Calcium is a secondary macronutrient absorbed as Ca2+. It cross-links pectins in cell walls, stabilises membranes and acts as a signalling ion. Calcium moves only upward with water in the xylem and cannot be moved from old leaves to new ones, so it is immobile.
In practice
Deficiency shows on new and fast-growing tissue: distorted or crinkled young leaves, brown necrotic spots, weak stems and dying growing tips. Because Ca rides the transpiration stream, low transpiration (very high humidity, poor airflow) can cause local shortage even when the feed is fine. Hydro feeds usually carry about 120-200 mg/L (ppm) Ca, most often from calcium nitrate. Coco binds calcium on its exchange sites, which is why coco needs buffering.
Don't confuse with
- Cal-Mag: a product class that supplies Ca and Mg, not the element.
- pH lockout: low root-zone pH reduces Ca uptake; check pH before adding more.
- Rust spots from pathogens or light stress: see the rust-spot article before blaming Ca.
Myth check
Calcium demand varies mainly by cultivar, water source and medium, not by light type (Evidence: house position, Myth for "LEDs need Cal-Mag"). See Cal-Mag.
The zone of dividing cells at the very tip of a shoot (or root) that produces new leaves, nodes and stem; removing it is what topping does.The evaporation of water from a plant, mainly through open stomata, which pulls water and dissolved nutrients up from the roots.Calcium nitrate is the standard hydroponic calcium source, supplying about 19% calcium and 15.5% nitrogen, mostly as nitrate.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.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.Nutrient lockout is when plants show deficiency symptoms although nutrients are present, because pH, salt build-up, ion antagonism or damaged roots stop uptake.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.Secondary nutrients are calcium, magnesium and sulfur: macronutrients needed in substantial amounts but not included in the N-P-K numbers on a label.An immobile nutrient stays where it is first deposited, so its deficiency shows on young leaves and growing tips first: Ca, S, Fe, Mn, B, Cu.Nutrient antagonism is when an excess of one element reduces the uptake or use of another, such as high potassium suppressing magnesium and calcium.
