How do I interpret my irrigation water report?
Many natural waters contain impurities that make them directly harmful to crops. This is particularly true for borehole water or well water in East Africa, especially where these waters are in heavy black cotton soils, which can be very poor quality irrigation water.
Is is essential for farmers or investors (big or small!) to test water chemistry as a first step, to assess the viability of an irrigation project. Poor quality irrigation water, when badly managed, is a major factor in failed irrigation projects.
This is because crops 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:
Electrical Conductivity or Salinity
- Total salinity
- Specific ions
- Iron for irrigation equipment that uses small water outlets
- Residual alkalinity
Precise standards for irrigation water quality are very difficult to set, since the conditions of use must also be taken into account and will modify the effects of the water. For our reports we use FAO Irrigation guides.
However, other factors that must be considered include:
- type of crops to be irrigated
- soils to be irrigated
- 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.
When salts dissolve in water, they dissociate into particles carrying either positive (cations) or negative (anions) electrical charges.
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 ppm, which are similar to milligrams/litre (mg/L).
The term pH is a measure of the acidity or alkalinity of a water sample.
|« Increasingly ACIDIC||Increasingly ALKALINE »|
Salinity (our Electrical Conductivity on our water reports)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.
High salts (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 nutrients, 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, and plant nutrients, 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.
The following table shows how the quality of irrigation water is classified based on its sality or conductivity.
Check from our water report
|< 0.65||1 - Low salinity water, suitable for use on all crops except tobacco, with all methods of water application, with little probability of a salinity problem developing.|
|0.650 - 1.3||2 - Medium salinity, suitable for use on all but very low salt tolerance crops. Water can be used if a moderate amount of leaching occurs. Plants with medium salt tolerance can be grown, usually without special practices for salinity control. Sprinkler irrigation with the more saline waters in this group may cause leaf burn on salt-sensitive crops, especially at higher temperatures in the daytime when evaporation may be high.|
|1.3 - 3.0||3 - High salinity - suitable for use on medium and high salt tolerant crops only. Water should not be used on soils with restricted drainage. Even with adequate drainage, special management for salinity control may be required.|
|3.0 - 5.0||4 - Very high salinity - suitable for use only on high salt tolerant crops. For use soils must be permeable, free draining, and water must be applied in excess to provide considerable leaching.|
|5.0 - 8.0||5 - Extremely high salinity generally unsuitable for irrigation unless soils are permeable, well drained and crops are of very high salt tolerance.|
|> 8.0||6 - Too saline for irrigation|
Sodicity (sodium levels)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 (see bottom of our irrigation water report), 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.
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.
|Sodium adsorption ratio (SAR from our water report)||Residual alkali||Sodicity class|
|Less than 3||Less than 1.25||No sodium problem|
|3 to 6||Less than 1.25||1. Low sodium, few problems except with sodium sensitive crops.|
|6 to 8||Less than 2.5||2. Medium sodium, increasing problems; use gypsum and not sodium sensitive crops.|
|8 to 14||Less than 2.5||3. High sodium - not generally recommended.|
|Greater than 14||Disregard||4. Very high sodium - unsuitable.|
|Less than 6||1.25 - 2.5||5. Medium R.A. - as for class 2.|
|Less than 14||2.5 - 5||6. High R.A. - as for class 3.|
|Less than 14||Greater than 5||7. Very high R.A. - unsuitable.|
ToxicityToxicity 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 -)
(from our water report)
|Suitability for irrigation|
|Less than 175 ppm||1. Suitable all crops except those highly sensitive to salt.|
|175 - 350 ppm||2. Suitable for crops except those moderately sensitive to salt.|
|350 - 700 ppm||3. Suitable for high and medium salt tolerance crops.|
|700 - 1300 ppm||4. Suitable for high salt tolerant crops only.|
|Greater than 1300 ppm||5. Too saline for irrigation of any crops.|
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.
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 local 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.
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 ppm 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.
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 ppm or SAR greater than 3.0.