Conductivity refers to the electrical conductivity of a solution, or in other words it is a measure of the electric current generated by charged ions in solution. These charged ions originate from all forms of salt* - not just table salt (sodium chloride - NaCl). Plants tend to take nutrients up in an ionic form, so soil conductivity of a soil solution provides clues to the nutrient content of a soil. Note: it does not indicate which nutrients are present, nor measure 'locked up' or un-ionised nutrients in soil.
What Does High or Low Conductivity Mean?
High conductivity indicates high concentrations of ions/nutrients, therefore high soil salt content. High ion concentrations interfer with plant and microbe growth by upsetting ionic/nutrient pathways via 1) reversing osmosis, 2) disrupting cellular ionic pumps, and/or 3) increasing the probability of nutrient substitution.
Low conductivity is a sign of low ionic activity therefore low soil nutrient content, ie. the soil is 'empty'. The addition of a fertiliser, particularly a high soluble (salt) fertiliser, can increase conductivity rapidly.
As a tool, conductivity alerts you to the salt load and therefore the potential quantity of 'nutrient' available in a soil, but it cannot tell you the quality or profile of those nutrients. Plants and microbes need different nutrients at different times at different rates and ratios. An excess or one or more ions/nutrients can be toxic, e.g. sodium (Na+) and/or chloride (Cl-), whereas missing ions result in mineral deficiencies and poor growth.
How Do You Measure Soil Conductivity?
There is a variety of DIY conductivity meters on the market that will measure soil conductivity. Most measure a solution made up of 1 part soil:1 part water - ideally distilled water to ensure no 'external' ionic contamination.
Commercial soil testing laboratories use a variety of methods which do not necessarily give the same result. EC1:5 (ie using a dilution of 1 part soil:5 parts water) is the most typical. EC1:2 is similar but tends to give a higher EC reading. Then there is ECs which measures conductivity at field capacity/saturation and ECse that measures conductivity of soil saturation extract - both depend on soil type, but can be converted to a water uptake weighted root zone EC which gives an indication of likely plant salt tolerance levels. Take note of what methodology is used before trying to compare it against another test - you may not be comparing apples with apples.
Conductivity is commonly measured using Siemens (S) or Mhos* (these terms are interchangable), though ergs has also be used.
It is generally agreed that soils with conductivity readings above 4 dS/m are saline, ie. salty. Note: this is not always due to sodium chloride; other salts can be culprits, especially accumulations of salt fertiliser residues that may have little or no sodium or chloride content. Sodic soils, while they are certainly saline, are distinghished by specifically high sodium, but low chloride levels.
Soil Conductivity Levels
Non-Saline Low Moderate High Severe
0-60cm 0-2 2-4 4-8 8-16 >16 dS/m
60-120cm 0-4 4-8 8-16 16-24 >24 dS/m
Non-Saline - no problems
Low - sensitive plants affected
Moderate - patchy, many legumes, perennial grasses, oats and wheat affected
High - only tolerant plants, trees dying
Severe - salt crystals evident, bare earth, dead trees
Plant Conductivity Tolerances (see Plant Salinity Tolerances)
* A salt is any substance which can dissassociate/dissolve into its cation and anion constituents in solution.
** Mhos is a measure of conductance, the inverse of resistance which is measured in Ohms (Note: Mho is the reverse spelling of Ohm).
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