Soils of the USA: The Broad Perspective

Abstract

In this chapter, the dominant suborders, soil parent materials, vegetation types, and climate of each Land Resource Region (LRR) are discussed. The chapter, therefore, constitutes a broad overview of Chaps. 5 through 17. There are 28 Land Resource Regions (LRRs) in the USA and its territories that are named for their location and primary land uses. Each LRR has been divided into from one to 23 Major Land Resource Areas (MLRAs; total of 278). The MLRAs are based primarily on physiography and geology but also on climate, soils, and vegetation type. The USA (excluding its territories) has a total land area of 9.3 million km². The LRRs range in area from 260 km² for the Hawaii Region (LRR V) to 1.4 million km² for the Western Range and Irrigated Region (LRR D). The number of soil series in a LRR is governed not only by its area but also by the diversity of the soil-forming factors. The number of established soil series ranges from 84 in the relatively small Mississippi Delta Region (LRR O; area = 100,710 km²) to 5467 in the massive Western Range and Irrigated Region (LRR D; 1.4 million km²). The ratio of land area to number of soil series per LRR ranges from 116 to 185 for LRRs A, B, C, L, and T (Pacific Northwest, California, Great Lakes region, and Atlantic and Gulf Coast), which represent high pedodiversity indices, to 593–754 for LRRs F, H, and P (northern and central Great Plains and southern Atlantic Gulf Slope), which represent low pedodiversity indices. There is a highly significant correlation (R ² = 0.92) between the number of soil series and the areas of soil orders (excluding the poorly represented Gelisols). From one to four suborders account for half of the soil series in each LRR. Soils of the USA occur in 12 orders, 65 suborders, 344 great groups, and numerous subgroups and families, yielding a total of around 23,000 named soil series. The distribution of soils in the USA is controlled primarily by climate (Aridisols, Gelisols, and Oxisols), vegetation (Histosols and Mollisols), parent materials (Andisols, Entisols, and Vertisols ), time (Inceptisols), or a combination of factors (Alfisols, Spodosols, and Ultisols ). The ranking of suborders by abundance is Ustolls > Udults > Udalfs > Turbels > Orthents > Udolls > Udepts > Xerolls ; these suborders comprise 53 % of the soils of the USA. Ninety-five percentage of the soils in the USA contain either an ochric or amollic epipedon. Soils with an argillic horizon comprise 49 % of the soils of the USA; 24 % of the soils (Entisols and Histosols) lack a diagnostic subsurface horizon; and 18 % contain a cambic horizon. The dominant soil-forming processes are argilluviation, melanization, base-cation enrichment, and cambisolization , which comprise 56, 42, 37, and 28 % of the soil area of the USA, respectively.