Buying a Fertigation Pump: Metering Head, Diaphragm or Eductor

A 1.2 ha greenhouse pulled a week of stock from a 60 L drum that was mixed by eye. The header duty was 8.0 L/h, which is 9.6 L per day and 67.2 L per week. The drum could not hold a full week of stock. The mix ranged from 10 to 18 percent strength while it lasted.

The dose then swung from 80 to 144 ppm around a 120 ppm target. Weak stock starves the crop the grower already paid to feed. Strong stock salts the root zone around young transplants. Both errors come from the injection set rather than from the emitters downstream of it.

The Snapshot

• 8.0 L/h is the header duty for 10 m³/h at 120 ppm from a 15 percent stock, from (10 × 120) divided by (15 × 10).
• 9.6 L per day and 67.2 L per week is the same duty spread over 1.2 hours of daily pumping, which exceeds a 60 L drum.
• 80 to 144 ppm is the band an eye-mixed 10 to 18 percent stock swings through on a fixed 8 L/h draw.
• 80 percent stroke on a 0 to 10 L/h head delivers the 8.0 L/h duty when a pulsing flow meter drives the stroke.

Why the Pump Is the Dose, Not the Transfer

Most fertigation injection systems use two pumps, and buyers tend to confuse them. The transfer pump lifts stock from a bulk tank to a day tank at whatever head the pipe offers. The injection unit moves a measured volume of stock into the irrigation main. A metering pump moves a precise volume of liquid in a specified time, and that duty is what the word metering means.

The distinction carries commercial weight. A transfer pump sold into a dosing duty holds a flow figure but not a ratio. When zone valves close, the main flow drops and the delivered concentration climbs. The dose then follows the hydraulics of the house instead of the setpoint on the controller.

So the buying question is not whether the site needs a pump. It is which class of injection unit can hold 8.0 L/h against a header that swings between two zones and eight. A fertigation pump in the metering family answers that question with arithmetic rather than with a larger motor.

The Dose Arithmetic: From ppm to Litres per Hour

One ppm is one milligram of active ingredient per litre of water. That identity carries the whole calculation. Multiply the treated water flow by the target in ppm and the product is the active mass per hour. Divide that mass by the stock strength in grams per litre and the stock flow falls out.

Collected in one line, dose rate in L/h equals (water flow in m³/h × ppm) divided by (stock percent × 10). The factor of ten folds two conversions into one. It covers the thousand litres in a cubic metre and the ten thousand milligrams per litre a unit density stock carries. Take the density from the supplier sheet for a brine, an acid or a caustic.

Water flow	Target dose	Stock strength	Stock flow
10 m³/h	120 ppm	15 %	8.0 L/h
5 m³/h	120 ppm	15 %	4.0 L/h
50 m³/h	2 ppm	10 %	1.0 L/h
20 m³/h	120 ppm	15 %	16 L/h

Read the table as one duty seen through four windows. The 50 m³/h row is the published site case for a weak stock in a large main. The 20 m³/h row is the same programme scaled onto a nursery line. It needs 16 L/h of stock from a 15 percent bottle.

Now close the mass rather than the volume. A 15 percent stock at 1.0 kg/L carries 150 g of active per litre. Metering 9.6 L per day therefore delivers 1.44 kg of active per day. The water route lands on the same figure, since 12 m³ at 120 ppm is 1.44 kg. Two independent routes agree, and that is the check that keeps a commissioning sheet honest.

The double check exposes a whole error class. Volume-only arithmetic hides a wrong stock strength. Mass arithmetic catches it, because the two routes diverge the moment the label strength changes.

Metering Pump Types: Piston, Diaphragm and Peristaltic

Piston and plunger heads are positive displacement units that hold a practically constant averaged flow across a wide range of discharge pressure. Manufacturers publish a maximum discharge pressure per model, so the head choice starts with the pressure the main can apply back to it. Each pump head runs an inlet check valve and an outlet check valve. The valves are often of ball type, and the stroke length or the piston speed sets the metering rate.

Diaphragm and peristaltic heads are the other common families, and each answers the corrosion question in its own way. A diaphragm head seals the stock behind a flexible membrane, so the wetted path stays inside the head and the drive stays dry. A peristaltic head squeezes a tube, and the tube is the only wetted part.

