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The south western state of Arizona is known for its arid rangelands. The NRCS (Natural Resources Conservation Service) has created several regions that US lands have been categorized under called LRR’s (Land Resource Regions). Arizona falls under LRR ‘D’ -Western Range and Irrigated Region. Within region D are several resource areas called MLRA’s (Major Land Resources Areas). Because Arizona is a large state with several hundreds of soil types, Areas of Interest (AOIs) were created to better understand some of these soils. The three chosen AOIs are located in 3 of the 5 MLRA’s in Arizona; 38, 40, and 41. Figure 1 shows where the AOIs are located within the state’s counties.

Introduction

The Soils in Arizona AOIs

Table 5 shows the top two soils in each AOI and the following information: pH (on a scale from acidic [1] to basic [14], with 7 being neutral), CEC7, the standard cation exchange capacity scale (units are mole equivalents per 100 grams), SOM (soil organic matter listed in a percentage), and SAR (sodium adsorption ratio), which is the amount of sodium in relation to the calcium and magnesium content.

Shown in Table 7 is the USLE (Universal Soil Loss Equation) for the top two soils within the selected AOI’s. The table only shows a few of the factors that contribute to measuring soil erosion. The K factor is the sheet/rill erosion due to water, the C factor stands for the cover and management factor, and the P factor is the support practices conducted on the soil. The A value is calculated by multiplying the K, C, P, R, and LS factors, which shows the predicted annual soil loss. The T factor is the maximum allowable rate of erosion due to water and wind in tons per acre per year. When the A value is divided by the T factor, it shows how great the predicted soil erosion will be. As shown in Figures 5, soils in category A<T will have low erosion, soils in category 2T>A>T will have medium erosion, and the soils in category A>2T will have extremely high erosion. The graphs in Figures 6 and 7 express that in both AOIs, all of the soil types have low predicted soil erosion.

Soil Erosion and the USLEDiscussion

It is important to understand the potential erosion because it reveals a great deal of information about the fertility and quality of the soil. The C factor chosen was for range grasses with no surface litter (0.17), due to the fact that these areas are mostly used as rangeland and grazing. The P factor (1) was chosen because the area is relatively flat and does not engage in strip-cropping practices. It can be inferred that these soils will have little loss of organic matter and topsoil due to the low expected erosion. The poor and fair ratings given for these soils mean that Arizona soils are not a good source of topsoil, mostly because it contains too much clay or too many rock fragments. The soils that were given a “very limited” rating are not areas where a basement should be built, mostly because the soil would shrink and swell too much. The Mohall, Pima, and McAllister soils would be suitable areas in which to add a basement. The Lonti series consists of 7 horizons (A [Ochric Epipedon], Bt1, Bt2, Bt3, Bt4 [Argillic horizons], Bk1, and Bk2) that extend down to 68 inches beneath the surface. The average annual temperature for this soil is 54-59 ºF. It has a slow permeability rate and gets about 15 inches of rain annually. The Barkerville series is comprised primarily of alluvial parent material, but it can also contain colluvial material. It has 5 horizons (Oi, A, Cl, C2, and 2R) which extend down about 40 inches. This soil has a fast permeability rate, and receives 12-16 inches of rain annually. The Contine series consists of 5 different horizons (Ap [Ochric epipedon], Bt [argillic horizon], Btkl , Btk2, and Btk3 [calcic horizon]) which extend down 60 inches. The precipitation rate is around 7 inches per year, and the soil temperature averages around 72-80 ºF. The Mohall series consists of 8 horizons (A, BA, Bt, Btk1, Btk2, 2Bk, 2Cl, 2C2) with a total depth of 98 inches. It receives about 2-10 inches of rain annually. It is mainly used for recreational purposes, grazing, and cropland. The Pima series is classified as a fine-silty soil that contains 5 horizons (Ap, Al, A2, Cl, and C2) with a total depth of 60 inches. It has a slow permeability, receiving about 10 inches of rain a year. The McAllister series consists of 5 horizons (Ap, Bt1, Bt2, Btk, and Bk) that stretch down to 60 inches in depth. It gets about 14 inches of precipitation annually. The Pima and McAllister series are used for irrigated crops and animal grazing.

