Growers are pushing fertigation (fertilizer via irrigation) and chemigation (pesticide via irrigation) for efficiency gains, but the chemistry is unforgiving: calcium, phosphate, sulfate, and pH shifts can turn nutrients into rock—literally—inside emitters. The fix is a disciplined mixing sequence, chelators, and pH control, backed by jar tests and filtration.
Industry: Agriculture | Process: Fertigation_&_Chemigation_Systems
Fertigation (injecting soluble fertilizers into irrigation) and chemigation (injecting pesticides) promise precise feeding and fewer passes. They’re also chemically sensitive. Clemson researchers put it plainly: “some fertilizer materials…react with minerals in irrigation water in tank mixes and form insoluble compounds and precipitates” (lgpress.clemson.edu). New South Wales DPI warns the result is emitter clogging and reduced nutrient availability (www.dpi.nsw.gov.au).
The field rule-of-thumb is as stark as it is simple: keep calcium, phosphate, and sulfate sources separate. Never tank-mix a phosphorus product (MAP, DAP, phosphoric acid) with calcium fertilizers (calcium nitrate or gypsum). Avoid pairing high-sulfate fertilizers with calcium as well (royalbrinkman.com; edis.ifas.ufl.edu). Mix calcium nitrate (Ca(NO₃)₂) with diammonium phosphate (DAP), and you’ll make calcium phosphate (Ca₃(PO₄)₂) that clogs lines (edis.ifas.ufl.edu).
Other pitfalls: urea with high‑bicarbonate water; urea hydrolyzes and raises pH to around 9, causing Ca/Mg bicarbonates to convert into carbonates (edis.ifas.ufl.edu). Liquid sulfur fertilizers like potassium thiosulfate are strongly alkaline and will precipitate iron and calcium salts if mis‑mixed (edis.ifas.ufl.edu). By contrast, potassium and nitrate fertilizers (KNO₃, NH₄NO₃, urea) are generally compatible with each other, though urea’s pH rise still needs monitoring (edis.ifas.ufl.edu; extension.uga.edu).
Chemical incompatibilities in irrigation
Calcium sources such as Ca(NO₃)₂ or CaCl₂ are incompatible with phosphates and sulfates. They can also precipitate as CaCO₃ when water is high in bicarbonate (royalbrinkman.com; edis.ifas.ufl.edu; www.dpi.nsw.gov.au). Phosphate sources (MAP, DAP, phosphoric acid) quickly form Ca/Mg phosphates when pH is ≥7.5; safe mixing involves acidifying water to pH ~5–6 first (www.dpi.nsw.gov.au; pubs.nmsu.edu).
Sulfate sources (K₂SO₄, (NH₄)₂SO₄, H₂SO₄) are incompatible with calcium or magnesium, forming gypsum or Epsom salts especially at pH > 7 (royalbrinkman.com; edis.ifas.ufl.edu). Potassium chloride (KCl) is generally compatible; potassium thiosulfate/polysulfide should not be mixed with calcium or Fe/Mn sources, or with UAN (edis.ifas.ufl.edu).
Pesticide injection constraints
Pesticide compatibility in chemigation is narrow. Only a few are labeled for drip injection (labeling is the legal authorization printed on pesticide products). Examples include Telone (1,3‑dichloropropene), chloropicrin, diazinon, aldicarb (Di‑Syston), imidacloprid (Admire), and oxamyl (Vydate) (pubs.nmsu.edu). Most foliar fungicides or herbicides are not injectable; growers are advised to inject only those explicitly permitted on the label (pubs.nmsu.edu).
Micronutrients or surfactants in pesticide tank mixes require fully solubilized, chelated forms; unchelated metals precipitate in alkaline solution (extension.uga.edu).
