Four questions, one lab report
Irrigation water quality comes down to four separate risks, and a standard analysis covers them all: salinity (yield), sodium (soil structure), specific ions (chloride, boron, bicarbonate: leaf and fruit damage), and equipment factors (pH, scaling, iron). South Africa's official yardstick is the DWAF irrigation guideline (Volume 4, 1996), which reports salinity as EC (electrical conductivity), in mS/m. If you think in TDS instead, multiply mS/m by about 6.5 to get mg/L; international tables use dS/m, which is mS/m divided by 100.
Salinity: the yield number
Plants pay for salty water by working harder to draw moisture, and the yield loss starts before anything looks wrong. The guideline's bands: under 40 mS/m, even salt-sensitive crops grow without loss; at 40–90 mS/m, moderately sensitive crops hold about 95% of yield with careful management; at 90–270 mS/m you are farming moderately tolerant crops at around 90%; at 270–540 mS/m those same crops hold about 80% only under high-frequency irrigation, and beyond 540 mS/m the choice of crops becomes short. Where yield loss begins depends heavily on the crop. The FAO's standard thresholds for the irrigation water itself (assuming normal leaching):
| Crop | Water EC where yield loss begins (mS/m) |
|---|---|
| Strawberry | 70 |
| Grapevine | 100 |
| Maize | 110 |
| Lucerne | 130 |
| Tomato | 170 |
| Wheat | 400 |
| Barley | 530 |
Two practical notes. Seedlings are more sensitive than the table suggests; germination can fail in water an established stand tolerates. And salt management is leaching management: salty water needs a deliberate fraction of extra irrigation to wash salts below the roots, which is water a drought does not give you. So treat a rising borehole EC in a dry year as a cropping decision.
Sodium: the number that seals the soil
SAR (the sodium adsorption ratio) measures sodium against calcium and magnesium. High-SAR water makes clay soils disperse: the surface seals, water stops infiltrating, and the ground bakes into crust. The guideline's safe target for sensitive soils is SAR below 1.5, with tolerance rising when the water also carries salts. Here sits the counter-intuitive trap: very pure water is its own infiltration hazard. Below about 20 mS/m, water is so "hungry" that it strips soil calcium and collapses structure even at modest SAR, a known problem with rainwater-fed and mountain-stream irrigation on sensitive soils. The classic fix is calcium: gypsum on the land, or dosed into the water, lifting both calcium and EC at once.
The ions that burn
- Chloride: safe to about 100 mg/L for almost everything, even sprayed on leaves. Sprinkled onto sensitive crops (citrus, stone fruit, vines), foliar burn starts between 100 and 175 mg/L; roots tolerate a little more. Drip irrigation sidesteps the foliar limit entirely, which is one argument for drippers on marginal water.
- Sodium on leaves: keep under about 70 mg/L where foliage is wetted by overheads in warm weather.
- Boron: the narrowest window in the report: lemons react below 0.5 mg/L; grapes, deciduous fruit and onions from 0.5–1 mg/L. There is no farm-scale treatment that removes boron cheaply; the answer is crop choice.
- Bicarbonate: above roughly 90 mg/L as HCO₃ (about 75 mg/L if your certificate reports it as CaCO₃, which is how South African laboratories normally report alkalinity), overhead irrigation leaves white lime deposits on leaves and fruit and pushes lime scale into pipes and drippers. The deposits are cosmetic on maize and a packing-line problem on table grapes and pears.
The equipment numbers
The guideline's normal pH window is 6.5–8.4. Beyond that, and whenever calcium, alkalinity and pH run high together, check the Langelier index (the scaling calculator in our tools does this): above +0.2 the water wants to deposit lime in pipes, valves and drippers; below −0.2 it tends to dissolve scale and can be aggressive to concrete and unprotected steel. And note the iron trap: the crop guideline tolerates iron to 5 mg/L, but drip emitters start clogging at a twenty-fifth of that. The next article covers why.
Rule of thumb
Test before every season, and mid-season in drought, because EC drifts up as sources shrink. Read the report in order: EC against your crop's threshold, SAR against your soil (remembering that very pure water misbehaves too), then chloride and bicarbonate if anything is overhead-irrigated, and boron for sensitive crops. Between about 20 and 40 mS/m with SAR under 1.5, irrigate freely; below 20 mS/m, watch for soil-structure loss; above 40 mS/m, every step is a management decision (leach more, switch to drip, add calcium, change the crop), and knowing early makes each one cheaper.