ReferencesBrady, N.C., and R.R. Weil. 2010. Elements of the nature and properties of soils. Pearson Education, New Jersey. NRCS -Natural Resources Conservation Service. 2012. MLRA Explorer. http://apps.cei.psu.edu/mlra/ (accessed 29 November, 2012). Penn State University, University park, PA.USDA- Natural Resources Conservation Service. 2012. Official Soil Series Descriptions (OSD). Available at http://soils.usda.gov/technical/classification/osd/index.html (verified 6 Dec. 2012). USDA, Washington, DC.USDA -United States Department of Agriculture. 2012. Web Soil Survey. http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx (accessed 29 November, 2012). Natural Resources Conservation Service, Washington, DC.

Dominant Soils Table 4 displays the top two soils from each AOI as explained within the AOI maps. The SMU name, how much area the soil occupies in the AOI (in acres), and the taxonomic family associated with each soil is displayed.

The following figures and tables show the three selected AOIs in Arizona. The maps are zoned according to each soil mapping unit (SMU). The legend contains the names for all of the SMUs, along with their acreages and relative percentages in the AOI as a whole.

In addition to providing informative data, the WSS (Web Soil Survey) website created by the USDA (United States Department of Agriculture) also includes information about the suitability of land for specific uses. Two land uses were selected, and the ratings for these uses are shown for all six soils. Table 8 shows the “rating for topsoil source”, and “rating for dwellings with basements”. Brief reasons for why these rating were given to particular soils are included.

MLRA 38 is known as the Mogollan Transition, and it stretches through central Arizona and just slightly into New Mexico. The dominant soils in this area are Aridisols, Alfisols, and Mollisols. They are from alluvium sources, having thermic and mesic temperature regimes as well as aridic and ustic moisture regimes. Most of the land is used as rangeland and a small portion is forest. The main management concern for this area is overgrazing. MLRA 40 is known as the

Sonoran Basin and Range, and it sits in the lower western and centralregion of Arizona. The dominant soils in this area are Entisols and Aridisols. They have thermic and hyperthermic temperature regimes as well as an aridic moisture regime. Most of the land is used as rangeland with small fractions used for urbanization and cropland. One concern in this area is losing soil sustainability in this desert land. MLRA 41 is known as the Southeastern Arizona Basin and Range, and it is in the lower eastern corner of Arizona with a slight portion extending into New Mexico. The dominant soils in this area are Aridisols, Entisols, Alfisols, and Mollisols. They have a thermic temperature regime, as well as ustic and aridic moisture regimes. Most of the land is used as rangeland, with small fractions used for urbanization and cropland. Maintaining the organic matter content is of high priority for this area. The soils in all three regions derive from alluvium sources.

Figure 1. Map of Arizona indicating AOI locations

The first AOI is located in Yavapai County, central Arizona. The two major SMU’s are BoF and LkD (Ro is actually the largest but is disregarded because rockland cannot be considered as a viable soil). BoF is Barkerville extremely rocky sandly loam, but will be referred to as ‘Barkerville’. LkD is Lonti gravelly sandy loam, and has also been shortened to ‘Lonti’.

Figure 2: Map of AOI 1

AOI 2 is located in Pinal County, southern Arizona (shown is Figure 3). The largest SMU’s are Co and Mv. Co is Contine clay loam, but will be referred to as ‘Contine’. Mv is Mohall loam, but this has been shortened to ‘Mohall’. As shown in Figure 4, the third AOI is located in Cochise County, found just above the southern border of Arizona. The major SMU’s are Pm and Mc. Pm is Pima loam, but has been shortened to ‘Pima’. Mc is McAllister loam alkali, but has also been shortened to ‘McAllister.’ These maps and tables showing the different SMUs help to determine the soil’s suitability for different uses, and it helps predict future obstacles that may occur in the area.

Table 2. Soil mapping units and taxonomic families for AOI 2

Table 1. Soil mapping units and taxonomic families for AOI 1

The physical aspects of the top two soils in each AOI are shown in Table 6. The factors such as saturated hydraulic conductivity (Ksat), available water capacity (AWC), drainage class and surface texture can help indicate the movement of water through soil. Bulk density and the restrictive layer can help determine how much water the soil is capable of holding. The linear extensibility (the percentage of change in clod length when going from wet to dry state) can show how expansive the soil is, which is an indicator of the amount of clay within the soil clod.

