General concepts EC

In order to develop their life circle, plants need a series of chemical elements that are called "nutritious elements". All these elements are equally important but plants need some of these elements in large quantities and these are called macro elements, while others are needed in small quantities and are called microelements or oligoelements.

All nutritious elements, which are essential for plants, are taken or assimilated in the form of ions. These ions can be positive (cations) or negative (anions).

The ways in which the concentrations of the ions can be expressed or measured in irrigation water are:

• Mol
• Milimol
• Miliequivalent
• Parts per million

Calculating the final conductivity of the nutritive solution

To calculate the conductivity of the nutritive solution that has to be prepared, there are various methods: 

The ion salt content method
This consists of expressing all the concentrations of the ions in ppm, and multiplying them by correction factors. Once the correct value for each ion has been obtained, all the values are added together and the result of this calculation will give the value of the conductivity of the nutritive solution expressed in microSiemens/cm..

Method of equivalents
In each saline solution it is necessary to calculate the sum of the number of miliequivalents of anions or the total number of miliequivalents of cations. This method consists of dividing the total number of miliequivalents of anions or the total number of miliequivalents of cations between a factor that varies between 10 and 12. The result is expressed in microSiemens/cm. The factor 10 should be chosen for low conductivities and 12 for high conductivities.

Gravimetric Method
It is similar to the first method. The total salt concentration in the solution is expressed in grams per litre and divided by a global and variable factor of between 0.7 and 0.9. The result is expressed in microsiemens/cm and 0.7 is the value selected for low conductivities and 0.9 for high ones..

Salinity in water

The salinity in water is an important factor for all types of vegetation. By "salinity" we mean "the quantity of salt dissolved in a litre of water".

From the measure of conductivity we can pass onto the dissolved salt content expressed in gr/litre and vice versa. The following sections express different ranges of conductivity and its applications.

0 to 750 mS/cm. Water with this range of conductivity has a low level of salt content. It can be used for any crop and on any type of land. Soil with low permeability, may occasionally requires irrigation to dissolve the deposited salt.

750 to 2250 mS/cm. Water of medium salinity. It can be used for watering ground that is sufficiently permeable and crops, which are on average tolerant of salinity. In areas of very low rainfall, it is necessary to carry out regular controls of the earth and the irrigation water in order to avoid salt deposits.

More than 5000 mS/cm. This water is not suitable for irrigation. It can be used occasionally on extremely permeable soil and if the crop is very tolerant of high levels of salt. This level of salinity in irrigation water requires constant control of the salinity in the soil and the adoption of special farming practices.