Soil analysis based on sa,ples withdrawn from different volumes: correlation versus calibration
Soil, particularly in forests, is replete with spatial variation with respect to soil C. Th e present standard chemical method for soil analysis by dry combustion (DC) is destructive, and comprehensive sampling is labor intensive and time consuming. Th ese, among other factors, are contributing to the development of new methods for soil analysis. Th ese include a near- and mid-infrared (NIR and MIR) spectroscopy, laser-induced breakdown spectroscopy (LIBS), and inelastic neutron scattering (INS). Th ese technologies overcome many of the state-ofthe- art DC method’s shortcomings and off er advances that it cannot. While NIR and MIR measure C bonds, the other two new methods, like DC, are more specifi c in measuring C and other elements based on chemical, atomic, and nuclear reactions. In addition to their fundamentally diff erent physical principles, these approaches vastly diff er in the volumes they typically sample: LIBS, 10-9 m3; DC, 10-7 m3; NIR and MIR, 10-6 m3; and INS, about 0.3 m3. Th us, extra care is needed when comparing the fi ndings from any two of these methods. Also, the high heterogeneity of the soil matrix, the nonuniformity of C distribution, and the presence of coarse fragments, particularly in forested ecosystems, further compound the diffi culties in making direct comparisons. We investigated the implications of these differences when correlating any two of these methods and reviewed the processes of comparing a volumetric measurement against a point measurement. We also conducted a detailed comparison of the INS method with the standard DC test. We found that the total (soil organic matter and roots) measured by the INS correlated better than its components with the DC analyses( r2 = 0.97, P = 10-7). Th e samples for DC analysis were taken from excavations of 40- by 40- by 40-cm plots, in 5- and 10-cm layers.