On many tomato and pepper plots that already run on drip irrigation, fertiliser is still spread by hand at the foot of each plant. Yet the network that delivers water to the right spot every day could deliver the nutrients too, in small doses and at the moment the crop needs them. That is fertigation. It does not call for a complicated installation, but it does not forgive guesswork: undissolved fertiliser blocks drippers, two incompatible products precipitate in the tank, and a badly timed injection leaves the fertiliser sitting in the pipes. Here is the method, step by step, for a vegetable farm or a cooperative in Cameroon, Côte d’Ivoire, Senegal or elsewhere in West and Central Africa.
What fertigation changes on a drip-irrigated plot
With fertigation, fertiliser is dissolved in water and injected into the main line. It travels with the irrigation water and reaches the wetted bulb, where the active roots are concentrated. That has three practical consequences for the farm manager.
- Nutrition can be split across the whole cycle, weekly or at every irrigation, instead of two or three heavy applications.
- Fertiliser goes where the water goes: if water distribution is poor, fertiliser distribution is poor too. An unbalanced network has to be corrected before you start.
- The irrigation equipment also becomes fertilising equipment. You need an injector, protection of the water source against backflow, and a second look at filtration.
The University of Arkansas extension service notes that a grower who irrigates often can either split the weekly rate across every irrigation, or prepare a stock solution for the week and inject it proportionally. The second option requires a good knowledge of the real injection rate of your equipment.
Four ways to inject fertiliser into the network
The Agriculture Victoria fertigation guide describes four families of injectors. The choice depends mainly on network size, available pressure and access to electricity.
| System | Strengths | Limits | Best suited to |
|---|---|---|---|
| Pressure tank (bypass tank) | Simple, no electricity, easy to maintain, quick to change fertiliser | Concentration falls as injection proceeds; needs a pressure loss in the main line | Small networks, preferably liquid products |
| Venturi injector | Simple to install and to adjust with a metering valve; proportional injection possible | Creates a pressure loss in the main line that can reach 30 percent, or needs a booster pump | Small to medium vegetable plots |
| Electric dosing pump | Accurate, no pressure loss, can be automated | Needs electricity; moving parts that wear | Larger networks, greenhouses, sites with power |
| Hydraulic dosing pump | Runs without electricity, portable, adjustable rate | Uses 2 to 3 litres of drive water per litre of fertiliser injected; regular maintenance | Small networks without power |
On a site supplied by solar pumping and an elevated tank, available pressure is often low. The pressure loss of a venturi then has to be taken into account at the design stage, otherwise the drippers at the end of the line receive less water during injection, and therefore less fertiliser.
Dissolving fertiliser: the rules that keep drippers open
The first risk of fertigation is mechanical: one undissolved particle is enough to block a dripper. The University of Arkansas extension service puts it simply: the fertiliser must be fully dissolved before injection, and if particles are still visible, add more water. Undissolved particles also damage the injector.
Agriculture Victoria gives concrete benchmarks for preparing the stock solution:
- general rule: 100 kg of fertiliser per 500 litres of water, a ratio of 1 to 5, which can be brought closer to 1 to 3 with good agitation;
- nitrogen fertilisers cool the water as they dissolve, which lowers solubility, so base your ratios on water at 10 °C whatever the air temperature;
- as an example, the solubility of urea at 10 °C is 30 kg per 100 litres;
- each 25 kg bag adds roughly 15 litres to the tank volume, so leave room to avoid overflowing;
- many organic products have low solubility and have caused unexpected clogging.
Potassium nitrate, widely used in vegetable production as a potassium source, can be slow to dissolve according to the University of Arkansas table. Give it time and stir before you start injecting.
Mixes that must never share a tank
The second risk is chemical. According to Agriculture Victoria, mixing a calcium fertiliser with a phosphate or sulphate fertiliser is the most common cause of precipitation in irrigation systems. The precipitate forms in the tank or in the pipe and ends up in the drippers.
| Fertiliser | Never mix with |
|---|---|
| Calcium nitrate (15.5-0-0) | Any product containing phosphate or sulphate |
| Ammonium sulphate (21-0-0) | Any product containing calcium |
| Monopotassium phosphate (0-52-34) | Any product containing calcium |
| Potassium sulphate (0-0-50) | Any product containing calcium |
Source: University of Arkansas extension service, fact sheet FSA6177.
The answer is to work with two separate tanks, one for calcium and one for phosphates and sulphates, or to inject these products at different times and rinse the equipment and network in between. The water itself can cause trouble too: hard water rich in calcium, or borehole water carrying iron, can also precipitate with some fertilisers.
The jar test, before every new mix
Agriculture Victoria recommends a simple test. In a clear jar, mix water from your own source with the fertiliser at the same ratio as in the tank, for example 1 to 5, and shake. If the fertiliser does not dissolve, do not inject it. If the solution turns cloudy, leaves a sediment or forms clumps within 24 hours, the mix is not suitable for injection.
A fertigation event, minute by minute
Fertiliser should neither go into a half-empty network nor stay in the pipes after shutdown. The sequence recommended by the University of Arkansas has three stages.
- Pressurise. Run the network on clean water, injector closed, until it is fully pressurised and the drippers at the end of the line are flowing normally.
- Inject. Open the injector and let the prepared dose pass into the network.