Head family	Flow character	Wetted parts	Main watch item
Piston or plunger	Steady, holds against high discharge pressure	Cylinder, piston and both check valves	Abrasive or salty stock wears the seal pair
Diaphragm	Steady, sealed drive	Membrane, head cavity, both check valves	Membrane fatigue and vapour lock on suction lift
Peristaltic	Smooth, low pressure	Tube only	Tube creep and wear set the recalibration interval

Turndown decides which head can hold a small zone. Every supplier publishes a turndown band for a metering head, the ratio between highest and lowest flow at rated accuracy, and accuracy collapses below the bottom of that band as the check valves stop seating cleanly. Size the head so the lowest zone duty stays inside the published band.

An 8.0 L/h duty on a 0 to 10 L/h head sits at 80 percent of capacity. That leaves the pump comfortable rather than stretched. The same head covers a single zone at 4.0 L/h, which is 40 percent of capacity.

Where a Metering Pump Fails: Vapour Lock and Backflow

Gas bubbles sitting in the head cause vapour lock. The pump compresses the bubble instead of moving liquid, and it can stop metering while still stroking. The controller sees stroke pulses and reports a healthy dose while the drip line receives water alone. Degassing the stock or priming the suction line prevents it, and a warm greenhouse day tank is the usual place where dissolved gas arrives.

Check the pressure order on every layout. If the outlet pressure sits below the inlet pressure, both check valves open and liquid flows through the head uncontrolled. A gravity fed day tank pushing stock into a low pressure main is the classic version of that fault. A correctly rated positive pressure differential check valve downstream of the head prevents it. Specify that valve for the full range of main pressure the site will run.

Suction side faults show up as a starving head. A clogged suction strainer, a hose that collapses under vacuum, or a lift the head was not rated for all starve the pump. The suction then drops below the vapour pressure of the stock, and the pump meters air in place of liquid.

Wear moves the dose in one direction only. A worn injection orifice that is 0.2 mm oversize roughly doubled a 0.8 ppm reservoir dose. It reached 1.5 ppm on a published site case. A fertilizer line fed through the same kind of fixed restrictor drifts rich until somebody re-measures it.

Diaphragm Pump or Piston Pump for Fertilizer Stock

Compare the two heads on five points rather than on price alone. The points are discharge pressure, stock viscosity, salt crystals, corrosion of the wetted path, and the turndown the zone schedule needs. A fertigation pump sized on the peak duty alone will sit at the bottom of its band on a single zone.

A piston head wins on pressure and on flow stability at that pressure. It holds its averaged flow across a wide range of discharge pressure. Its seals see the stock, and a fertilizer that dries on the seal faces leaves salt there between runs.

A diaphragm head wins on corrosion and on maintenance. The membrane keeps the stock away from the drive, so the wetted set is small and easy to specify in a compatible plastic. A fertilizer injection pump of this family covers most greenhouse and nursery duty below the high pressure band.

A peristaltic head wins on small flows and on stocks that carry solids. It holds a smooth pulsing flow down to very low figures. It can also pass a suspension that would foul a ball check valve. The price is the tube, a wear item with a finite life the site has to schedule.

For a 10 m³/h greenhouse header the decision usually settles on one of the two positive displacement heads. Choose the diaphragm head for the fertilizer stock in most greenhouse work. Hold the piston head for a high pressure chemical injection pump duty, such as a sanitizing line on the same water source.

The Eductor Route: Centrifugal Pump With Venturi

A centrifugal pump on its own is a transfer pump and not a dosing unit. It answers head and flow, and it says nothing about concentration. The common buying error is to couple a centrifugal pump to a fixed orifice and call it a dose system. The delivered ratio then tracks main flow with a square law error.

The correct version of that idea is a centrifugal pump driving a venturi injector. An eductor pump draws and mixes stock in one pass. Motive driven eductors entrain stock in a 1:1 to 4:1 band, quoted as litres of motive water per litre of stock drawn. A greenhouse can then inject stock with no power at the point of injection. That suits an irrigation injection system with a large main and few zone changes.

The limit is that an eductor pump holds a ratio and not a ppm figure. Change the motive pressure and the entrained flow changes with it. A fixed motive setting cannot follow a header that swings from two zones to eight. Drive the motive line from a flow proportional signal, or accept a ratio that drifts with the pump curve.

