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How to Do Soil Testing via SME (Saturated Media Extract)

Updated: Jul 4, 2023

In this brief guide we will cover the following topics:

How to collect the soil sample (+ Best Practices)

To get the most accurate soil test, it is critical to collect representative samples of the field to be analyzed. Since soil composition is highly variable, we recommend collecting different soil samples in a random pattern across the field to have a homogeneous representation.

Best Practices for Soil Sampling:

  • Make sure that all soil samples are collected within the same day

  • Sampling consistency, always take the sample in the same manner

  • Collect at least 10 samples per acre at randomly selected locations

  • Use composite sample: mix samples collected in one composite sample

  • Collect sample at crop roots level.

Soil Sampling Protocol

  1. Using a soil sampling probe, collect the sample at a depth of 6 inches (note that this is a typical root depth, but it can change depending on the crop type. Make sure you verify the roots' depth for your specific crop.).

  2. For each sample, collect around 1 ounce (around 30g.)

  3. Place all the samples collected in a plastic bag.

  4. Once all the soil samples have been collected, mix the soil well inside the plastic bag in order to obtain one composite sample.

What is Saturated Media Extract (SME): definition of SME

The Saturated Media Extract uses water to extract the media sample. When using this method, we do not recommend sieving the sample before analysis.

How to prepare Saturated Media Extract

  1. In a large container, place the composite soil sample (all samples collected on the same lot) and mix to obtain homogeneous media.

  2. Add the soil extractant (we recommend adding 5 drops per each 5g of soil)

  3. Add deionized water until water just barely stands on the surface

  4. Then mix the soil media with the water for 1 minute and let the mixture rest for at least 5 minutes

  5. Extract the soil solution by using the lysimeter (Soil Solution Access Tube) or by filtering the mixture with the filter paper. (instructions below)

The filtrate recovered is now ready for analysis!

NB: When using the Saturated Media Extract method, your results do not need to be corrected by any dilution factor.

Should you filter SME paste? (+ instructions on filtering the paste)

The Saturated Media Extract samples can be measured directly (without filtration) on HORIBA LAQUAtwin meters, but samples need to be filtrated before measuring Phosphorus with HANNA HI-706 Checker. Also, for more accurate analysis, sample filtration is also recommended to measure the ions (Nitrate, Potassium, Calcium and Sodium) with HORIBA LAQUAtwin meters.

To perform a filtration use a lysimeter (Irrometer SSAT) that comes with the Sampling Expert kit or you can also purchase it separately.

Remember that a lysimeter (SSAT) should be soaked for at least 6 hours in water prior to the first use.
  1. Connect the vacuum syringe to the inner tube 

  2. Open the finger clamp 

  3. Pump with the syringe and close the finger clamp 

  4. Disconnect the syringe and chase the air 

  5. Repeat these steps two more times to have enough vacuum / negative pressure

  6. Wait for 10 minutes for soil solution to be absorbed through the ceramic tip

  7. Collect the soil solution accumulated in the tube, open the clamp

  8. Connect the syringe and extract the solution.

The soil solution can either be extracted from the SME preparation using a lysimeter and a vacuum syringe or it can be filtrated using the funnel, beaker and filter included in the Sampling Expert kit.

​Reference for Nutrient Concentration in Saturated Media Extract (SME)

I have prepared a short overview of key nutrient (NPK and Calcium) concentrations in SME in the table above. Of course, the values could change slightly according to your type of crop.

If you want to consult comprehensive reference tables for various crops, then consider using our NutriCheck guide that comes with any of our Agronomist Kits. There is a huge advantage in comparing the testers' results with specific data relevant for your crop: you can see if the nutrients available in the soil correspond to your crop instant need.

The Saturated Media Extract - is it the right soil testing method for you?

The Saturated Media Extract is the easiest way to prepare soil solution, as it needs less equipment when compared to other methods. This technique is also the preferred technique for the soilless medium such as coconut coir. This method works for the vast majority of growers for its simplicity.

The SME compensates the possible fluctuation of the soil moisture, as the saturation point is the same regardless of the initial sample being wet or dry. The soil paste obtained by this method can be measured directly with the pH and conductivity meters, however if you wish to measure the nutrients in soil, we highly recommend filtrating the paste before the measurement.

If you have any questions on the procedure, please feel free to contact me and I'll be happy to help.



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