Many natural waters contain impurities that make them directly harmful to crops. Plants vary in their ability to tolerate and use poor quality water; so do soils vary in their resistance to its effect. Therefore, knowledge of a water supply's quality, assessed by chemical analysis, is essential in determining its usefulness in an irrigation system. However, unless you have some understanding of the analysis, the reasons why a water is suitable or unsuitable may not be clear.

In general, water for irrigation of agricultural crops can be assessed in terms of five criteria, which, together, indicate its potential to harm crop, soil or equipment. They are grouped into categories:

Salinity

• Total salinity
• Specific ions
• Iron for irrigation equipment that uses small water outlets

Sodicity

• Sodium
• Residual alkalinity

Precise standards for irrigation water quality are virtually impossible to set, since the conditions of use must also be taken into account and will modify the effects of the water. Factors that must be considered include:

• type of crops to be irrigated
• soils to be irrigated
• climate
• method of irrigation
• management of irrigation and drainage.

A water sample analysis serves the purpose of making you aware of what the main problems with your water supply are likely to be. The comments made about a water analysis are based on predictions of the likely effects the water will have when used for irrigation, and should be considered as guidelines only.

Chemical analysis

When salts dissolve in water, they dissociate into particles carrying either positive (cations) or negative (anions) electrical charges. For example calcium bicarbonate becomes:

Ca (HCO₃)₂   = Ca⁺⁺ + HCO₃^¯ + HCO₃^¯

Much of a water sample´s chemical analysis depends on the determination of the ion concentration of elements in the sample. Ionic concentrations of salts dissolved in a water sample are listed on the analysis report in metric units as milligrams/litre (mg/L), which are similar to the imperial unit parts per million (ppm).

pH

The term pH is a measure of the acidity or alkalinity of a water sample.

The pH of natural waters normally falls between the range of pH 4.0 to pH 9.0. Soils generally are highly buffered systems and the pH of the soil would not be significantly affected by the application of irrigation water within this range.

Waters having pH values greater than 8.0 would be expected to contain sodium carbonate and bicarbonate and the waters usefulness would depend on the amount of these salts present.

Corrosion is more rapid in acid than in neutral or alkaline waters. Irrigation with strongly acid water may dissolve iron, aluminium and magnesium from the soil in amounts that could be toxic to plant growth.

Salinity

All natural water contains water-soluble chemicals known as salts. They may be plant foods or other organic or inorganic salts gathered as it travels over or through sub-soil rock or through the soil. The total salt content of water is its salinity, but it is the types of salts that make up this salinity that will determine the suitability of a water for its intended use.

Large quantities of salt may be added to the soil each year in saline irrigation water, eg. 1 milligram per litre (mg/L) of salt as Total Dissolved Ions (TDI) is equal to 1 kilogram of salt per megalitre (ML) or 1 000 000 litres of irrigation water. Even though it may not be immediately harmful to the plant or soil structure, this salt has to go somewhere. Depending on the soil texture, its depth and the amount of leaching that occurs, this salt will accumulate at some point in the soil profile.

Salinity can have the following effects on plant and soil

•  Plants may grow well in moderately saline soil when soil water is in good supply. When soil water is removed, by plant transpiration, evaporation and drainage, the salt concentration in the remaining soil water increases. Salts also attract and absorb water so as the available soil water declines plants have to exert more energy to satisfy their water needs. In this way salts compete directly with plants for available moisture and reduce the amount available to the plant.
• Reduce the availability of certain plant foods, for example, high levels of magnesium or sodium may induce calcium or potassium deficiencies in plants growing on soils low in these elements.
• As a plant root absorbs water from the soil it also absorbs the salts, plant foods etcetera, dissolved in it. High concentrations of undesirable salts introduced into the soil irrigation water may become toxic to salt sensitive plants.
• Salt accumulation on plant foliage after spray irrigation may burn the leaves.
• Sodium reacts with the soil to change the soil structure in a detrimental manner.

Conductivity

A simple test for quick assessment of a water´s salinity or total dissolved ion content. The conductivity test does not identify the dissolved salts, or the affects they may have on crop or soil, but it does indicate fairly reliably the degree with which a salinity problem is likely to occur.

Most laboratories - including the Water Testing Laboratory, Natural Resource Sciences Chemistry Centre, Department of Natural Resources and Mines - measure conductivity in microsiemens per centimetre (uS/cm^-1). Some laboratories measure conductivity in decisiemens/metre (dS/m^-1). Microsiemens/centimetre (uS/cm^-1) can be converted to decisiemens/metre by dividing by 1000. ie. 1 dS/m^-1= 1000 uS/cm^-1. Water of high purity strongly resists the passage of electric current, a poor conductor of electricity. When salts are dissolved in water they improve its conductivity, so the greater the quantity of dissolved salts a water contains the higher will be its conductivity reading.

