2.2.1 Microscale Heterogeneity
Within-sample matrix heterogeneity is due to the particulate nature of soil. Grain size variation within each sample is a major contributor to microheterogeneity affecting concentration measurements. As shown in Figure 2-2, soil particle sizes span several orders of magnitude, from less than 0.002 mm for fine clay-sized particles to 1–2 mm for very coarse sands (USDA 2010). Commonly, the maximum grain size considered to still qualify as part of soil is 2 mm. This section devotes a great deal of attention to grain size and its effect on concentration heterogeneity because (a) unless specific field and laboratory sampling and subsampling procedures are followed, routine sampling can lead to concentration estimates that are biased high or low due to grain size effects and (b) ISM sampling guidance detailed in later sections offers techniques to reduce the error due to grain size effects.