Mixing sequence and injection control
A disciplined sequence, often using separate “A” and “B” tanks, prevents precipitates (royalbrinkman.com; lgpress.clemson.edu):
1) Water and acid first. Fill the stock tank with clean irrigation water, then inject acid (typically H₂SO₄ or HCl) to reach pH 5–6. “When acidifying, acid must always be added to water, never water to acid” (edis.ifas.ufl.edu). Lower pH suppresses carbonate and phosphate precipitates (edis.ifas.ufl.edu; edis.ifas.ufl.edu). Using weak acids like citric or phosphoric creates buffers, so more acid is needed for the same pH change (edis.ifas.ufl.edu).
2) Major nutrients. In a dual‑tank system, dissolve calcium fertilizers in the A‑tank and phosphate/sulfate fertilizers in the B‑tank (royalbrinkman.com). Add the most soluble materials first (ammonium or potassium nitrate and urea) and stir until fully dissolved (extension.uga.edu). Next, add other macros like potassium sulfate and ammonium sulfate. Ammonium sources can slightly acidify (NH₄⁺ → ammonia + OH⁻), so they’re often added early. Allow insoluble prills to settle and decant; UGA recommends 6–8 hours settling before injection with coated dry fertilizers (extension.uga.edu).
3) Phosphorus sources and acids. Add polyphosphate or MAP/DAP last, into acidified water. For concentrated phosphoric acid, inject slowly into the tank, beyond any stainless steel plumbing, to avoid corrosion (www.dpi.nsw.gov.au). Alternatively, schedule phosphoric‑acid injection after water has passed through emitters via a downstream acid injector (www.dpi.nsw.gov.au).
4) Micronutrients. Add chelated micronutrients (Fe, Zn, Mn) into the finished fertilizer solution just before injection. They need acidic water to remain soluble and should not be mixed with lime‑forming fertilizers. In two‑tank rigs, chelates often go in the A‑tank (with Ca nitrates), ensuring the A‑tank solution is acidified (cropnuts.helpscoutdocs.com).
5) Pesticides and adjuvants. Inject separately from fertilizers. Use dedicated injection pumps and run a full irrigation cycle to clear the system before/after pesticide croph. Test any pesticide‑fertilizer tank mix in a jar first (extension.uga.edu).
6) Filtration and flow. Use fine filters—130–120 mesh—before injection and immediately before emitters. Daily flushing of tank solids and backflushing filters is recommended. For injection, calibrate so emitter concentration matches the desired lbs/acre; steady injection (constant‑rate pump or venturi) is preferred to pulse dosing. A grower quoted in NMSU emphasizes reliability over complexity: “we thought we had everything right…drip does it, you don’t need all that complicated equipment” (pubs.nmsu.edu). For injector calibration and steady feed, growers rely on equipment like a dosing pump. For fine pre‑emitter filtration, a cartridge filter or an automatic screen filter is commonly positioned where the guide calls for “fine filters (e.g., 130–120 mesh).”
7) Monitoring. Use a jar test: mix each chemical with irrigation water at planned concentration, agitate, and let stand overnight. Cloudiness or crystals predict field clogging (lgpress.clemson.edu).
Chelators and pH buffer roles
Chelating agents (organic ligands that bind metals) keep micronutrients soluble in fertigation. Fe, Mn, Zn otherwise precipitate as hydroxides/carbonates at moderate pH (edis.ifas.ufl.edu). Ferrous iron (Fe²⁺) begins oxidizing to ferric (Fe³⁺) at pH ≈ 5.3; above pH ≈ 7.5, Fe³⁺ precipitates as Fe(OH)₃ (rust). Fe‑EDDHA remains soluble up to pH ~9–10 (edis.ifas.ufl.edu), so chelated Fe is crucial in alkaline irrigation water.
pH strongly affects chelate chemistry. EDTA is only soluble as Na‑EDTA above pH ~3; at very low pH it can form insoluble salts (patents.google.com). Using citric or phosphoric acid “builds pH buffer systems,” so more acid is required to reach a target pH compared with HCl or H₂SO₄ (edis.ifas.ufl.edu). Polyphosphates buffer toward pH ~7–8; acidification before or co‑injection is advised. Under high pH, even chelates can fail (e.g., EDTA precipitates at low pH) (patents.google.com).
