Choosing the injector
Four ways to get fertiliser into an irrigation line, in rising order of control:
- Pressure-differential tanks — cheapest, but the concentration fades through the run; fine for orchard blocks, wrong for anything needing a steady feed.
- Venturi injectors — no moving parts and water-powered, but they need a solid pressure difference across the device to suck, they cost that pressure permanently, and the rate drifts as system pressure and tank level change.
- Proportional water-driven injectors — dose in step with flow, holding a constant concentration even as blocks valve in and out.
- Electric metering pumps — the precision option, and the same machines our chemical-dosing section covers: the sizing rule there (keep the duty in the pump's comfortable mid-range, dilute the stock solution if it is not) applies unchanged when the chemical is fertiliser.
Whichever injector, two placement rules hold: inject upstream of the system filters, so undissolved fertiliser and any precipitate is caught before the drippers rather than delivered to them, and give the line enough distance to mix fully before the flow splits.
The mixing chemistry: calcium is antisocial
The classic fertigation disaster is made in the stock tank: calcium sources must never share a tank with sulphate or phosphate sources. Calcium nitrate plus ammonium sulphate makes gypsum; calcium plus phosphate (MAP, MKP) makes insoluble calcium phosphate — in both cases a milky precipitate that plugs emitters wholesale. That is why serious fertigation runs A and B tanks (calcium in one, phosphates and sulphates in the other) or alternates products between cycles.
Solubility sets the other limit. Urea dissolves at over 100 kg per 100 L, potassium nitrate and potassium chloride nearer 32–34 kg, potassium sulphate a mere 11 kg — and urea and potassium nitrate chill the water as they dissolve, so cold winter water dissolves less and slower. The universal safeguard costs nothing: the jar test. Mix the intended fertilisers in the actual irrigation water — at line concentration, and at stock strength for shared tanks — and stand it overnight (12–18 hours). Cloudiness, milkiness or sediment means do not inject; hard water can fail the test on its own, which is worth knowing before the season, not during it.
Protecting the water you drink
A fertigation line connected to any supply that also serves people or stock is a contamination path running backwards: shut a pump at the wrong moment and the system can siphon fertiliser solution into the borehole or the house line. South Africa's plumbing standard, SANS 10252-1, treats agricultural installations — dairies and nurseries by name — as a category needing backflow protection, rated by hazard: substances hazardous to health call for a reduced-pressure-zone (RPZ) valve or a full air gap into a break tank; a simple double-check valve is only for nuisance-grade risks. Municipal by-laws repeat the duty where municipal water is involved — Cape Town specifies an RPZ immediately after the private stopcock. Good chemigation practice adds the operational layer: a check valve and low-pressure drain on the irrigation main, and the injector electrically interlocked with the irrigation pump so the injector can never run alone, quietly filling a dead line with concentrate.
After the feed, flush
Fertiliser left standing in warm laterals is a nutrient broth, and the slime that grows on it binds silt into exactly the plugs the filtration was meant to prevent. Structure each event in quarters: irrigate the first stretch on plain water, inject through the middle, and keep the final stretch for flushing the lines clear before shutdown. Flush the injector and its fittings with clean water too — fertiliser solutions are corrosive, and acid fertilisers (phosphoric, urea-sulphuric) belong in pumps built for them, with acid-resistant heads and seals. When lines do slime up despite it all, the remedies are the drip-maintenance pair from the previous article: chlorine for the biology, acid for the scale, never mixed.
Rule of thumb
Inject before the filters, mix before the manifold, and jar-test every new recipe in your own water overnight. Keep calcium away from sulphates and phosphates — two tanks, not one. Fit the RPZ before the inspector or the incident asks for it, interlock the injector with the pump, and end every fertigation with clean water through the lines: feed the crop, not the biofilm.