An A/B nutrient is a complete mineral feed sold or mixed as two separate concentrates. Part A usually contains calcium nitrate, often with potassium nitrate and iron chelate. Part B contains the phosphates, sulfates and the remaining micronutrients. Kept apart in concentrated form, they stay clear; mixed together at concentrate strength, calcium phosphate and calcium sulfate drop out as sludge.
In practice
Add part A to the water, stir, then add part B, never pouring one concentrate straight into the other. Equal doses of A and B are typical, but check the label. The limitation is that both parts scale together: cutting the dose to reduce nitrogen also reduces everything else. That is why per-element dosing exists, and why a late-flower nitrogen taper is hard to run with a bottled two-part alone.
Don't confuse with
- One-part nutrient: a single concentrate that avoids the precipitation problem by formulation.
- Bloom and grow bottles: a stage split, which may or may not also be an A/B split.
- A/B testing: a trial method, not a feed.
Calcium nitrate is the standard hydroponic calcium source, supplying about 19% calcium and 15.5% nitrogen, mostly as nitrate.Potassium nitrate (KNO3) is a soluble fertiliser salt supplying about 13% nitrogen as nitrate and 38% potassium, used in both veg and flower feeds.An iron chelate is iron bound to an organic molecule (EDTA, DTPA or EDDHA) that keeps it dissolved and available; each type holds up to a different pH.Phosphate is the ionic form in which plants absorb phosphorus, mainly H2PO4- at the normal root-zone pH of 5.5-6.5.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.Per-element dosing builds a feed from individual salts or single-element stocks so each nutrient can be set independently, such as cutting nitrogen without cutting potassium.Gradually cutting nitrogen in late flower, toward zero near the finish, while still feeding phosphorus, potassium, calcium, magnesium, sulfur and micronutrients; GenetixBay's alternative to flushing.A one-part nutrient is a complete mineral feed in a single concentrate or powder, formulated to avoid the calcium-sulfate and calcium-phosphate precipitation that forces most feeds into two parts.A stock solution is a concentrated nutrient mix, often 100x or 200x final strength, diluted into the reservoir; calcium and sulfate/phosphate stocks must stay separate.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.