Chelators double as cleaners. Low‑concentration citric acid (0.5–1%) is used to flush emitters and dissolve Ca/Mn scales, with 24–48 h soaks for severe clogging (edis.ifas.ufl.edu).
Troubleshooting precipitation and clogs
Physical clues help. White/gray chalky residues (often caught in filters) point to Ca/Mg carbonates or phosphates; brown or red‑brown films indicate oxidized iron; black can indicate manganese oxide or organic film (edis.ifas.ufl.edu; edis.ifas.ufl.edu; edis.ifas.ufl.edu).
Test water chemistry. Elevated pH (≥7.5–8) and bicarbonate >1–2 meq/L (milliequivalents per liter; a unit expressing concentration by charge) predict CaCO₃ precipitation. Iron/manganese, even at a few tenths ppm, will precipitate in aerobic, high‑pH systems (edis.ifas.ufl.edu; edis.ifas.ufl.edu; edis.ifas.ufl.edu).
Jar‑test suspect mixes at working concentrations and hold overnight; haze or sediment confirms incompatibility (lgpress.clemson.edu). Adjust pH where needed: sulfuric or phosphoric acid injection neutralizes bicarbonates and lowers pH (pubs.nmsu.edu; edis.ifas.ufl.edu). In one NMSU case, 1.2 gal/hr sulfuric‑acid injection lowered pH from ~7.5 to ~6.5 and halted emitter scaling (pubs.nmsu.edu; pubs.nmsu.edu). Always add acid to water, not water to acid (edis.ifas.ufl.edu). Where Ca²⁺ > 50 ppm, avoid phosphoric acid; use sulfuric or HCl instead (edis.ifas.ufl.edu).
Use chelated Fe/Mn sources or lower pH if metal hydroxides are clogging lines. A small amount of chelant (e.g., EDTA) in an injectate can dissolve nascent scale; routine chelation prevents re‑precipitation when pH drifts (extension.uga.edu). Flush systems and clean filters; a citric‑acid soak (0.5–1%) loosens Ca/Mn scales (edis.ifas.ufl.edu). For routine backwash and debris capture in the “fine filter” positions specified above, producers deploy equipment such as an automatic screen filter.
Verify pumps and injectors are drawing from the well‑mixed layer (8–10 inches above settled solids) and calibrated correctly; overconcentrated “shots” can exceed solubility (extension.uga.edu). If crystals persist in stock tanks, dilute or switch to liquid fertilizers. For upstream media filtration where guidance calls for “filters,” growers align that need with solutions like a sand-silica filter before the fine mesh stage.
Data, outcomes, and safeguards
Used correctly, fertigation can deliver comparable or better yields with 20–50% less fertilizer (extension.uga.edu). Drip systems also use roughly half the irrigation water of furrow systems in grower reports (pubs.nmsu.edu). The trade‑off is precision: one NMSU trial tied poor chemigation sanitation to root‑knot nematodes that caused a 50% yield loss in chile plants (pubs.nmsu.edu). Even modest clogging can skew application by >10–20%.
Demand for precision fertigation is growing (~7–8% CAGR through 2030), driven by yield and environmental goals (extension.uga.edu; pubs.nmsu.edu). Regulators and extension services emphasize safeties (venturi check valves, pressure monitors) and accurate calibration to meet label concentrations (agris.fao.org). One caution from those same sources: “chemigation of chemicals at higher concentrations could also lead to leaching and contamination” (agris.fao.org).
The takeaway is consistent across universities and grower case studies: confirm compatibility with jar tests, control pH, rely on chelates for micros, and keep disciplined records of water analyses, recipes, and injector settings. Done right, nutrient use efficiency can improve by >30% while avoiding downtime (extension.uga.edu; pubs.nmsu.edu).
