Hi, expertise and researchers!
I’m currently working on analyzing exchangeable cations and cation exchange capacity (CEC) of soil samples as part of my study. I’d really appreciate some guidance and insight, especially as I’m still learning and navigating the methods.
To quantify soil health, I’m also applying the Tropical Soil Quality Index (TSQI) and examining the relationship between soil texture and CEC as in the attached files. The soil texture in my study area is predominantly loam, which would typically suggest moderate to high CEC.
For the analysis, I followed the standard procedure:
- Extraction using 1M NH₄OAc (pH 7.0)
- Measurement using ICP-OES
- Dilution of 1ml aliquot to 50 mL before analysis
- Included blanks and standard concentrations (2, 4, and 8 ppm) for calibration
From what I’ve learned, when a sample contains characters like “o” or “u”, it must be adjusted or standardized to match the concentration range of the method used for ICP detection.
However, my results are puzzling. Many studies report exchangeable cation concentrations in the range of 100+ ppm, resulting in reasonable values when converted to meq/100g or cmol(+) /kg. In contrast, my values are all below 10 ppm, and when converted, they yield extremely low CEC values — averaging around 0.5 meq/100g, which seems inconsistent with the loamy texture of the soil.
I’m wondering:
- Am I missing a step or misapplying the conversion?
- Could there be an issue with the dilution or detection range?
- Is it normal for ICP results from NH₄OAc extracts to show such low ppm in undisturbed forest loam soils?
Any advice, feedback, or experience shared would be truly appreciated. Thank you in advance!
References:
Article Proposing a suitable soil quality index for natural, seconda...
https://ohioline.osu.edu/factsheet/anr-81
Your unusually low exchangeable cation and CEC values likely result from errors in dilution calculations, ICP calibration range, or unit conversions rather than the soil itself. Double-check these steps and validate with standards to obtain accurate, expected results for loam soils.
@Abdelhak Maghchice
Understood. Regarding the ICP calibration range, I was able to set it correctly, as all of my sample concentrations fall within the standard range of 2–8 ppm.
However, I did dealt a lot with unit conversions, particularly from ppm to mg/kg and then from mg/kg to meq/100g. Upon consulting with a few lecturers, I was advised that converting ppm to mg/kg is necessary when dealing with soil samples. The issue was partially resolved using Samsuri's formula for conversion to mg/kg (Ref: https://www.researchgate.net/post/Converting_mg_l_to_mg_kg/1). The calculated concentration (mg/kg)
then apply in converting to meq/100g to find exchangeable cation and CEC. The results provided slightly lower but more reasonable results compared to earlier attempts.
Some of my friends also suggested to multiply the concentration value (ppm) with dilution factor first before using the ppm to mg/kg formula. After doing so, I managed to obtain values for exchangeable cations and CEC that are within an acceptable range, as shown in the attached image.
Somehow, we are still uncertain whether this approach is entirely acceptable, as it is try and error process. We also concern since the ICP results were already based on diluted solutions.
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