Contrasting soil development on the sedimentary Kootenai Formation and granitic Boulder batholith in southwestern Montana

Abstract

Characteristics of soils on the Kootenai Formation and Boulder batholith were compared to demonstrate relationships between soils and geologic units in Montana. The Early Cretaceous Kootenai Formation includes clayey shale, fine-grained sandstone, limestone and a basal conglomeratic sandstone. The batholith is composed of a uniform coarse-grained granitic rock, 25 pedons on each parent material were selected by point intercept transects for description, sampling and laboratory analysis. Thirty of 44 soil properties contrasted sharply between the two parent materials. Boulder batholith soils have sandy loam or loamy sand textures; 40 percent higher frequency of a C horizon in the upper 100 cm; mean pH 0.6 unit higher; larger structure size and weaker grade; and 2-22 percent coarse fragment content. Kootenai Formation soils are more variable, having 7 textural classes ranging from sandy loam to clay and a clay content range of 3 to 74 percent. They have 20 cm greater mean solum thickness; more variable consistence; redder color hues; and a 0-65 percent coarse fragment content. There are 6 family textural classes identified in Kootenai Formation soils compared to 2 on the Boulder batholith. Less than 10 samples (generally < 5) are required to estimate mean pH; coarse fragment content; sand fraction contents; and sand, silt and clay contents in all horizons of Boulder batholith soils within + or - 5 units (0.5 for pH) about the mean. Thickness of the B2t and C horizons and depth to the C are the most variable properties, requiring up to 50 samples. In Kootenai Formation soils, about 25 percent of these horizon properties require 10-35 samples and 25 percent require > 30 samples. Coarse fragment content, depth and thickness of the B2t and C horizons, and fine sand, total sand and clay contents of the C horizon require 34-72 samples; clay content being the most variable. From laboratory analyses of five soils on each parent material, Kootenai Formation soils have 1/3 larger available water holding capacities in the B2t and C horizons; higher extractable Ca, CEC and percent BS; and more variable clay mineralogy ranging from dominantly montmorillonite to kaolinite, illite and interstratified 2:1 clays. Boulder batholith soils are dominated by illite clay with < 25 percent montmorillonite. This documentation of soil differences related to geologic units will allow more accuracy and efficiency in soil mapping and management .