How to interpret water analysis for irrigation

How to interpret water analysis for irrigation

The reading of an analysis of water for irrigation and therefore of its bulletin is an activity that needs particular professional skills (belonging to the Agronomists and Forest Doctors) and that allows us to understand, sometimes in a preventive way, the suitability for use of that water and compatibility with crops and soil.
The following is a bulletin of analysis, to understand how to interpret water analysis for irrigation, with some of the most important determinations in order to understand the most salient features of the water. In this case the following parameters and values ​​were determined: pH, Electrical Conductivity, calcium ions, sodium, magnesium, chlorides and sulfates as well as the Total Hardness and Sodium Adsorption Ratio (SAR “Sodium Adsorpion Ratio”). More details or investigations can determine other parameters that may concern potassium, other elements and microelements or parameters that help us to understand possible characteristics of our waters for irrigation.

In the case of our bulletin we are in the presence of water with a subacid pH, with an electrical conductivity in μS of 1,897; this value gives an indication of the electrical conductivity of the water which in turn is linked to the salts dissolved in it. Water for irrigation use should never exceed the indicative value of 800 μS but this parameter can fluctuate a lot depending on the quality of the dissolved salts. Among the dissolved ions, sodium (Na) plays a fundamental role. Waters with high contents of this ion are absolutely not recommended for irrigation use; in this case of our bulletin it is already quite high; it should be borne in mind that the high presence of sodium carries out two negative actions: with regard to the absorption of elements (and microelements) in plants, with the determination of induced chlorosis and with regard to the soil with its destructuring; consider that many of the misdiagnosed landslides of our soils are due to the high contributions of this element both with water and with fertilizers and pesticides. Calcium ions, magnesium, chlorides and sulphates, if not present in high concentrations, are however interesting in their relationship, characterizing the qualities and characteristics of the water with regard to the soil and the absorption of the elements.
Finally, a particular mention should be given to the simple SAR, which is a function relationship between three elements (Ca, Mg and Na), giving us a very immediate indication of the capacity of soils to absorb Sodium compared to Calcium and Magnesium.
The SAR = [CNa] / [√ (CCa + CMg) / 2]
where is it:
(C): Concentration of ions in mol / m3
Na: Sodium
Ca: Football
Mg: Magnesium
If the units are meq / L, the sum CCa + CMg must be divided by half before taking the square root.
SAR values ​​below the unit characterize waters with a better balance of the three elements with regard to the absorption capacity of the soil; gradually increasing SAR value denote a dangerousness of water to “release” sodium into the soil with all the negative phenomena we have mentioned.
In the bulletin in question, a further classification (EC-S) is made, which relates the Electrical Conductivity with the SAR and which classifies the irrigation characteristics of our water.
As usual, the fundamental thing of the analysis reports is that they report a minimum classification of the same in addition to the main quantitative and qualitative parameters.

Guido Bissanti

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