Fertigation — Feeding Through the Drip

Fertigation is fertiliser applied with the irrigation: dissolved into the water, injected at a known ratio, delivered to the root zone with almost no waste. Get it right and nitrogen losses drop, growth evens out and you stop hauling bags through wet blocks. Get the chemistry wrong and you plug every emitter on the farm.

It assumes a drip system and a nutrient budget from the fertiliser calculator; this guide covers the hardware, the chemistry and the schedule that connect the two.

Water flows left to right: the pump draws through a suction basket where the venturi injector draws stock solution, then a filter, then fertigated water down the mainline to the drip laterals. The two stock tanks — nitrogen–calcium and phosphate–potassium — are never mixed in one tank.

The system, component by component#

Component	What it does	What goes wrong without it
Injector (venturi, dosing pump or proportional)	Draws stock solution into the line at a set ratio	No injection, or a ratio that drifts with pressure
Stock tanks (two, not one)	Hold concentrated fertiliser solution	Precipitates and plugging (see compatibility below)
Filter (screen or disc, ≥120 mesh)	Catches precipitates and particulates	Emitters block within weeks
Check / control valves	Prevent backflow into the tank or bore	Contaminated water siphoned back into stock
Flush points	Clear fines and biofilm from laterals	Progressive flow loss along the lateral
Mixer / agitation	Keeps solution homogeneous	Strength drifts as solids settle

Injector ratios#

An injector expressed as 1:100 draws 1 litre of stock per 100 litres of water. To choose a ratio, work backwards from the dose you want:

Stock strength  = desired ppm in field × dilution ratio
Example         = 100 ppm N desired, 1:100 injector
                = 100 × 100 = 10,000 ppm N in the stock tank (1% solution)
                = 10 g N per litre of stock → 37 g urea (46% N) per litre

A dosing pump removes this arithmetic — it doses millilitres per hour directly — but the field concentration is still governed by injector rate ÷ water flow. Verify with the fertigation calculator whenever flow changes seasonally.

Stock solution chemistry: the rules that matter#

Compatible together: urea, KNO₃, ammonium nitrate, potassium sulphate (with no calcium present), most acidified solutions.
Incompatible with calcium: phosphates (MAP, DAP, polyphosphates), sulphates (potash magnesium sulphate), micronutrient sulphates.
Acid injection (phosphoric, sulphuric or nitric acid) for pH control is its own circuit — never acidify a tank containing calcium without expecting precipitation.
Never dissolve fertiliser in the stock tank by warm water hacks on a live system — pre-dissolve in a bucket, let it settle, decant the clear liquor, strain it into the tank.

Injection schedule#

Bananas take up nitrogen steadily from establishment onward, and fertigation matches that with small, frequent doses — typically 2–4 injections per week, each wetting 6–15 mm at the emitter.

Check	Typical guidance
Share of N via fertigation	50–70% of the season budget; balance through splits
Potassium	Most or all K suits fertigation — it is highly mobile and defies banding
Phosphorus	Poor candidate for fertigation (immobile, precipitates); keep it in the planting hole and basal applications
Concentration ceiling	≤250 ppm N at the emitter; halve it if the irrigation water is already saline — see salinity
Weekly N	4–6 kg N/ha in the wet/growth season; verify against the season budget

Keeping emitters alive#

Filter to 120 mesh minimum, sized for the flow — check differential pressure monthly.
Flush laterals every 2–4 weeks, opening the end caps until the water runs clear.
Acid-clean (phosphoric or sulphuric, pH 4 for 30 minutes) quarterly in hard-water areas to dissolve carbonate scale.
Chlorinate (10–20 ppm free chlorine) where algae or bacterial slime dominate — especially with open stock tanks.
Strain every bucket of stock before it enters the tank. Every time.
Record what was injected, when, at what strength — this is also what residue audits and certification ask for.

Common mistakes#

One tank for everything — the fastest route to plugged emitters.
Injector ratio set once and forgotten — pressure and filter condition move actual injection rates; verify quarterly with a jar test.
Fertigating phosphorus — it precipitates, plugs, and barely moves in the soil anyway.
Fertigation replacing soil tests — the tool applies; the soil test interpreter decides.
Skipping the flush between tanks — sequential injection still mixes in the mainline without it.

Next steps#

Dose the event: fertigation calculator.
Set the season budget: fertiliser calculator and nutrition reference.
Size and schedule the water: irrigation calculator, irrigation scheduler.
Water quality limits that cap concentration: salinity and water quality.
System layout and filtration: water management.