Effects of water quality on soil, plants and irrigation equipment

Using a salt tolerant plant does not solve every problem when it comes to using salt-laden water for irrigation. It is important to be aware that salts in the water can build up through evaporative concentration and damage both plants and soil.

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Sodicity

Sodicity is the effect the irrigation water will have on the physical properties of the soil due to an accumulation of sodium.

Sodium can affect plants and soils in three ways:

  1. By destroying soil structure. In the presence of moisture and exchangeable sodium1, clay particles disperse rather than cling together as small peds (friable soil aggregates). This reduces water movement (permeability) and aeration in the soil. Soils with a poor structure will have a coarse blocky or powdery texture and surface crusts will form after rain or irrigation.
  2. By poisoning sodium sensitive plants when absorbed by either their roots or leaves, and
  3. Calcium and/or potassium deficiencies may occur if the soil or irrigation water is high in sodium.

Sodium absorption ratio

The sodium adsorption ratio (SARw2) is an indicator of the relative proportion of sodium ions in a water sample to those of calcium and magnesium. SARw is used to predict the sodium hazard.

The sodium adsorption ratio is used to predict the potential for sodium to accumulate in the soil, if sodic water was in constant use. A water sample with a high SARw and a low residual alkalinity (RA - see later) usually has high sodium content due to the predominance of sodium chloride.

In order to calculate the SARw from water analysis data, it is essential to convert the units from parts per million or milligrams per litre to milliequivalents per litre:

sodium absorption ratio formula

sodium absorption ratio formula

Where equivalent weights are:

  • Calcium = 20
  • Sodium = 23
  • Magnesium = 12.2

SAR parameter

SAR parameter

This parameter qualifies the ratio of sodium to calcium and magnesium in terms of the ability of the sodium to dominate the soil. The lower SAR the less likely the water is to cause structural degradation of susceptible soils. Table 1 outlines the levels at which SARw indicates a hazard to soil structure. The susceptibility of differing soil types to degradation is then further qualified in Table 2.

Table 1: Hazard levels for SARw

SARw

Hazard

<10 Safe to irrigate with no structural deterioration but salt-sensitive plants many be affected depending on EC/TDS (see tables 2 and 4)
10-18 Hazard on fine textured soils with a high cation exchange capacity. Suitable on course textured soils with good drainage (see Table 2)
18-26 Hazard on most soils. Need to manage with amendments and drainage (i.e. leaching)
26 Not suitable for irrigation
Table 2. SARw limits based on soil type

Soil

No Hazard

Slight - Moderate hazard

Severe hazard

2:1 clays

<6

6-9

>9

1:1 clays

<16

16-24

>24

Sand ECw3>1.5

<16

16-24

>24

Sand, ECw<1.5 dSm-1

<6

6-9

>9

2:1 clays such as montmorillonite, illite and smectite are the common clay minerals found in black earths and yellow sodic soils. 1:1 clays such as kaolinite are commonly found in Ferrosols (formerly known as krasnozems). The SAR at which a 2:1 clay is at risk is lower than for a 1:1 clay, as the bonds holding the 2:1 clay platelets together are more unstable in water than those of a 1:1 clay mineral.

Residual alkalinity (RA) or Residual sodium carbonate (RSC)

Residual alkalinity (RA) represents the amount of sodium carbonate and sodium bicarbonate in the water and is said to be present in a water sample if the concentrations of carbonate and bicarbonate ions exceed the concentrations of calcium and magnesium ions. Residual alkalinity is usually expressed as milliequivalents per litre (meq/L) of sodium carbonate, or on some analysis reports as calcium carbonate.

When irrigation water containing residual alkalinity is used on clay soils containing exchangeable calcium and magnesium, sodium from the residual alkalinity in the water will replace calcium and magnesium in the soil. An increase in the sodium content of a clay soil may cause structural damage.

Residual Sodium Carbonate (RSC) predicts the accumulation of sodium in the soil based on the potential precipitation of calcium/magnesium carbonate.

RSC = (CO3 + HCO3) - (Ca + Mg)

A negative RSC indicates water is unlikely to cause structural degradation. An RSC greater than 1.25 indicates a potential hazard to soil structure. Additions of a calcium source, such as gypsum, or acidification of the water prior to use may be required.

Leaching requirement

It is possible to ensure that salt levels in the soil do not exceed that of the irrigation water by leaching the salt beyond the rootzone. Adequate drainage should ensure that this salt laden water does not cause further environmental damage.

The fraction of irrigation water that must pass through the rootzone to control salts at an acceptable level is described as the leaching requirement or leaching fraction, derived from the following equation.

LR = ECw ÷ (5ECec - ECw)

Where:
ECw = irrigation water salinity (dS/m)
ECec = Threshold salinity (dS/m) a user specified value, based on knowledge of plant tolerances and soil types

Tolerances of plants to salt

The Electrical Conductivity (EC) of irrigation water can affect which plants can be grown in an otherwise suitable site. Plant tolerance to high salinity is species specific. This is shown in Table 4 with special reference to turfgrass species.

Table 4. The tolerance of plants, particularly to turf, to different levels of salinity hazard in irrigation water.

Salinity hazard

Effect on plants

Suitable turfgrasses

Low (EC<0.8) No detrimental effects Suitable for most plants
Medium (EC = 0.8-1.6) Sensitive plants show salt stress Not suitable for blue couch or Kikuyu.
Sensitive varieties of green couch and buffalo may show signs of stress.
High (EC = 1.6-3) Salt tolerant plants only Not suitable for green couch or buffalo grass.
Zoysia matrella may start to show signs of distress.
Very high (EC >3) Very salt tolerant plants only Halophytes such as Paspalum vaginatum; Sporobolus virginicus and; Distichlus spicata are the only grasses likely to survive.

Saturation Index (SI)

The saturation index (SI) of water is a relationship between pH, salinity, alkalinity and hardness. It assesses the potential of the water to cause scaling and precipitation or corrosion. Scale build-up and corrosion both damage irrigation equipment.

Table 5. Interpreting Saturation Index
SI Range Likelihood of scaling Likelihood of corrosion

-0.5 to 0.5

Not likely

Not likely

0.5 to 1.5

Moderate risk

Not likely

>1.5

Strong risk

Not likely

-0.5 to -1.5

Not likely

Moderate risk

<-1.5

Not likely

Strong risk

Footnotes and references

  1. Positively charged sodium ions capable of adhering to clay particles
  2. The subscript w in the SAR indicates that the SAR is for water rather than soil
  3. ECw is the electrical conductivity for water

Reference: D.S. Loch, R.E. Poulter, M.B. Roche, C.J. Carson, T.W. Lees, L. O´Brien and C.R. Durant (2006) Amenity Grasses for Salt-Affected Parks in Coastal Australia, Horticulture Australia Ltd project number TU02005.

Last updated 30 July 2012