Greenhouse Fertigation Stock Tank Mixing

A stock tank is a measured concentrate an injector turns into irrigation water—not a fertilizer recommendation. State the selected nutrient basis, label analysis on that basis, final stock volume and calibrated injector ratio. The arithmetic cannot choose a crop nutrient program, prove compatibility or replace a water analysis.

Use the Fertigation Injector & Stock Tank Calculator to document inputs and outlet concentration. Record product, lot, water source and injector setting with every mix.

The target is ppm of one selected nutrient, not ppm of total product. Keep target, background and label percentage on N, P₂O₅ or K₂O. Oxide label bases are not silently elemental P or K. A tested 0 ppm background is a measurement. Unknown background is different: it is an explicit zero-contribution assumption. EC is a separate verification record with no universal conversion to ppm N, P₂O₅ or K₂O.

This guide uses 1:R as stock : resulting dilute solution. Manufacturer definitions can differ, so read the manual and calibrate the installed injector.

For this concentration calculation, 1 ppm in water is approximately 1 mg/L. The equations assume final stock volume after topping up, complete dissolution and uniform mixing.

Follow product labels for dissolution order, maximum concentration, compatibility, safety, storage and backflow protection. Calibrate draw and delivery at operating pressure with measured volumes; a nominal dial setting is not verification.

Stock mixing does not set crop demand, all nutrients or water depth. The crop-side water depth remains a separate calculation: use the NPK Calculator for product allocation, the Irrigation Water Requirement Calculator for water depth, and the Water Cost Calculator for entered water and pumping costs.

• Selected-nutrient mass balance, homogeneous stock and stable injector draw.
• Background is measured on the same nutrient basis, or explicitly assumed zero when unknown.
• The model does not cover precipitation, alkalinity, emitter uniformity, uptake, leaching, sanitation or label-specific mixing rules.

• Calling unknown water contribution measured zero or mixing nutrient bases.
• Misreading 1:R rather than verifying stock : resulting dilute solution.
• Using EC as a ppm conversion, ignoring compatibility limits, or trying to lower excessive background by adding fertilizer.

• University of Georgia Extension, Fertilizer Injectors: Selection, Maintenance and Calibration — ratio and calibration context.
• University of Maryland Extension, Care and Calibration of Injector Pumps — measured injector performance.
• Virginia Tech, The Basics of Fertilizer Calculations for Greenhouse Crops — label-analysis context.