Deciphering a water analysis


Precision agriculture involves conducting soil, water and plant analyzes. Only in this way will we be able to squeeze the fertilizer supply and the irrigation that we have to do to the maximum. In the long run it is good for our pockets. As our goal is to spread the word about agriculture we can, today we begin this line of articles by “deciphering” a water analysis .


An irrigation water analysis will give us many interesting parameters. One of the most important is its mineral content, its pH, the salts it houses and its conductivity. However, not everything stays in these data, there are also others that, knowing how to understand them, are important to try to change some cultural tasks that harm the soil.

For example, regarding pH we have commented at length in Gardenprue, although we have studied it from the point of view of plants and soil:

The ph of plants

The behavior of nutrients according to pH

How to change soil pH

This last article is closely linked to the pH of the water, since it is the one that, in the long run, can modify the pH of a soil, no matter how much buffer it has.

If the pH of the water is basic, over time, the pH of the soil will be too. It is useless to make additions of iron sulfate, acid peat or others if we will continue in the future adding alkaline water to the soil.


Although there are laboratories better than others, erring is human, and many times we can be surprised. There are some parameters that we, at the foot of the field, cannot know with certainty if they have been very calculating, but others have.

The cations (sodium, potassium, calcium and magnesium) and anions (carbonates, bicarbonates, nitrates, sulfates and chlorides) that an analysis of irrigation water shows , must be equated. That is, the sum of cations and anions must equal 0, or at least with an error of less than 10%. Other times only 5% is accepted, when you want to be more precise.

A difference of more than 10% between anions and cations would mean requesting a new analysis in another laboratory (or having it repeated by the same).

Another way to check if the electrical conductivity is between adequate values ​​is to see that the EC ( divided by the sum of the values ​​of anions and cations (meq · L) is equivalent to 100, without exceeding the values of 110 and 80.

For example: let’s imagine that the conductivity in the water analysis gives us 2,650 (us · cm-1), and the sum of cations and anions (meq · L) gives us 26. If we divide these values ​​we obtain a result of 101.92 , a number quite close to 100. We can take this electrical conductivity as valid .


Most water analyzes give a series of introductory parameters (before moving on to the volume of anions and cations in the solution). These are usually the following:

  • pH: the acidity or alkalinity of the water, measured in the concentration of hydronium ions [H 3 O] +.
  • Conductivity (CE):  the ease with which the electric current can pass through said solution. Higher salt content, higher conductivity. Very interesting to control the fertilizers that can be added to the soil without causing damage to the crops.
  •  SAR:  the sodium adsorption ratio, that is, the ratio between sodium, calcium and magnesium. Sometimes the corrected SAR also appears.
  • Scott index  an index to know the height of the water when it evaporates, to analyze the amount of salts that remain in the soil.
  • Hardness:  the amount of the calcium cation present in the soil, measured in French degrees (ºF).

These, a priori, are the data that any analysis of normal irrigation water could give us, although there are some more precise ones where they offer us other interesting parameters, such as:

  • EATON Residual Sodium Carbonate (CSR) Index
  • Kelly Index
  • Langellier index


Important, when it comes to understanding a soil analysis , is to know between what values ​​we can move or what values ​​are considered normal. It is useless to be able to see on the sheet that the laboratory sends us that the water with which we irrigate has a Scott index (IS) of 4.

Is this a lot or a little?

Let’s see what Urbano Lump says (1995), in Stabler’s classification:

  • IS> 18: good water. No precautions of any kind are necessary.
  • 6 <IS <18: tolerable water. Certain precautions must be employed.
  • 1,2 <IS <6: dangerous water. Use only on soils with good drainage conditions.
  • IS <1,2: water not usable.

According to the value that we have commented previously of the Scott index, 4, we see that we could consider this water as dangerous. We will have to pay special attention to the drainage of our soil, and improve it, if necessary.

How to improve the drainage of a soil.

Solutions for compacted soil.



We can see it in this table:

  • EC <0.7: no problem. [(millimhos per centimeter)]
  • 0.7 <EC <3: growing problem [(millimhos per centimeter)]
  • CE> 3: serious problem. [(millimeters per centimeter)]

Also tell you that in an article on electrical conductivity , there is a table where you see the reduction in crop yield according to the accumulation of salts.


We have said that the SAR was the sodium adsorption ratio , but that to many people we may not be saying anything. We want mean values ​​to compare with our analysis! Okay!

This is closely related to soil permeability problems, where a high SAR value means that we are promoting a loss of soil structure.

Also, in many analyzes we see how a second SAR value appears, that is, the corrected one. This value takes into account the calcium precipitated in bicarbonates , a very real situation in our soil. We will pay more attention to this value.

Let’s see the SAR values, to compare them with ours:

  • 0 <SAR <10: low
  • 10 <SAR <18: medium
  • 18<SAR<26: alto
  • SAR> 26: very high


Many times we see how some of our appliances (washing machine, iron, coffee makers, etc.) deteriorate due to the accumulation of lime in some of their parts. This is closely linked to the hardness of the water , and is related to the calcium cation .

Let’s see a table to be able to compare it with our water analysis :

  • Very soft waters: <7ºF
  • Soft water: 7> º F <14
  • Medium soft waters: 14 <º F <22
  • Medium hard waters: 22 <º F <32
  • Hard waters: 32 <º F <54
  • Very hard waters: º F> 54

And well, although there are more ways to interpret the quality of the water from an analysis, these are more than enough to make corrections in the fertilizer, improve our soil or find other means to irrigate.

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