The eductor range covers the suction and inline mixing variants. The eductor sizing calculator turns motive pressure and back pressure into an entrainment figure. The water eductors guide works the throat pressure balance in detail.

The injection and mixing guide sets out where the stock enters the pipe and how far the mixture must travel to even out.

Sizing the Pump for Greenhouse Zones

Take the published reference shape for this class of site. It is a 1.2 ha multi span house with eight irrigation zones. Each zone runs 1.5 m³ of water per day and two zones irrigate at once. The header peak is 10 m³/h, and the daily volume is 12 m³ over 1.2 hours of pumping. A greenhouse fertigation injection retrofit works this shape end to end.

The zone sum closes at the header. Eight zones at 1.5 m³ per day is 12 m³ per day. That volume over 1.2 hours is 10 m³/h at the header. Split across two irrigating zones, each zone sees 5 m³/h, and each needs 4.0 L/h of stock. Two zones together add back to the 8.0 L/h header duty.

Size the head on the worst habit of the schedule rather than on the average. The required 8.0 L/h on a 0 to 10 L/h head means 80 percent stroke at full frequency. That is a comfortable working band.

Size the fertigation pump on the header duty and the stroke band rather than on the drum the site happens to own. Size the day tank on the daily mass as well. The daily draw is 9.6 L, so a 20 L day tank carries about two days of stock at 19.2 L.

A 60 L drum holds under a full week. The seven day need is 67.2 L, so the drum is 7.2 L short. That shortfall caused the drift in the opening example. The last day of every week drew below target as the bottle emptied.

Signal the pump from flow rather than from time. A pulsing flow meter on the header sends a pulse per unit of water. A proportional controller converts that pulse train into a stroke rate. The fertilizer injector for irrigation system then follows the flow meter through every zone change, so the ratio holds while the flow moves.

Materials: What Survives the Stock

Material choice is a chemistry decision before it is a strength decision. A wrong choice shows up as a drifting dose or a swollen head rather than a burst pipe. That makes it easy to defer and expensive to ignore.

Start with the fertilizer stock itself. Many greenhouse blends carry chloride or a low pH, and the supplier datasheet is the only sound source for the wetted material list. Take the room temperature of the worst case as well, because a warm injection room shifts some plastics inside their softening range.

Where the same skid also doses a sanitizing solution, the published hypochlorite data applies. Sodium hypochlorite solutions above 500 ppm available chlorine are corrosive to some metals and to many thermoplastics, including acetal resin. Hypochlorite attacks stainless steel and aluminium. The listed compatible materials include PE, HDPE, PP, PVC, PTFE, PVDF, Viton and titanium, and glass is safe. The same list covers chemical injection pumps used for sanitizing between crops.

Strength and density both move with concentration and with storage. A 5 percent solution has a density of 1.093 g/mL at 20 C. A 14 percent solution reaches 1.21 g/mL at the same temperature. Solutions are alkaline, at pH 11 or higher. Household bleach is a 3 to 8 percent solution at manufacture, and its strength falls in storage.

Sanitizing duty on a shared skid carries its own numbers. Food contact surfaces may be sanitized at 200 ppm available chlorine or less without a rinse. The historic reference for continuous chlorination is a dose of 0.2 to 0.35 ppm, used at Boonton Reservoir in 1908.

Backflow Prevention and Injection Practice

An irrigation injection system ties the stock tank to the water source, so protection is a design decision rather than an accessory. Fit a reduced pressure zone assembly, or hold a physical air gap between the source and the injection line. Both protect the well or the municipal supply if main pressure fails while the pump keeps stroking.

Keep the stock side separated as well. Leave an air gap over the day tank, and never plumb a tank fill line straight from a potable main into a stock tank.

The positive pressure differential check valve has a second job beyond vapour lock. It also blocks a reverse path from the main back into the head when the pump stops. Specify it for the full main pressure range, and place it at the head rather than at the end of a long branch. A fertilizer injection pump should never depend on the check valve in its own head for source protection.

Name the practice standards in the order rather than inventing a rule set. ASAE EP734 and ANSI/ASABE F444 cover injection system practice. ISO 9906 and EN 12224 give the performance test basis for a pump curve. A supplier who can name the test standard behind a quoted curve is easier to hold to the metering band you specified.

