First evidence for the presence of Andisols in the dry-hot environment of the Arabian Shield

Abstract

Saudi Arabia has many volcanoes and extensive basaltic lava fields known as harrats located along the western part of the Arabian Shield sub-parallel to the Red Sea. The soils developed on these volcanic materials have not yet been investigated. Forty representative soil profiles were selected from six harrats and their morphological and physicochemical properties as well as their mineralogical compositions were characterized using standard chemical, XRD, SEM, and TEM techniques. The main objective of this study was to determine if the soils developed from basaltic rocks in the harrats of the Arabian Shield are Andisols. The results revealed that all representative profiles had volcanic glass content that ranged from 15 to 40%, P-retention capacity >35%, and acid oxalate extractable A1o + 1/2 Feo that ranged from 0.44 to 2.20%. The crystalline clay minerals were dominated by smectite, kaolin (kaolinite/halloysite), and mica (illite), whereas, chlorite-hydroxy-interlayered vermiculite (Ch-HIV), palygorskite, Ca-zeolites, and pseudo-chlorite were present as minor components. Small amounts of allophane, imogolite, and ferrihydrite made up the poorly-crystalline clay minerals. All representative profiles satisfied the criteria for andic soil properties, and were classified as Torrands due to the presence of an aridic soil moisture regime in the area. Paleoclimatic conditions that existed in the Arabian Peninsula over the last few thousand years are consistent with the formation of andic and/or vitric soil properties. Because of this, Andisols developed from pyroclastic parent materials and are now present in dry-hot conditions. The presence of Andisols in the study area was demonstrated, and was mainly attributed to the influence of Pleistocene paleoclimatic conditions.