Water quality for irrigation of citrus trees

Water salinity

The first quality parameter that must be considered is total dissolved solids (TDS), which measures the total quantity of various inorganic salts dissolved in water. The TDS is assessed by measuring the electrical conductivity (EC) of the water. The principle of EC measurement is based on the fact that salts in water increase the rate at which electricity is conducted in the water. Therefore, the TDS is typically calculated mathematically by laboratories from the EC by using a factor that will differ according to the unit in which the EC is expressed. The relevant factors are provided in Table 1 below, with the TDS being expressed as milligrams per litre (mg/), which is equal to parts per million (ppm).

Interpretation of the total salt content of water is done using a scale of EC values – the higher the EC, the less suitable the water becomes for citrus production. Different EC ranges are used to classify the water quality – the different classes that are internationally used as an indication of the suitability of the water for citrus production is provided in Table 2. The EC, however, does not indicate the type of salts that are present in the water.

The sodium content of the water

Even though the water’s total salinity places it in salinity Class C1, C2 or C3, it can still be unsuitable for irrigation due to disproportionately high sodium (Na) in the water. In addition to being a salt that can damage plant tissues in extremely high concentrations, Na is harmful to soils since it displaces Ca and Mg on clay particles, resulting in clay dispersion and breakdown of soil structure. A decreased water infiltration and/or drainage, with an exponential increase in the soil salinification rate, is the end-result.

To evaluate the risk that Na poses to the soil, the ratio of Na in relation to calcium (Ca) and magnesium (Mg) in the water is used, being expressed as a factor called the sodium adsorption ratio (SAR). The SAR is a value that is mathematically calculated by the laboratory from the water Na, Ca and Mg analysis results. An interpretation of the SAR values is provided in Table 3.

Irrigation water analysis reports will therefore often contain a so-called irrigation class, which is expressed with C and S symbols denoting the salinity (C = conductivity) and sodium status (S = sodium). For example, highly saline water (EC > 2.25) with a low SAR (< 5) will be classified as a C4S1 water. These classifications can be interpreted from tables 2 and 3.
Crust formation that typically occurs on the surface of soil that is irrigated with water that has a high residual sodium carbonate index or an excessive SAR. The result is poor water infiltration, i.e. excessive run-off.

Residual sodium carbonate index

When the pH of the irrigation water exceeds 8.0, a useful alternative measure of the risk that sodium poses is the residual sodium carbonate (RSC) index. In this index the bicarbonate (HCO-3) and carbonate (CO32) concentrations in the water are brought into consideration. The logic being that precipitation of soil Ca and Mg with HCO-3 and CO32- in the irrigation water subsequently leads to an increase in the relative proportions of sodium to the other cations in the soil solution. This situation, in turn, will increase the sodium hazard of the soil-water to a level greater than indicated by the SAR value of the irrigation water. Therefore, a high RSC of irrigation water results in an increase in the soil’s exchangeable sodium percentage (ESP), with the consequent dispersion of clay, as referred to above.

On their own, high HCO-3 and/or CO32- levels in irrigation water also increase the water pH, cause clogging of irrigation systems due to calcite or lime deposition, and a whitish deposition forms on leaves and fruit from the droplets of irrigation water.

In arid and semi-arid regions, underground water often has a high pH and RSC. The higher the pH of the water, the higher the HCO-3 – and CO32- concentration will be, with a progressive shift towards CO32- as the pH increases. Clogging of irrigation systems increases as the HCO-3 and CO32- concentration and water pH increase. The guidelines in Table 4 can be used to assess the risk of drip irrigation systems becoming blocked due to calcite/lime deposition.

To assess the risk of sodification of soil, a calculation of the RSC should be done if the HCO-3 and CO32- concentration of the water exceeds 120 mg/ and 15 mg/ respectively and the SAR>3. The RSC can be calculated from normal irrigation water analysis results, using the following formula:

RSC index = {(HCO-3/61) + (CO3/30)} – {(Ca/20) + (Mg/12)},

where the concentrations are in mg/, and the resulting RSC index value is expressed in milli-equivalents per litre (meq/).

Management and mitigation possibilities

From the above tables, it is implicit that while it is possible to use a wide variety of water quality types for irrigation, management procedures become more demanding as the water quality decreases – the suitable range of soils for citrus also becomes more restrictive. Possible management options to prevent build-up of salt in the soil, and ensure long-term tree performance, are:

Soil drainage: To avoid build-up of salts, the soil must be permeable – a permeable soil can be leached to wash out the salts. In this regard, sand is much more forgiving than heavy soil.

Leaching: Salt build-up in soil is avoided through leaching. Through regular monitoring of salt levels in the soil (measuring the soil’s EC or resistance), the need for leaching (or its effectiveness when practiced) can be determined. Generally, the higher the salt level of the water, the more arid the climate, and the greater the leaching requirement will be.

Irrigation frequency and duration: Irrigation scheduling has a significant impact on salt accumulation or reducing levels in the soil. Short, regular cycles will more likely result in salt build-up in the topsoil because of evaporation. On the other hand, long cycles leading to deep water penetration mitigate build-up of salts in the root zone.

Soil and water amelioration: Irrigation water can be modified with gypsum to improve its SAR. The quantity of gypsum needed for adding to irrigation water depends on the quality of water (the SAR and RSC) and the annual quantity of water required for irrigation of the trees. The addition of gypsum increases water salinity, so this is only an option when the water’s salinity is not too high, and when the soil is well drained. Soils can also be modified using gypsum to ensure proper drainage (it helps to maintain and improve the soil’s structure), but this should only be used when the soil’s exchangeable sodium percentage exceeds 10%. Otherwise the soil is actually further salinified.

In this article the three most important water quality parameters were discussed, i.e. total salinity, SAR and RSC. These are the most common restricting factors that must be considered. Water quality problems can also be associated with the presence of other constituents, like excessive levels of iron (Fe) or manganese (Mn), that can result in drip irrigation systems being blocked.

Literature cited

Prasad, A., D. Kumar, D.V. Singh. 2001. “Effect of residual sodium carbonate in irrigation water on the soil sodication and yield of palmarosa (Cymbopogon martinni) and lemongrass (Cymbopogon flexuosus).” Agricultural Water Management 50, 161-172.

Wilcox, L.V. 1955. “Classification and the use of irrigation waters.” Circular No. 969, USDA, Washington.

Zaman, M., S.A. Shahid, and L. Heng. 2018. “Guideline for salinity assessment, mitigation and adaptation using nuclear and related techniques.” IAEA, Vienna. https://doi.org/10.1007/978-3-319-96190-3.

Featured Image: Drip irrigation lines become blocked when irrigation water with high bicarbonate and carbonate concentrations is used.