To perform the comparison, soil samples were exposed to Solvita and an IR detector simultaneously. The technicians employed the standard Solvita protocol of approximately 30 g soil moistened to 50% water-filled-pore-space. The correspondence represented by the plot x – y where the regression slope is essentially 1 (see figure) provides strong evidence that Solvita chemistry is virtually comparable to electronic measurement of CO2 performed under similar conditions.
So what causes the variability many persons ascribe to Solvita? Confounding forces were due to a CO2-burst application adopted from ARS-TX, now known to be the cause of over-saturation of soils (a factor reducing respiration and increasing variance). Another identified issue, also due to the ARS lab, was using too much soil during the test and in tiny 1/2 pint jars. Solvita did originally develop a small jar method but for basal and not CO2-burst procedures. Basal rates are much lower than CO2-burst which triggers a tripling of the normal respiration in 24-hrs. Apparently time was needed for the complexity and confoundment to become more evident, but even so the method should have been adapted to this, but wasn’t. The current excellent correlations observed between Solvita and IR result from these very simple controls on the test conditions.
Woods End believes that ultimately soil health testing methods must be isolated from commercial soil nutrient labs with methods appropriately scaled to biology: not too much soil, not over-moistened and never machine-ground or over-sieved prior to analysis. Similar concerns are being voiced about proper microbiological culturing and even PFLA tests are being found to be damaged by over-handling soils.
For New England dairy farm soils, the new Solvita method produces results comparable to more costly infrared methods and indicates that by either test, these soils and farms exhibit a wide range of microbial activity clearly influenced by manure additions and favorable crop rotations mixing hay and row crops.