Conductivity in water and soil.

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Conductivity in water and soil.

How are salts in water and soils measured? A quick way is to use a conductivity meter and read off the electrical conductivity. The idea being that a salty solution, because it is full of charged particles will conduct electricity. Most conductivity meters give readings in micro Siemens per cm (ÁS/cm). Conductivity is made up of different salts. Some are plant nutrients like potassium, calcium and magnesium. Others are less desirable salts like sodium and chloride. Other plant nutrients like phosphates and nitrates may be present. They are usually low in agricultural soils but can be at high concentrations in hydroponics solutions. By using a conductivity tester there is no way to measure the individual salts. Still its a useful measurment to take because different plants and plants at different stages of growth can only tolerate certain salt levels.

If you live in an area where there is salt in groundwater or soil or if you are using salty irrigation water then you should measure salts in both soil and water to better understand how to manage the salts. The amount of salt that accumulates in the soil depends on several factors. How much salt is in irrigation water, amount of leaching by natural rainwater or by irrigation water, amount of evapouration and the structure of the soil including water holding capacity.

In hydroponics solutions typical conductivities can be in the 3000 - 3500 microS/cm range. Normally this salt level in a normal soil would be too high for most plants but plants seem to do well in hydoponics solutions because most of the salts are nutrients. In soilless growing media conductivities can be in the range 2500 - 300 microS/cm. In natural, agricultural soils, conductivities in the actual soil solution should usually be no higher than 1000 microS/cm. At Apps Laboratories we measured salinity (as conductivity) in the soil solution at a vineyard in central Victoria in Autmun 2009. In some places the soil was very dry and salinities had gone very high - several thousand microS/cm. We also found that the irrigation water being used had high salinity - around the 2900 microS/cm range. It was likely that salinity was the most significant factor affecting vine growth. If we had just looked at nutrients we sould have missed the issue of what was causing poor growth of the vines.

The conductivity of a soil means conductivity of the soil water or soil solution. If the soil is not irrigated or there is not too much sodium and chloride in irrigation water or the soil then a conductivity measurement can give a rough indication of nutrients - like a fertility index. In soils and irrigation water there is quite often but not always similar amounts of sodium and chloride. Sodium is hard to measure directly without complicated equipment. However chloride can be tested more easily on location with a test kit. Chloride levels are often directly related to overall salt levels but just to make sure tolerance limits for chloride a set separately for plants and crops.

Salt buildup in soil has two undesirable effects. The overall salt level stresses the plants and if there is a high proportion of sodium in irrigation water then some of that sodium will displace more desirable nutrients like calcium and magnesium from the soil. In soils one of the key things to consider is the proportion of cations like sodium, potassium, calcium and magnesium held in the soil - that is, bound to soil or humus particles. Getting those salts off the soil particles to measure them requires mixing the soil with chemical extractants like dilute acids or other more complex extractants. The balance of sodium to calcium plus magnesium is found using a slightly tricky calculation and is called the sodium adsorption ratio or SAR. A figure similar to SAR can also be calculated for irrigation water and although it is not strictly applicable to water, at least it gives a good indication of any imbalance between sodium and calcium / magnesium. In other words it indicates if there is a 'sodium hazard'. Measuring calcium and magnesium in water is possible for ordinary folks but then you need a sodium value. Therefore for those tests a sample is best sent to the lab.

Measuring salts that are dissolved in the soil solution can be done with a direct conductivity probe. That is, the probe of a conductivity meter is pushed into the soil. The alternative is to mix the soil with a known volume of water and take a conductivity reading using a conductivity tester suited to testing water. Because many farmers and landowners already own a conductivity tester it is worth knowing how to do this dilution test. For the procedure and calculations please see Testing salts in soil and water

Read more about the different units used to measure salinity and how salinity is influenced by different salts at measuring salinity.

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