- Flush. Close the injector and keep irrigating for another 5 to 10 minutes to clear fertiliser from the pipes and drippers.
Agriculture Victoria adds two points. First, inject as late as possible in the irrigation, while leaving enough time to flush. Second, fertigation events and leaching irrigations should be kept separate, so you do not push the fertiliser below the root zone.
Measure the travel time of your own network
The 5 to 10 minute flush applies to a small network. On a longer one, water takes more time to reach the last dripper. A University of Florida fact sheet on citrus fertigation gives 20 to 30 minutes from the injection point to the farthest emitter, recommends injecting for at least 30 to 45 minutes for uniform distribution, and flushing for at least 30 minutes, warning that flushing much longer can leach nutrients below the roots.
The right figure is the one for your plot. Agriculture Victoria suggests measuring it with a dye trial: inject a food dye and time its arrival at the farthest dripper. Another method is to follow the conductivity of the water leaving that last dripper with an EC meter. The flush should last at least that travel time.
Filtration and backflow: the order of equipment at the head of the network
Adding an injector changes the control head. Two rules complement each other. Agriculture Victoria says to inject downstream of the main filters, so fertiliser does not foul the head filter. The B.C. Trickle Irrigation Manual asks for a filter after the injectors, to catch precipitates that may form at the point of injection. In practice, this gives the following order:
- check valve and anti-siphon device on the water source side;
- main filter, matched to water quality;
- fertiliser injection point;
- screen or disc safety filter after the injection point;
- pressure gauges and section valves.
The check valve is not optional. Agriculture Victoria stresses that check valves and anti-siphon devices are needed to stop the solution flowing back, and that the fertigation circuit must never be connected to household, livestock or any other water supply. How to choose the main filter depending on whether your water comes from a borehole, a river or a pond is covered in our article on choosing a drip irrigation filter.
Monitoring pH and conductivity
Agriculture Victoria places the target pH of the injected solution generally between 5.5 and 7, depending on crop and situation. pH probes need regular recalibration. Many growers also monitor electrical conductivity so as not to exceed the crop’s salt tolerance. A pocket EC meter and pH meter are enough to start: measure the solution leaving a reference dripper, at the same time of day, and record the values in the crop logbook.
Checklist before the first injection
- The network is balanced: the flow of the first and last dripper on a line has been measured and is close.
- The check valve and anti-siphon device are installed and tested.
- A safety filter is in place after the injection point.
- The jar test has been done with water from the plot and the planned mix.
- Calcium on one side, phosphates and sulphates on the other: never in the same tank.
- The travel time to the farthest dripper has been measured.
- The programme runs pressurise, inject, then a flush at least as long as the travel time.
- pH and EC meters are calibrated.
- The crop nutrition plan is set, ideally from a soil analysis.
Fertigation feeds the roots. It does not replace a targeted foliar application when a crop goes through a critical stage: they are two different tools, applied with two different pieces of equipment. We explained in another article what a foliar fertiliser really supplies, and the full crop plan for tomato, where fertigation fits naturally, is in our tomato farming guide. For the design side of the network itself, see our guide to drip irrigation systems for African farms.
Equipping your plot for fertigation with JOGOO
JOGOO Agriculture designs and installs drip networks with their control head: filtration, fertiliser injection, backflow protection and pumping, including solar. See the Irrigation & Pumping page for equipment, and the Fertilization & Plant Nutrition page for fertigation, fertilisers and agronomic advice. Alongside fertigation, JOGOO offers its own AgriBiosol foliar fertiliser range, applied by spraying. For a quote matched to your area, crop and water source, go through the Contact page.
Frequently asked questions
What is fertigation?
Fertigation is the application of fertiliser dissolved in irrigation water. On a drip network, the solution is injected into the main line and reaches the foot of each plant with the water, so nutrition can be split across the whole cycle.
Which injector suits a small vegetable plot?
A venturi injector or a pressure tank suits small networks and works without electricity. A venturi creates a pressure loss in the main line that can reach 30 percent according to Agriculture Victoria, which must be built into the design, especially with solar pumping.
Can calcium nitrate and potassium sulphate go in the same tank?
No. Calcium precipitates with sulphates and phosphates. Use two separate tanks or two separate injections with a rinse in between, and always run a jar test with water from your plot.
How long should I flush the network after injecting?
At least as long as water takes to travel from the injection point to the farthest dripper. The University of Arkansas gives 5 to 10 minutes for a small network; on a large network the travel time can reach 20 to 30 minutes according to the University of Florida. Measure it with a dye.
Where should the injector go relative to the filter?
After the main filter, with a screen or disc safety filter downstream of the injection point to catch any precipitate. A check valve protects the water source.
Can a foliar fertiliser be injected into drip lines?
A foliar fertiliser is formulated to be sprayed on leaves. Follow the product label: fertigation relies on fully soluble fertilisers, and many low-solubility organic products have already blocked drippers.
Sources
- Agriculture Victoria, Fertigation, Drip irrigation
- Ernst T., Barney K., McWhirt A., Herrera L., Carr L., Fertigation Calculations That Grow Results, FSA6177, University of Arkansas System Division of Agriculture, Cooperative Extension Service
- Zekri M. et al., Fertigation for Citrus Trees, HS1306, UF/IFAS Extension, 2024
- B.C. Trickle Irrigation Manual, Chapter 10, Filtration, 1999
