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Irrigation Water Quality: What to Test For and Why
Soil & Fertigation · Water Quality
Irrigation Water Quality: What to Test For and Why
Irrigation water quality shapes every other input, and water is the single largest input a crop receives, and in most of this region it is also the least examined. A water analysis costs a fraction of one fertilizer application, yet it determines whether that application works at all.
Key takeaways
- Water chemistry decides salinity load, nutrient availability at the emitter, and whether your irrigation system stays clear.
- The four figures that drive most decisions are EC, SAR, bicarbonate and pH.
- Bicarbonate is the most commonly overlooked, and the main cause of both emitter blockage and reduced availability at high pH.
- A water analysis is a prerequisite for designing fertigation, not an optional extra.
Why water is the starting point
Every irrigation adds dissolved salts and minerals to the root zone. The crop takes up water and leaves most of that load behind. Over a season the soil solution reflects the water far more than it reflects the original soil.
This is why two identical soils under different water sources diverge within a few seasons, and why a nutrition programme designed without a water analysis is built on an assumption.
The figures that matter most
| Measurement | What it tells you | What it affects |
|---|---|---|
| EC (electrical conductivity) | Total dissolved salt concentration | Salinity build-up, leaching requirement, crop selection |
| SAR (sodium adsorption ratio) | Sodium relative to calcium and magnesium | Soil structure, infiltration rate |
| Bicarbonate and carbonate | Alkalinity of the water | Emitter scaling, pH at the root, micronutrient availability |
| pH | Acidity of the water itself | Fertilizer solubility and compatibility in the tank |
| Chloride, sodium, boron | Specific ions that damage sensitive crops | Leaf scorch, quality loss in sensitive species |
EC: the salinity load
Water EC sets the floor for soil salinity. Soil EC will always exceed water EC, because plants remove water and leave salt behind. The gap between them depends on how much extra water is applied for leaching.
A water source with low EC gives flexibility in crop choice. A high-EC source narrows it, and no amendment changes that — the practical responses are leaching, drainage and selecting tolerant crops.
SAR: the structural risk
Sodium behaves differently from other salts. Where it dominates the exchange sites relative to calcium and magnesium, clay particles disperse, pores collapse and infiltration falls. The field becomes hard to wet and slow to drain, which then makes leaching harder — a self-reinforcing problem.
SAR must be read alongside EC, and the two interact in ways that are not intuitive. Water with a moderate SAR but very low EC can be more damaging to structure than water with the same SAR at higher EC, because dilute water accelerates dispersion. Gypsum is the standard correction where sodium is the issue.
Saline and sodic are different problems requiring different treatments. A saline soil has excess soluble salts and responds to leaching. A sodic soil has excess sodium on the exchange complex and requires calcium, usually as gypsum, before leaching will work. A water analysis distinguishes them.
Bicarbonate: the overlooked figure
Bicarbonate is common in groundwater across Egypt and the Gulf, and it causes two problems at once.
Inside the irrigation system, it precipitates as calcium carbonate scale. Drip emitters have narrow apertures and block progressively, which shows up as uneven growth long before anyone inspects a line.
In the root zone, it raises pH around the emitter. Iron, zinc and manganese become less available exactly where the roots are concentrated, so micronutrient deficiency appears despite adequate soil reserves.
Acid injection addresses both. The rate depends on the bicarbonate concentration, which is why the analysis comes first. This requires proper equipment, dilution and safety procedures, and should be set up with technical support.
Specific ion effects
Some crops are damaged by particular ions at concentrations well below general salinity thresholds. Chloride causes leaf-margin scorch in citrus, stone fruit and grapevine. Boron has a narrow window between sufficiency and toxicity, and irrigation water is a common source of excess. Sodium can cause direct leaf damage under sprinkler irrigation where it contacts foliage.
These effects are crop-specific rather than general, which is why a water report should always be read against the intended crop rather than against a universal standard.
Sampling irrigation water quality correctly
- Sample from the point of use, not the source. Water changes as it moves through storage and pipework.
- Run the system first. Let water flow for several minutes before collecting, so you sample water rather than what settled in the line.
- Use a clean container supplied or specified by the laboratory, and fill it completely.
- Deliver promptly. Some parameters shift within hours of collection.
- Repeat seasonally where the source varies. Groundwater chemistry can change through the year and between years.
Blending and treatment options
Where water quality is poor, several options exist before accepting the constraint.
Blending sources. Mixing a saline well with a fresher canal or municipal source lowers the EC of the applied water. The blend ratio can be calculated from the EC of each source, and even a partial improvement widens crop options.
Acid injection. Addresses bicarbonate, reduces scale and improves nutrient availability at the emitter. The rate follows from the bicarbonate concentration in the analysis.
Filtration. Removes suspended solids, algae and sand. It does not affect dissolved salts, and confusing the two is a common and expensive misunderstanding.
Reverse osmosis. Removes dissolved salts, and is used in high-value protected cultivation where water quality is the binding constraint. Capital and running costs limit it to crops that can carry them.
Maintaining the system
Water chemistry damages irrigation systems gradually rather than suddenly, which is why the damage is usually discovered late.
Emitter blockage from carbonate scale shows first as uneven growth along a line, not as a visible blockage. By the time plants differ visibly, flow rates have often been declining for weeks.
Periodic flushing of laterals, checking flow rates at the start and end of runs, and inspecting emitters at intervals catch the problem while it is still correctable. Where bicarbonate is high, acid treatment of the system at intervals prevents accumulation rather than removing it later.
Frequently asked questions
How often should irrigation water be tested?
At least annually, and more often where the source is a well whose chemistry shifts seasonally, or where you blend sources. Test before designing any new fertigation programme.
Can I use water with high EC if I leach more?
Up to a point, and only where drainage allows the leachate to leave the root zone. Without functioning drainage, additional leaching moves salt to the water table, which then returns it. Drainage capacity sets the practical limit.
Does a filter solve bicarbonate problems?
No. Filtration removes suspended particles. Bicarbonate is dissolved and precipitates after passing the filter, inside the lines and emitters. Acid injection addresses the chemistry; filtration addresses the particles.
What if my water quality is genuinely poor?
Options are blending with a better source where available, selecting tolerant crops, increasing leaching where drainage permits, and adjusting fertilizer forms to avoid adding to the problem. Water quality constrains what is achievable, and planning around it beats ignoring it.
Send us your water analysis
Our agronomy team will assess your irrigation water against your crop and soil, and design a fertigation programme that works with the chemistry you have.
Related reading:
Fertigation guide ·
Soil salinity management ·
Reading a soil analysis report
