Why are the conductivity measurements from my Pulse Meter different from my normal extraction tests?

With any extraction method (e.g. using 1:1.5 method, 1:2 method, SME, pour through) you add water to the media to extract the conducting ions from the soil then measure the resulting solution. The media is generally shaken or stirred in some way to help speed this up, then may or may not be filtered.

The first source of variation between different common extraction methods is the amount of water used. For example, a 1:1.5 test will use say 100 ml of soil to 150 ml of water, while a 1:2 (also very common) will use 100 ml of soil to 200 ml of water. As different amounts of water are used the EC will be diluted by a different amount.

The next source of variation is the agitation method, shaking or stirring, how vigorously, for how long etc. Changing any of these will vary the number of conductive ions you extract from a given soil sample.

This means that even when using two laboratories to do tests for the same soil sample you may well get different conductivity readings. Neither are wrong, just the measurement method was different.

For this reason, it is key that when you decide on a test method or lab you stick to it and if you are going to change, do some comparative measurements to understand the differences and how they track. It is likely that it is as simple as lab A give 0.5 EC higher than lab B.

This is also true for any direct measurement method such as the Pulse. The results you will get are accurate, but they were created in a different way to an extract method and have different factors, such as moisture content and temperature, that affect them. As such, if you do currently test regularly and know what you are looking for in terms of a conductivity measurement, take some measures with Pulse, then test the same samples with your current method to understand what the difference is and how they compare. Direct measures of Bulk conductivity tend to be lower than most extraction methods.

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