In science it is often the case that increasing the sample size decreases the variance of the result, meaning large samples produce better estimates of content than do small samples. Soil laboratories use only a few grams of soil per sample for regular nutrient analyses, employing equipment that makes up for soil minutiae with low-level detection capability. However, for biological assays like CO2 respiration, more soil is preferable for reliable results.
For Solvita the minimum amount of dry soil was set at 40g for lab assays, which is at the lower end of reliable quantity. But for field soil testing with as-is moist soil, it is desirable to use 80-100g of soil, and even more would be preferable.
The unusual design of Solvita probes along the lines of the Ideal Gas Law means that detector probes can be placed in jars with differing weights of soil, and still obtain the same result, as shown at right. All the samples at right gave similar gross respiration of 22 ± 2.6 ppm CO2-C! How can that be? The range of soil weight shown here is from a low of 100g (standard basal method) to 780g or about 1¾ lbs of soil. Repeated again after 3 days the 4 jar sets gave 19.4 ppm ± 3.1 – about the same results.
The secret is to keep the ratio the same for total air volume to “dead volume” (occupied by pure soil and water ). That’s because Solvita registers the headspace concentration of CO2 and the calculator in the DCR has already done the work of converting via the Ideal Gas Law to respiration per as-is sample. Write to us at solvita@woodsend.com if you want to learn more about using differing weight to improve your results!