Secondary nutrients are calcium (Ca), magnesium (Mg) and sulfur (S). They are true macronutrients, needed in amounts comparable to phosphorus, but historically "secondary" because field soils usually supplied enough and fertiliser labels led with N-P-K. The term is about labelling tradition, not importance.
In practice
In soilless growing they are often the first elements to run short. RO water strips the calcium and magnesium that tap water brings, coco binds calcium and magnesium on its exchange sites until buffered, and high potassium can crowd them out. A complete hydro base supplies all three: calcium from calcium nitrate, magnesium and sulfur from magnesium sulfate. Fertiliser labels list them under the guaranteed analysis, below N-P-K, when present.
Don't confuse with
- Micronutrients: needed in trace amounts only.
- Cal-Mag: a supplement covering two of the three.
- Primary nutrients: N, P and K.
The guaranteed analysis is the legally required label statement of minimum nutrient content, giving P as P2O5 and K as K2O; multiply by 0.436 and 0.830 for elements.The N-P-K ratio is the three numbers on a fertiliser label giving percent nitrogen, phosphate (P2O5) and potash (K2O) by weight, not elemental P and K.A soilless medium contains no mineral soil, such as peat, coco, rockwool or perlite blends, and relies mostly on the grower to supply nutrients.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Calcium nitrate is the standard hydroponic calcium source, supplying about 19% calcium and 15.5% nitrogen, mostly as nitrate.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.A macronutrient is an essential element plants need in large amounts, measured in tens to hundreds of mg/L: nitrogen, phosphorus and potassium, plus calcium, magnesium and sulfur.Calcium (Ca) is an immobile secondary nutrient that builds cell walls and membranes; it travels only in the transpiration stream, so deficiency appears in new growth.Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.Sulfur (S) is a secondary nutrient taken up as sulfate, used in the amino acids cysteine and methionine; deficiency yellows new growth fairly evenly.