Geometry belongs in the same schedule as the devices. Keep 10 pipe diameters of straight run from the injection point to the nearest bend or mixer. Closer than that, the stock arrives as a slug the pipe cannot smooth. On a 100 mm main, 10 diameters is 1.0 m of straight pipe.

Verification: Proving the Dose Without a Lab

Three checks close the loop on an installed system, and all three use instruments a grower already owns. The checks are a tank drawdown, a mass close on the arithmetic, and a tracer step at the far end of the line.

Commissioning fertigation injection systems starts with the drawdown. Measure the day tank before and after a full irrigation cycle and compare the fall with the computed 9.6 L. A tank that drops 12 L during a normal cycle is over-dosing. The cause is usually a stroke left at its commissioning figure after a stock change.

Then close the mass. Metering 9.6 L per day of a 15 percent stock at 1.0 kg/L delivers 1.44 kg of active per day. Water at 12 m³ per day and 120 ppm carries the same 1.44 kg. When the two figures agree on paper and the tank draw matches on site, the dose is real rather than assumed.

Run a tracer step for the mixing claim. Step the dose from zero to the working figure at steady flow. Log a downstream conductivity or colorimeter reading at the far end of the run. The interval to reach 95 percent of the final value is the mixing time constant. It should sit inside the transit time of the pipe.

Transit time sets the floor. A 10 m length of 100 mm pipe holds 78.5 L. At 50 m³/h that gives 5.6 seconds of residence time, the entire mixing window with no static mixer. A fertigation pump with no mixer downstream relies on that window.

Protect the injection hardware with a screen sized to the passage it feeds. The working rule is a screen opening near one quarter to one third of the smallest free passage. A 1.0 mm passage wants a screen of 250 to 330 microns, about 50 to 60 mesh. A 0.5 mm passage wants 125 to 165 microns, near 100 to 120 mesh.

Choosing Between the Three Pump Classes

The class follows the duty once the duty is written down. Fill the table with the site figures before any quotation is requested, and the shortlist usually falls out of three rows.

Duty on the site	Class that fits	Why it fits	Watch item
Header near 10 m³/h, one stock, flow varies with zones	Metering head, diaphragm family	Holds a fixed L/h against changing main pressure	Vapour lock and low stroke valve seating
Large main, stock drawn at the point, power limited there	Eductor on a motive pump	Draws and mixes in one pass with no local power	Ratio tracks motive pressure, not ppm
Flow under about 1 L/h, stocks carrying solids	Peristaltic head	No check valves to foul on a gritty stock	Tube life sets the calibration interval
Bulk stock moving to a day tank	Centrifugal pump	Cheap, robust, tolerant of a dirty tank	Cannot meter, so it never touches the dose line

Two rows in that table can be true on one site. A greenhouse often runs a diaphragm head on the dose line and a centrifugal unit on transfer. The two connect only through the day tank. A fertilizer injector for irrigation system sized on the header duty still needs a transfer pump for the bulk stock.

Revisit the class when the programme changes. Moving from one stock to a two part programme changes the required flow. A higher ppm target can push a head below its seating band.

What to Send Us Before You Order

Send five figures and the selection becomes a calculation rather than a guess. They are the water flow in m³/h, the target dose in ppm, and the stock strength in percent.

Add the stock density, the main pressure at the injection point, and the smallest zone flow the schedule will run. Add the two physical facts that decide the hardware as well. They are the room temperature at the injection point and the wetted materials the stock datasheet allows.

With those in hand, the duty in L/h, the head size and the stroke band all follow from the arithmetic. That set of figures is what sizes a fertigation pump, and it takes a grower about ten minutes to collect.

If the daily draw already points at a drum that runs short, say so. A 67.2 L weekly need against a 60 L drum is a sizing problem and not an operator problem. Solve it with a 20 L day tank and a flow proportional drive rather than with a second drum.

The agricultural nozzle range covers the emitters and spray hardware downstream. The eductor range covers motive driven injection and tank mixing. Send your five figures and two physical facts to the application team. They will return a stock flow in L/h, a head recommendation and the verification steps that prove the dose on site.