Using conductivity meters in agriculture
Ian Walker, Department of Primary Industries and Fisheries
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Introduction
Electrical conductivity (usually referred to as EC) is an extremely useful and easy measure to use for monitoring agricultural waters, soils and potting mixes. It indicates the level of dissolved salts by measuring the ability of a solution to carry an electric current by ions. A high electrical conductivity will stress your plants and cause productivity losses. It may produce leaf tip and margin burns and in extreme cases wilting, collapse and death of the plant. The salts responsible for this may come from irrigation waters, soils, potting media, rising water tables or from fertilisers. They are mainly sodium, magnesium, calcium, chloride, sulphate and bicarbonate.
EC will not, however, tell you which salts are contributing to the reading. This is a matter for your intuition unless detailed laboratory analysis is undertaken. If your groundwaters are known to have a sodium chloride (sea salt) problem and you have a high conductivity reading, then most of it is probably due to sea salt. If you have a potting mix with a high conductivity and you know you have good quality water, then you would suspect that excess fertiliser or poor media components are the culprit.
This DPI&F Note is intended to guide you through the proper use of conductivity meters. For interpretation of results consult one of the references listed at the end of this document. Keep in mind that different irrigation methods allow the use of different quality waters. Trickle irrigation, for instance, tends to push salts away from the root zone which allows you to use irrigation waters with higher conductivities than if you used furrow irrigation which tends to push salts into the root zone. If the water you have is too saline, it is possible to mix it with less saline water to obtain suitable water for irrigation.
Units
The preferred unit of EC is deciSiemens per metre - dS/m or dS m-1. Other units are microSiemens per centimetre (abbreviated m S/cm or m S cm-1) and milliSiemens per centimetre (mS/cm or mS cm-1). Conversions between the units are listed below.
A Siemen is an inverse ohm (also called a mho), a measure of electrical resistance. The distance term (centimetre or metre) comes from the way conductivity is measured. Classically this is done between two platinum electrodes of known surface area and a known distance apart.
-
1 dS/m = 1 mS/cm
-
1 dS/m = 1 000 m S/cm
-
1 000 m S/cm = 1 mS/cm
The EC of distilled or deionised water is approximately 0.002 dS/m because most of the salts have been removed. The EC of seawater is approximately 58 dS/m.
Temperature effect
EC readings are affected by temperature and ion interactions which change with concentration of salts. Because conductivity changes by about 2% per degree Celsius (see table below), readings are adjusted to a standard temperature, 25 ° C. Instruments will do this automatically but it is a good idea not to have your water samples either very hot or very cold. Don't leave your sample in a bottle on a car dashboard as they get extremely hot.
Conductivity for a dilute potassium chloride (KCl) solution at various temperatures in dS/m
|
Temp (° C) |
0.01 M KCl |
|
5 |
0.896 |
|
10 |
1.020 |
|
15 |
1.147 |
|
16 |
1.173 |
|
17 |
1.199 |
|
18 |
1.225 |
|
19 |
1.251 |
|
20 |
1.278 |
|
21 |
1.305 |
|
22 |
1.332 |
|
23 |
1.359 |
|
24 |
1.386 |
|
25 |
1.413 |
|
26 |
1.437 |
|
27 |
1.462 |
|
28 |
1.488 |
|
29 |
1.513 |
|
30 |
1.54 |
Additionally, conductivity does not change linearly with concentration e.g. a 0.01M solution of KCl has a conductivity of 1.413 dS/m whereas a solution of 0.02 M (double the concentration) has a conductivity of 2.76 dS/m which is not double the EC. This is due to interaction between the salts in solution and is an important point to remember.
Other measures of salt concentration
A common unit of measurement of salt concentration called Total Dissolved Salts (TDS) is parts per million (ppm) or more correctly, milligrams per litre (mg/L or mg L-1). This is a measure of how much salt is in the solution by weight obtained by evaporation. This is different from conductivity which gives a measure of how much electricity the solution will conduct as a result of all the salts present.
Conversions from EC to TDS can be made using an approximation. If you want to relate your new EC readings back to previous readings in ppm, then you can multiply your dS/cm reading by 550 to 900. All of these conversions are quoted in various sources.
