Mineralogy and Geochemistry of Volcanic Soils Under Ustic Conditions in Mexico.

Volcanic soils (Andisols) have unique chemical and mineralogical properties that affect their agriculture uses and management. The most common minerals in andisols are kaolinite, Fe oxides, gibbsite, imogolite, and allophane. High surface area and high variable charge of andic soil particles determine their strong chemical sorption capacities for heavy metals, phosphate and other anions. The main objective of this study was to characterize the chemical and mineralogical properties of Mexico volcanic soils under ustic conditions to elucidate their weathering stage. Representative soils were selected after field observations around the Pico de Orizaba or Citláltepetl (19.03°N, 92.27° W). With an elevation of 5765 masl, the active volcano is the highest peak in Mexico and the third highest in North America. The rainfall in the region ranges from 800 to 1000 mm/year, and the highest temperature is 9 °C.

Fieldes and Perrott field test was positive for all soils samples. Soil particle density increased from 2.0 g/cm³ in horizon A to 2.4 in horizon B; soil bulk density increased from 0.9 to 1.6 g/cm³. Texture was sandy loam. Average organic matter content decreased from 10% (mass%) in surface horizons,  0.9% in B horizons, to less than 1% in the rest of the profile. The pH difference ΔpH (pHKCl-pHwater) ranged from -0.83 to -0.35, suggesting high acidity of the variable charged soils. The soils are dominated by short-range-ordered minerals or amorphous materials. Amphiboles, albite, andesine, and anorthite were identificed in the soils. More amphiboles were found in the lower horizons of the profiles. Magnetic minerals increased from 1 to 7 % (mass%) from the top to the bottom. Quartz was absent in the samples. The mineralogical results suggest that the soils experienced weak but unique weathering under ustic conditions.