
• mmol/L × atomic mass = mg/L (N 14.0, P 31.0, K 39.1, Ca 40.1, Mg 24.3, S 32.1)

Don't confuse with

• EC meter ppm (ppm500 / ppm700): an estimate of total dissolved salts converted from electrical conductivity, not a count of any element. 1.0 mS/cm reads about 500 or 700 "ppm" depending on scale.
• Oxide ppm: some charts quote P2O5 or K2O instead of P or K; convert first.