The Effects of Petrocalcic (Caliche) Horizons on Desert Agriculture


The Effects of Petrocalcic (Caliche) Horizons on Desert Agriculture

            Caliche is an idiomatic term for a layer within a soil horizon that is rich in calcium carbonate (CaCO3) and has been solidified by periods of intense flooding and extended periods of drought (VI). The term was adopted by the Spanish but stems from the Latin word calx which means lime (IV). Although caliche and sometimes the names duripans or "lime hardpans" are commonly used to describe these cemented horizons found within a soil profile, the term petrocalcic horizon is used by the USDA Soil Taxonomy and World Reference Base for Soil Resources. It is given the designation Bkm if found as a dominate layer and noted if found as nodules (VIII). While these layers are often falsely confused with duripans that are in rich silica they are similar in how they dictate land use (IV). A simple hydrochloric acid (HCl) test can be used to differentiate between the two (VIII).

            Petrolcalcic horizons are typically found in semiarid and arid regions of the southwestern United States including the Sonoran Desert of Arizona (I). Since water infiltration is the most dominant factor that dictates how deep these layers form, annual precipitation of an area is a good predictor of where these horizons will be encountered (IV). Tucson receives on average about 25 cm of water every year and the CaCO3 accumulates at depths of around 25 cm. Yuma receives approximately 10 cm of rain annually and calcic horizons can be unearthed at or near the surface (V). In general though, caliche is found around 50-100 cm below the soil surface and can be 15-100 cm thick as seen in figure 3. In other parts of the US that receive ample rain, CaCO3 is leached out of soil profiles into various surface and ground water sources (I and III).

            While caliche is formed through various processes, it is mostly a chemical process (figure 1). First carbonate layers form on the surface of the soil and is transported most commonly by wind erosion of clastic rocks (V). However, anthropogenic sources can introduce carbonates as well (III). Then heavy precipitation events (monsoons) percolate carbonate saturated water along with dissolved carbon dioxide (CO2) into the soil. In locations where there is little rain this calcium carbonate is unable to be moved any further into the ground and turns into a concrete like substance (V). Over thousands of years of dry and wet cycles these calcic horizons undergoe four stages of development as they get thicker. In stage one calcium carbonate forms on the under sides of rocks and soil particles and then gradually fills pores between them in stage two. By stage three, the CaCO2 has completely filled all the pores in the soil profile. Once the petrocalcic horizon has fully formed in stage four it can rapidly get thicker because water is unable to infiltrate it. Although some evidence shows these horizons can form quickly they are usually indicative of older soils (V and I).

            While not all desert soils in Arizona have caliche, it is still a huge problem for many gardeners, landscapers, ranchers, and farmers in the region. The hardened layer is impenetrable to most plant roots so they may be unable to adequately uptake water and nutrients. In addition, the plants may form shallow roots that make crops more susceptible to being uprooted by high winds. The calcrete layer may also not allow water to drain properly which can increase the occurrence of root diseases. These anaerobic saturated conditions can "drown" or "suffucate" a plant too because crops do need oxygen in the root zone to carry out respiration. Although Arizona growers usually have high quality irrigation water low in salts (NaCl) the small amount can build overtime which can stress plants. The caliche can increase the salinity of soil at a faster rate because the water and salt is unable to be leached out of the profile. Moreover, calcium carbonate can increase the soil pH which makes many nutrients bio-unavailable to plants to include most commonly iron and zinc (II and VII).

            There are several methods to directly and indirectly manage caliche on both small and larger scales. The first thing that should be done is to break holes through the petrocalcic horizon so that water can drain out of the root zone. The holes should not be directly under crops because roots will find their way through it and girdle themselves as shown in figure 2. Nodules of caliche should be removed if feasible. Once the soil is able to properly drain gypsum (CaSO4) should be used to address soils that are saline, alkali, saline-alkali, and sodic. Sulphur or sulfuric acid can be used to lower the pH of soil into the 6.8-7.2 range but should be avoided in most native desert plants. This will make certain key nutrients more bioavailable to plants. On smaller scales top-soil can be added to increase the distance between the surface and the top of the caliche layer. Raised beds can increase the root zone as well if caliche can't be removed. There is some evidence that daikon radishes can break up shallow thin layers of caliche as well. Economics is in large part which drives what method to choose (II & VII).

 

References

 

      I.         Phillips, S. J., Comus, P. W., & Dimmitt, M. A. (2015). A Natural History of the Sonoran Desert. Berkeley: University of California Press.

 

     II.         Kelly, Jack, Walworth, Jim, and Plant Sciences, School of. Managing Caliche in the Home Yard (2011). Web.

 

   III.         Breazeale, J. F, and Smith, H. V. Caliche in Arizona (2011). Web.

 

   IV.         Almasmoum, ALI. Classification System for Excavating Caliche in Tucson, Arizona (1985): ProQuest Dissertations and Theses. Web.

 

    V.         Penn, Chad, Andrew Whitaker, and Jason Warren. Surface Application of Oil-Base Drilling Mud Mixed with Gypsum, Limestone, and                              Caliche. Agronomy Journal 106.5 (2014): 1859-866. Web.

 

   VI.         DuHamel, J. Caliche Natural Concrete, Can You Dig It? https://arizonadailyindependent.com/2016/06/05/caliche-natural-concrete-can-                       you-dig-it/

 

 VII.         Western Plant Health Association (Author). Western Fertilizer Handbook. Waveland Press, 2010.

 

VIII.         USDA-NRCS. 2014. Keys to Soil Taxonomy (12th Edition).