Conversion factors
|
To convert from: |
To: |
Multiply by: |
|
m S/cm |
mS/cm |
0.001 (or divide by 1000) |
|
mS/cm |
m S/cm |
1000 |
|
m S/cm |
dS/m |
0.001 (or divide by 1000) |
|
dS/m |
m S/cm |
1000 |
|
mS/cm |
dS/m |
1 (i.e. they are the same) |
|
m S/cm |
ppm or mg/L |
0.6 (approximately) |
|
mS/cm |
ppm or mg/L |
600 (approximately) |
|
dS/m |
ppm or mg/L |
600 (approximately) |
Conductivity meters
There are basically two types of meter commonly available - small "stick" types and larger, more complex and more accurate bench top and portable models. A stick type will cost about $100, a portable around $700 including electrode and laboratory bench top models are about $1500 including electrode.
Stick types, e.g. the TDScan4, are quite inexpensive and fit in your pocket and as such are quite useful. It should be remembered that these are not as accurate as the larger models and tend to irretrievably break down after a season or two depending on use. There are also more expensive waterproof versions that can last much longer. The digital reading on these is usually in dS/m thus a reading of 1.4 is 1.4 dS/m or 1400 m S/cm. Because they only have one decimal place it means that each step is 0.1 dS/m (100 m S/cm) which makes for reasonably coarse readings and less than perfect calibrations. There is usually a small screw on the meter that is used to calibrate it (see below).
Larger models often have a number of functions (often combined with a pH meter). They usually give readings to 0.001 dS/m (1 m S/cm) and have a more convenient knob to calibrate them and are more accurate. The electrode is separate and plugs into the instrument.
Electrodes can be the classic two separated platinum plates (these are black as the plates are given a layer of platinum black) with a separate reference electrode or other arrangements that are often round and encased in a protective PVC sheath. These latter models are meant for environmental monitoring and are built to withstand field conditions. Both models cost about $270.
Electrodes come with various cell constants (written as "K"). Different cell constants are suitable for measuring different strength solutions. For agricultural measurements K=1 is suitable. This will be written on the electrode or the cable.
Calibrating the instrument
It is very important that you calibrate your instrument every time you use it otherwise you will get inaccurate readings. Calibration means reading a solution of known conductivity and adjusting your meter to read the same. The adjustment is done with a screw or knob. The commonly used standard solution is potassium chloride (KCl). A solution of 0.01 M gives a conductivity of 1.413 dS/m and a 0.02M solution gives 2.76 dS/m. It is good practice to calibrate using a solution that approximates the readings you expect. Agricultural applications are usually in the 0.5 to 4.0 dS/m range and both 1.413 and 2.76 dS/m would be suitable. After calibration, never return the standard to the bottle as it will contaminate the standard. Discard it.
These solutions can be bought from scientific supply companies but tend to be quite expensive. A cheaper method, if you have access to an accurate balance and volumetric glassware, is to make up a stock solution of 1M KCl and then use that to make up your calibration standard a litre at a time using a 1:100 dilution (e.g. use a 10 mL pipette and a 1 L volumetric flask). It is important to ensure that the solutions are kept airtight to avoid evaporation as this will concentrate your salts and so change the conductivity. Provided the solutions are tightly sealed, kept from extremes of temperature and out of the light (e.g. in cupboard) then they will last for years.
To make up a stock solution of 1M KCl use 74.555 g of dry reagent grade KCl and make up to 1 litre with distilled water. Keep this as your stock and do 1:100 (0.01 M KCl) for 1.413 dS/m or 1:50 (0.02 M KCl) dilutions for 2.76 dS/m standard solutions.
Measuring the conductivity
Taking a conductivity reading is easy, simply immerse the electrode in the solution, jiggle the electrode up and down a few times to remove any bubbles. Wait thirty seconds or so for the sample and electrode to come to the same temperature and then read the measurement. The instrument will do the temperature correction for you. The electrode should be immersed over the plates and reference electrode or up to the hole in the protective PVC sheath. With the stick types, immerse the pins to the level indicated in the instructions. It is most important that the level of the liquid is not above the waterproofed section of the electrode or meter.
If you are making a series of measurements, you should rinse the electrode with distilled water and dab dry with a tissue between samples (without rubbing the electrode itself).
Ensure that the battery is in good condition for portable types. Most modern instruments will give a battery warning if it is low. If you suspect it is low, then replace it.