Selected compatibility chart
| Mix category | Compatibility | Notes / References |
|---|---|---|
| Ca(NO₃)₂ + DAP | Incompatible – precipitates Ca₃(PO₄)₂ | CEPST: Calcium & phosphate mix blutatanker; precipitate－ (edis.ifas.ufl.edu) |
| Ca(NO₃)₂ + MAP | Incompatible if water has high Ca/Mg | Precipitates at pH ≥ 7.5; use acid during fertigation – (www.dpi.nsw.gov.au; pubs.nmsu.edu) |
| CaCl₂ + phosphates/sulfates | Incompatible – CaCl₂ reacts with H₃PO₄ or SO₄²⁻ | General rule: “never mix phosphate or sulfate with Ca” (royalbrinkman.com) |
| Potassium phosphate + UAN/AN | Compatible, with pH caution | Ammonia raises pH causing CaCO₃ if water is hard (edis.ifas.ufl.edu) |
| Urea + Ca/Mg‑hard water | Initially compatible; pH rise precipitates CaCO₃/MgCO₃ | Urea hydrolysis raises pH (~9) (edis.ifas.ufl.edu) |
| K₂SO₄, (NH₄)₂SO₄ + Ca‑source | Incompatible – gypsum (CaSO₄) precipitate | Avoid co‑injection (same rule as Ca–P) |
| Fe‑EDTA chelate + phosphates | Mostly compatible; FePO₄ “lock‑up” risk | Keep chelated Fe away from high‑phosphate inputs (extension.uga.edu) |
| Fe‑EDDHA + Ca‑rich water | Compatible – stable to pH 9–10 | EDDHA effective at high pH (cropnuts.helpscoutdocs.com) |
| KNO₃ + any nitrate | Compatible | No known precipitate formation with carbonates |
| KCl + any non‑metal | Generally compatible | Cl⁻ largely inert in these mixes |
| Amino‑acid or humate complexes | Compatible | Humates are stable and mildly buffer (edis.ifas.ufl.edu) |
| Soil amendments (citric acid, potassium bicarbonate) | Varies | Citric acid flush 0.5–1% (edis.ifas.ufl.edu); KHCO₃ raises carbonate content, precipitating Ca/Mg |
Practical guardrails and footnotes
Never mix Ca‑containing fertilizers with phosphate or sulfate fertilizers—use separate tanks or injections (royalbrinkman.com; edis.ifas.ufl.edu). Acidic fertilizers (sulfuric, phosphoric) can be injected to adjust pH, but only into water/tank before adding precipitating ions; if irrigation water has Ca > 50 ppm, avoid phosphoric acid and use sulfuric instead (edis.ifas.ufl.edu; edis.ifas.ufl.edu). Micronutrients should be fully chelated; unchelated metals or metal sulfates will precipitate after mixing (extension.uga.edu).
For Fe specifically, Fe‑EDTA is only stable up to pH ≈ 6.5, while Fe‑EDDHA remains soluble to pH ≈ 9–10 (edis.ifas.ufl.edu). Liquid polyphosphates (ammonium polyphosphate, MAP) should be injected slowly into neutral/low‑pH water, ideally after any H₂SO₄ dosing (www.dpi.nsw.gov.au).
Source notes and further reading
Citations include Clemson Univ. Land‑Grant Press on fertilizer mixtures and jar testing (lgpress.clemson.edu; lgpress.clemson.edu), UF/IFAS guidance on clogging, pH control, and chelates (edis.ifas.ufl.edu; edis.ifas.ufl.edu; edis.ifas.ufl.edu), University of Georgia on injection practices (extension.uga.edu), NMSU on drip pesticide registrations and operations (pubs.nmsu.edu; pubs.nmsu.edu; pubs.nmsu.edu), NSW DPI on acid injection and polyphosphates (www.dpi.nsw.gov.au), FAO/UF on chemigation safeties and concentration control (agris.fao.org), and Royal Brinkman on tank separation rules (royalbrinkman.com). Additional notes include EDTA solubility behavior (patents.google.com) and a chelate deployment note (cropnuts.helpscoutdocs.com).