Interpretations of Soil Uses

Chemical Data

Physical Data

NREM 221- Soil Resources Chelsea Smith, Nimisha Krishna, and Vanessa Shipley

The Soils of Arizona

Chemical Data TableSoil Name pH CEC7 (mEq/ 100g) SOM (%) SAR

AOI 1- Lonti 7.2 10 1.5 0

AOI 1- Barkerville 7 7.5 1.5 0

AOI 2- Contine 8.2 15.5 0.75 0

AOI 2- Mohall 8.2 11.5 0.25 0

AOI 3- Pima 7.9 17.5 2 3

AOI 3- McAllister 8.8 15 1.5 9

Physical Data Table

Soil Name Surface TextureSurface Bulk

Density (g/ cm3)Depth to and Type

of Restictive Layer (cm) Ksat (µm/Sec) AWC (cm/cm)Linear

Extensibility (%) Drainage Class

AOI 1- Lonti Cobbly Loam 1.4 >200 3 0.1 1.5 Well drained

AOI 1- Barkerville Cobbly Sandly Loam 1.48 >200 28 0.14 1.5 Well drained

AOI 2- Contine Clay loam 1.4 > 200 2.7 0.2 7.5 Well drained

AOI 2- Mohall loam 1.5 > 200 3 0.18 4.5 Well drained

AOI 3- Pima Loam 1.27 >200 2.7 0.2 1.5 Well Drained

AOI 3- McAllister Loam 1.32 >200 9 0.15 1.5 Well Drained

Soil Name K FactorT Factor

(tons/acre/yr) C Factor P FactorCalculated

A ValueAOI 1- Lonti 0.17 5 0.17 1 4.51AOI 1- Barkerville 0.15 2 0.17 1 15.24AOI 2- Contine 0.32 4 0.17 1 0.11AOI 2- Mohall 0.32 3 0.17 1 0.11AOI 3- Pima 0.32 5 0.17 1 0.22AOI 3- McAllister 0.32 5 0.17 1 0.22

USLE Table

Soil Mapping Units Area in AOI (Acres) Taxonomic Families

AOI 1Lonti gravelly

sandy loam264

Fine, mixed, mesic Ustollic Haplargids

Barkerville extremely rocky sandy loam

206Sandy-skeletal, mixed, mesic Typic Ustorthents

AOI 2 Contine clay loam 453

Fine, mixed, hyperthermic Typic Haplargids

Mohall loam 215

Fine-loamy, mixed, hyperthermic Typic Haplargids

AOI 3 Pima Loam389.9

Fine-silty, mixed, thermic Cumulic Haplustolls

McAllister Loam; alkali208

Fine-silty, mixed, thermic, Cumulic Haplustolls

Dominant Soils in Selected AOIs

Figure 3. Map of AOI 2

Table 3. Soil mapping units and taxonomic families for AOI 3Figure 4. Map of AOI 3

Table 4. SMUs and taxonomic families for the dominant soils in each AOI

Table 5. Soil chemical properties for the dominant soils in each AOI

Table 6. Soil physical properties accounted for the dominant soil in the three AOIs

Table 7. USLE factors and predicted annual soil loss for dominant soils in each AOI

Figure 7. Chart showing degree of predicted soil erosion for AOI 3

Figure 6. Chart showing degree of predicted soil erosion for AOI 2

Figure 5. Chart showing degree of predicted soil erosion for AOI 1

Soil NameRating for

Topsoil Source Reasons for RatingRating for Dwellings

with Basements Reasons for Rating

AOI 1- Lonti PoorHard to reclaim rock fragments Very limited

Slope and insuffi cient shrink-swell rating

AOI 1- Barkerville Poor Shallow depth to bedrock Very limitedSlope and shallow depth to bedrock

AOI 2- Contine Poor Too Clayey Very limited Severe shrink-swell rating

AOI 2- Mohall FairInsuffi cient carbonate

conent Somewhat limited Moderate shrink-swell rating

AOI 3- Pima Fair Too Clayey Not limited Favorable soil conditions

AOI 3- McAllister PoorToo clayey, low sodium content and low salinity Somewhat limited Insuffi cient shrink-swell rating

Interpretation Table

Table 8. Ratings for suitability of topsoil source and dwellings with basements including reasons for the ratings