MINERALOGICAL VARIABILITY OF THE CLAY IN A MAP DELINEATION OF BRANDON SOIL

Abstract

The mineralogical variability of the clays from the Ap and BCg horizons of 20 pedons in a delineation of Brandon soil near Ottawa was measured. The soil has level topography, is dominantly poorly drained, and has developed from marine clay during the past 8000 yr. Smectite, vermiculite and mica were the dominant clay minerals, whereas quartz, feldspars and amphiboles were generally present in smaller amounts. The coefficients of variation (CV) among pedons were generally less than 20% indicating that the lateral distribution of minerals in the clay was relatively uniform. This was supported by the low variability (CV of 10% or less) of the chemical properties of the clay, such as cation exchange capacities and K contents, which are generally related to clay mineralogy. There were more significant differences in clay mineralogy between Ap and BCg horizons. Differences appear to have resulted from pedogenic weathering. The clay from the Ap horizon contained a higher proportion of smectite and lower proportions of mica, albite and amphiboles. Presumably, some mica had expanded to form smectite and some primary mineral grains had weathered. Decreased total amounts of Na, K and Ca in the Ap clay also reflected this weathering. The lateral uniformity of the clay mineralogy was accounted for by the relatively uniform soil forming factors and processes in this map unit. The confidence limits for variations from means of mineral contents in replicated clays as well as other results indicate that the determination of spatial variability of clay mineralogy is limited by the precision of analytical methods for mineral quantification. Key words: Mineralogical variability, map delineation, Brandon soil