Remember:
-
Calibrate your meter (see above)
- Ensure that the sample you have is truly representative of what you want to measure (see below).
Storing the electrode
After making your last measurement, rinse with distilled water and dab dry and then store it according to the manufacturers instructions. This may be storing in air or in a solution. Ensure that it is out of strong light and away from temperature extremes and dusts. Remember that fertilisers are very corrosive so keep all instrumentation protected from them. The stick types usually have a protective cover for the electrodes.
Maintenance of electrodes
Maintenance is only possible on the separated plate type electrodes. As electrodes age or if they are not rinsed thoroughly after each reading they will eventually get a build up of dirt and scale on them and this will result in inaccurate readings, readings that drift or very low readings. Never attempt to touch, scrub or rub that electrodes as this will remove the platinum coatings. If you suspect that you may have contaminated your electrode with fats or oils, then these can be removed by soaking in methylated spirits for 10 to 15 minutes, rinse in distilled water and recalibrate. Scales (accumulated salts) can be removed by dipping briefly in a solution of one part concentrated hydrochloric acid and ten parts distilled water and then rinse thoroughly with distilled water and recalibrate. If neither of these procedures rectify your problem then your electrode may need replatinisation. This is done at the factory and costs about $30, far cheaper than the $270 replacement cost of the electrode.
Sample preparation
The essential thing to remember when preparing a sample is that it be representative of what you are trying to measure.
Water samples
If you are taking a sample from a bore ensure that you run it for at least half an hour beforehand to get a true indication of the water you will be using. This is particularly important for bores that have remained unused for a long time. Be careful not to contaminate your sample with the previous contents of the bottle you are using to take the sample. Wash it thoroughly before using and rinse the bottle three times with the water you are sampling before taking the final sample. Avoid temperature extremes before reading your sample. When sampling running waters sample from a place where the water is flowing, not a backwater. When sampling still surface waters, e.g. dams, lakes and swamps, it is important to remember that as saline water is denser than fresh, these waters often stratify with the denser saline water on the bottom and fresh at the top. Sample from the level where your intake is. This is also the case for tidal creeks and rivers.
Soil and potting media samples
Soil and potting media should be collected from a number of sites and depths throughout the sampling area and these should then be well mixed and sub-sampled to obtain a representative sample to test. The standard for soil samples is 1:5 w/v (weight for volume) soil to water. Take a weight of soil and add to it five times that volume of distilled or deionised water. A very handy size bottle is a 455 g Vegemite bottle using 60 g of dry soil and 300 mL of water in this. These bottles have nice broad mouths and good screw-top plastic lids. Cap your bottle and shake it 50 times, allow to stand for 15 minutes then shake 50 times again and allow to settle for another 15 minutes before reading. Place the electrode near the boundary of the water and settled soil.
Potting mixes are done the same way but use one volume of mix and one and a half volumes of water e.g. 100 mL of damp mix gently tamped down and 150 mL of water.
Remember that when comparing results they must have been obtained using the same extraction method.
Further information
Your first information source should be any manuals you have with your equipment. These will give the most relevant and specific information on your own particular instrument. Other good references are:
Handreck, K. & N.Black Growing Media for Ornamental Plants and Turf New South Wales University Press This Australian classic has been through several editions and has excellent information on theoretical and practical aspects of conductivity and many other aspects of agriculture.
Bodman, K. & K.V.Sharman (Editors) Container Media Management DPI 1993 This book is sold by the Queensland Nursery Industry Association and gives a very practical approach the subject.
Lorenz, O.A & D.N.Maynard Knott's Handbook for Vegetable Growers 3rd Edition John Wiley & Sons, New York. This American book is an invaluable source of facts and figures on all aspects of vegetable production and gives a table of salinity tolerances of various vegetables.
NRM Water Facts Irrigation Water Quality - Salinity & Soil Structure Stability This fact sheet is produced by the Department of Natural Resources and Mines, Queensland and is available on the DPI&F Prime Notes CD, on the Internet at www.nrm.qld.gov.au or as an NR&M free faxback on 1800 240 691. It contains an extensive list of fruit, vegetable, field crop, pasture and ornamental species and their tolerance to salinity in irrigation water as measured by conductivity in dS/m.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last updated 10 February 2004
