Jul 26, 2023
Arizona consistently produces the highest yields and quality of crops found anywhere. This is a testimony to not only the skill of Arizona farmers and the capacity to irrigate and produce crops in the desert but also to the productive soils that we are fortunate to have in this region. Soils are commonly considered to serve as the foundation of agriculture (Parikh and James, 2012) and that is certainly true in Arizona.
Many people think Arizona is composed of nothing but rocks and sand. While we do have an abundance of rocks and sand in Arizona, this region also has many areas with soils that are geologically young, with high natural fertility, and extremely productive if provided with sufficient water and good management.
Most Arizona crops are produced on alluvial soils that are formed from the deposits of sediments from periodic flooding of rivers and streams over thousands and millions of years. The alluvial soils used in Arizona crop production vary tremendously across the state and the entire desert Southwest. They vary in texture, depth, and structure. Our crop production capacities in this region are supported by the irrigation water and these incredible soil resources.
A great example of the rich alluvial soils of this region can be found in the lower Colorado River valleys including Mohave Valley, AZ; Parker Valley, AZ; Palo Verde Valley, CA; Imperial Valley, CA; Bard Valley, CA; Yuma Valley, Arizona; and the lower Gila Valleys of Arizona. The alluvial sediments that have formed the soils we encounter in the lower Colorado River system are estimated to have been deposited to several hundred meters (several thousand feet) in depth in the current river channel approximately 4.63 to 4.80 million years ago (Crow et al., 2021 and Kimbrough et al., 2015). These sediments are geologically “young” in contrast to the ancient crystalline rocks at the bottom of the Grand Canyon, often referred to as the “Vishnu Basement Rocks” that are 1,680 – 1,750 Ma (1.68 – 1.75 billion (b) years ago), (Crow, et al., 2021 and Mathis and Bowman, 2018).
In 1941, Dr. Hans Jenny from the University of California – Berkeley, published Factors of Soil Formation: A System of Quantitative Pedology. [Pedology is the scientific study of soils and their weathering profiles, including the formation, nature, ecology, and classification of soil.] This book is a classic and fundamental piece of work in the study of soil science due to Jenny’s capacity to translate the basic concepts into a quantitative scientific framework through a rigorous definition of the soil system and its genesis, including the identification and separation of dependent and independent variables.
Jenny described soil formation as a function of five factors: climate, biota (plants and animals), relief (topography), and parent material, all operating together over time (Figure 1).
Alluvial soils tend to be highly variable both horizontally across the landscape and vertically down through the soil profile. This can sometimes present challenges in field-level management for crop production, particularly when soil types change within a given field or relatively small area, which is rather common.
Figure 1. Hans Jenny and the factors of soil formation, 1941.
We commonly find that soil formation will vary tremendously across a landscape with natural soil variation across the landscape in both the horizontal and vertical dimensions with the distinct organization of soil horizons (Figures 2 and 3).
It is important to recognize soil variability in a crop production system because different soil types will have unique physical and chemical properties that can affect things like soil-water holding capacity, drainage, leaching capacities, soil fertility, and plant root development.
The United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) has conducted extensive soil surveys across the United States for each state by county. Soil surveys can be accessed on-line at the following site: https://websoilsurvey.nrcs.usda.gov/app/
A broad map selected from NRCS Soil Survey of the Yuma area graphically depicts both the alluvial nature and variability of the soil resources commonly encountered in this area. https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx
It is important to recognize the importance and high value of these natural soil resources that our agricultural systems are developed and dependent upon and be alert to the natural variability associated with the alluvial soils in this region.
Figure 2. Natural variation of soils across a landscape
Figure 3. Layers of soil, basic organization of soil horizons.
Figure 4. Soil volume, soil texture, and water holding capacity
relationships. Source: Kansas State University Agronomy Department, Soil
Crow, R.S., J. Schwing, K.E. Karlstrom, M. Heizler, P.A. Pearthree, P.K. House, S. Dulin, S.U. Jänecke, M. Stelten, and L.J. Crossey. 2021. Redefining the age of the lower Colorado River, southwestern United States: Geology, v. 49, p. 635–640, https://doi.org/10.1130/G48080.1
Jenny, H. 1941. Factors of Soil Formation: A System of Quantitative Pedology. McGraw-Hill Book Company. New York and London.
Kimbrough, D.L., M. Grove, M., G.E. Gehrels,, R.J. Dorsey, K.A. Howard, O. Lovera, A. Aslan, P.K. House, and P.A. Pearthree. 2015. Detrital zircon U-Pb provenance of the Colorado River: A 5 m.y. record of incision into cover strata overlying the Colorado Plateau and adjacent regions:
Geosphere, v. 11, p. 1719–1748, https://doi.
Kansas State University Agronomy Department, Soil Laboratory Manual: Soil and water relationships. https://kstatelibraries.pressbooks.pub/soilslabmanual/chapter/soil-and-water-relationships/
Mathis, A. and C. Bowman. 2018. Telling Time at Grand Canyon National Park. United States Department of the Interior, National Park Service.
Parikh, S. J. and B.R. James. 2012. Soil: The Foundation of Agriculture. Nature Education Knowledge 3(10):2