Effective conductivity

As salts of low solubility are likely to precipitate out of solution in the soil and not contribute to the salinity of the soil water, some allowance must be made for this. The main salt of concern is calcium carbonate and an estimate is made of the amount of CaCO₃ that would precipitate from the water. The conductivity measurement is then corrected accordingly. Waters high in calcium carbonates and bicarbonates are fairly common in Queensland. Correcting the conductivity for loss of these salts allows a wider range of waters to be considered suitable for irrigation use. The conductivity corrected for CaCO₃ loss is then referred to as the effective conductivity.

Total dissolved ions (TDI)

Is the sum of all the ions present in a sample of water and represents the total salt content of the water.

Sodicity

This 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 in three ways:

• By destroying soil structure causing clay particles to disperse rather than cling together as small peds (coarse blocky texture, crust formation after rain or irrigation) and reducing water movement (permeability) and aeration in the soil.
• By poisoning sodium sensitive plants when absorbed by either their roots or leaves.
• Calcium and/or potassium deficiencies may occur if the soil or irrigation water is high in sodium.

Sodium adsorption ratio (SAR)

The sodium adsorption ratio measures the relative proportion of sodium ions in a water sample to those of calcium and magnesium. The SAR is used to predict the sodium hazard of high carbonate waters especially if they contain no residual alkali.

The sodium adsorption ratio is used to predict the potential for sodium to accumulate in the soil, which would result from continued use of a sodic water, referred to as the Exchangeable Sodium Percentage (ESP). A water sample with a high SAR and a low RA usually has high sodium content due to the predominance of sodium chloride.

Effective SAR

Similarly to conductivity, the SAR can be corrected to allow for calcium carbonate precipitation. It usually raises the reading for SAR because the presence of calcium can cause the calculation for SAR to understate the importance of sodium in a water.

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

Residual alkalinity represents the amount of sodium carbonate and sodium bicarbonate in the water and is said to be present in a water sample if the concentration 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 a clay soils sodium content may cause structure damage.

Toxicity

Toxicity is the detrimental effect that certain specific ions have on plants. The toxicity problem differs from the salinity and sodicity problems in that it occurs within the crop itself and can occur in sensitive crops even if the total salinity of the water is low.

Chloride (Cl ^-)

Chloride toxicity is taken into account with salinity evaluation as chloride sensitive plants are all in the low salt tolerance range.

Irrigation water high in chloride may cause foliage burn, especially on salt sensitive plants, starting at the leaf tip and progressing back along the leaf margins or edges. Foliage burn may be aggravated by hot dry weather.

The chloride ion can be toxic to plants with a low salt tolerance when taken up by their roots and absorbed through their leaves. Crops mentioned under sodium are also most sensitive to chloride.

Iron (Fe⁺⁺⁺)

Dissolved iron in water is present in the ferrous state. Except at low pH values, ferrous iron is readily oxidised to ferric iron, an insoluble reddish brown precipitate on exposure to air and sunlight.  For information on how to take water samples for iron analysis, check with the laboratory that will be conducting the analysis.

For localised irrigation schemes that use very small outlets, the presence of iron in the irrigation water has proved to be a problem. It has been found that iron bacteria flourish in water that contains as little as 1.0 mg/L of iron. The bacteria extract the iron out of solution and convert it into a rust coloured sludge, which quickly blocks filters and outlets.

Nitrate (NO₃^-)

For many crops nitrate in the irrigation water will provide them with some extra nitrogen, but nitrate sensitive crops could be affected by concentrations greater than 22 mg/L nitrate and problems may occur with increasing concentrations up to 133 mg/L nitrate, above which severe problems could arise. Ammonium nitrogen is seldom found in significant amounts in natural waters.

Nitrogen could be found in dams containing decaying organic matter; or in underground water contaminated with seepage from soils, that have had large quantities of nitrogen fertiliser applied, or by effluent from cattle feed lots or piggeries.

Sodium (Na⁺)

Symptoms of sodium toxicity appear as burning or drying on the outer edges of older leaves. Progressing inwards towards the centre. Sodium can be absorbed through roots or leaves if sprinkler irrigation is used. Tree crops and woody perennials are most affected. With the exception of beans annual crops are not so sensitive.

Beans, avocado, citrus, deciduous fruits and nut trees are very sensitive to sodium and may show its toxic affect when:

• flood irrigation water has a sodium adsorption ratio (SAR) as low as 4.5
• spray irrigation water that wets the foliage, has a sodium content greater than 70 mg/L or SAR greater than 3.0.