EFFECT OF STRUCTURAL WATER IN CLAY MINERALS ON THE ESTIMATION OF SOIL ORGANIC MATTER CONTENT BY THE LOI ANALYTICAL METHOD

The loss on ignition (LOI) analytical method traditionally has been considered reliable in estimating organic matter content in soil samples. To evaluate the impact of structural water loss (SWL) on the accuracy of these types of analyses, 14 terrestrial loamy soil and 3 marsh soil samples were collected from various depths within the A and B horizons at 5 different sites in New Jersey. In addition, five “pure” clay samples, with no organic matter, were analyzed by LOI to obtain the SWL for each “pure” clay mineral. These results then are multiplied by the measured abundance, expressed in weight percent, of all identified clay minerals in each soil sample to determine total SWL. Clay mineral abundances for each soil sample were measured by a quantitative X-ray powder spectra analysis. Abundance values ranged from 27% to 48% for the 14 loamy soil samples, and from 32% to 44% for the 3 marsh soil samples. After sequentially heating the “pure” clay samples at temperatures of 200°, 300°, 400°, 500°, and 550°C for 4 hours each, SWLs (measured as LOI) for “pure” clay minerals (no organics) are 10.6% for kaolinite, 1.7% for bentonite, 4.1% for illite, 6% for vermiculite, and1% for chlorite; soil samples were heated in the same way to determine their LOI. Calculated total SWIs for the 14 loamy soil samples were between 0.87% and 1.82% and between 0.97% and 1.45% for the 3 marsh soil samples. Based on initial results, the average SWL/LOI ratio for the A horizon is 0.37 at a depth of 0-20 cm and 0.48 for the B horizon at a depth of 30–50 cm. The average SWL/LOI ratio is 0.48 for the 14 loamy soil samples and 0.15 for the 3 marsh soil samples. The former result indicates a potential 48% overestimation of the organic matter content by the traditional LOI method when SWL is not considered. The potential 15% overestimation of organic matter content for the three marsh soils suggests it might be less of an issue for soils from this environment. Although the LOI method alone may be adequate for estimating the organic matter content of soil samples with large organic contents, such as marsh soils, it appears to cause significant errors for terrestrial loamy soils where organic matter content typically is less than 6%.