Solvita CO2 compared to Infrared CO2 IR gives surprising uniformity in results with a virtually 1:1 relationship over a range of 0 – 125 mg kg. But miniaturized CO2 tests did not.

Solvita is a leading commercial test worldwide for soil CO2 respiration (and compost maturity) and is often in comparisons with newer soil respiration methods vying for market share.  Such is the case in a recent study by Oklahoma State University which proposed a semi-automatic Gas Chromatography (GC) method for soil respiration. The team, under Jason Warren, Associate Professor and Soil Management Extension Specialist, selected soils from 9-regions representing paired-farms with long term No-Till or conventional management.  They ran Solvita and their proposed method and samples were shared with Woods End Laboratory and were split with the University of Maine for basal and CO2-Burst modes of respiration by IR and Solvita. While OK State did report a very high correlation between GC  and Solvita (CSA News August 2018), there were differences in magnitudes that are unexplained. Using a GC should not alone grant imprimatur that the results are absolutely correct. Therefore Woods End and the University of Maine hypothesized that other protocols run independently (IR-infrared and Solvita) should give similar results. So far, Woods End Lab and the University of Maine obtained virtually identical results revealing that their methods are capable of giving nearly identical quantitation.  The team is now zeroing in on what may have caused the differences with the GC approach. A crucial clue appears to be that OK State reduced the soil quantity from 40 g down to 5 g to fit into the GC auto-sampler tubes.

Soil respiration experience in the past has suggested that small soil samples (<10 g) in microbial response are problematic. Could this explain the differences between OK State and Solvita (and other micro-sample methods being proposed)?  GC is not necessarily a problem per se. Early favorable comparisons of Solvita with GC have been made using fairly large samples by John Doran’s team at University of Nebraska/ARS (1996). And regarding CO2 detectability, early studies by Dr. Bern at Iowa State University (2006) and Dr. Stroshine at Purdue University (2008) confirmed Solvita gel chemistry is capable of a strong linear relationship to standardized CO2 over a range of 0 – 3% CO2 – translating into a linearity of about 0 – 140 ppm in soil samples at the given method.  The Woods End and UME team agree that increasing the jar size to 475 cc (1 pt) reduces discrepancies between methods and have published a CSA- News Letter to Editor discussing this and other factors.

Current trends of (unnecessary) infighting over soil CO2 methodology for soil health are likely muddying the waters for soil respiration in the short term –  a soil health community problem. If soil CO2 respiration issues and equivalency of methods can’t be resolved, then the testing industry may go back to quick chemistry as ersatz biology  – which would be unfortunate. That’s because the more we learn, the more we find it is the living substructure of soils and plants and interactions between them and microbes that is sustaining and driving soil qualities.

Postscript: The Woods End team ran ANOVA on all the results from all three labs for the Oklahoma soils series study. Regardless of between-lab variation, all separated the improved No-Till from conventional management at a very high degree of statistical certainty.