Soils of the USA: The Broad Perspective
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 Regi
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Soils of the USA: The Broad Perspective James G. Bockheim
4.1
Soil Suborders by Land Resource Region
Soils form in response to climate, organisms, relief, parent materials, and time. In view of the heterogeneity of these factors in a country as large as the USA, the soils are extremely diverse. The purpose of this chapter is to provide a broad overview of soils in each Land Resource Region (LRR) as related to soil-forming factors. There are 28 LRRs in the USA and its territories that are named for their location and primary land uses (NRCS 2006; Figs. 1.1 to 1.3). Each LRR is divided into from one to 23 Major Land Resource Areas (MLRAs; total 278). The MLRAs are basedprimarily on physiography and geology but also on climate, soils, and vegetation type. The USA (excluding its territories) has a total land area of 9.0 million km2. The LRRs range in area from 260 km2 for the Hawaii Region to 1.4 million km2 for the Western Range and Irrigated Region (Table 4.1). 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; area = 1424,480 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, which represent high pedodiversity indices, and 593–754 for LRRs F, H, and P, which represent low pedodiversity indices (the ratio of land area to number of soil series per LRR). The number of soil orders per LRR ranges from 5 for various regions in Alaska to 10 in the Hawaii Region (LRR V) and averages 7 (Table 4.1). The number of suborders ranges from 9 in western Alaska to 44 in the Northwestern Forest, Range, and Specialty Crop Region (LRR A) and averages 21. Much of Alaska has been mapped only at a
J.G. Bockheim (&) University of Wisconsin, Madison, WI 53706-1299, USA e-mail: [email protected] © Springer International Publishing AG 2017 L.T. West et al. (eds.), The Soils of the USA, World Soils Book Series, DOI 10.1007/978-3-319-41870-4_4
reconnaissance scale so that the number of soil series is not consistent with its large area. The number of soil great groups ranges from 24 in the Mississippi Delta to 118 in the Western Range and Irrigated Region (LRR D) and averages 60. The dominant soil taxa by LRR or MLRA can be determined through three techniques: (i) by overlaying the LRR or MLRA map on the soil order and suborder maps, (ii) identifying all of the series in an MLRA using the NRCS soil classification database and the accompanying areal extent maps, and (iii) from the number of soil series per taxa. There is a highly significant correlation between the number of soil series and the areas of soil orders (Fig. 4.1). From Table 4.1, it can be seen that from one to four suborders (those in bold face) account for half of the soil series in each LRR. Mollisols are the dominant order in the USA, accounting for 23
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