Quantification of soil carbon with the Elementar soli TOC® cube: the impact of sample weight and grain size on precision

The need for soil carbon measurements is increasing as global efforts to combat climate change intensify. Soil health, soil fertility and climate change mitigation are drivers for ongoing analytical testing. Accurate and precise soil carbon data for applications such as carbon credit programs is crucial for transitioning from estimated based approaches to direct measurement technologies. This study evaluates the precision of soil carbon measurements using the Elementar soli TOC® cube, focusing on the critical factors of sample grain size and sample weight.  

 

We analyzed two different soil samples, ALP SRS-1810 and ALP SRS-2308, using the DIN EN 17505 standard for temperature-dependent differentiation of carbon fractions, including TOC, ROC, and TIC, without the need for time-consuming acid pre-treatment. Samples were ground and sieved to various grain sizes, from <2.00 mm down to <0.10 mm, and then analyzed in different weights ranging from 0.025 g to 1.0 g. Our results show a clear trend: precision, as measured by the absolute standard deviation (SD), significantly improves with decreasing grain size and increasing sample weight. However, the data also reveals a sweet spot that balances sample preparation effort with measurement precision. 

 

Based on our findings, a sieve size of 0.25 mm to 0.50 mm combined with a sample weight of 0.1 g to 0.25 g provides a highly efficient and effective working range (Figure 1). This combination delivers very good to excellent precision (relative standard deviation, SD rel.  < 1.80%) while minimizing the labour and time associated with excessive grinding and weighing. The soli TOC® cube's ability to handle this optimal range, along with its automated sample feeder, makes it an ideal instrument for high-throughput laboratory analysis, providing reliable and reproducible soil carbon data that exceeds required precision specifications.