Availability and fractionation of trace elements in arid calcareous soils

Thirty seven soil samples were collected from seven agricultural regions in Saudi Arabia to investigate trace element availability and fractionation distribution. Di-ethylene tri-amine penta acetic acid extractable micronutrients ranged from 1.1 to 11.5 µg g-1 for Fe, 0.2-3.7 µg g-1 for Zn, 0.48-13.0 µg g-1 for Mn, and 0.2-3.7 µg g-1 for Cu. Based on published critical levels, Cu was sufficient for most crops in all soils. Four soil samples were low in Mn, 28 were low in Zn, and 31 were low in Fe. Fractionation of micronutrients (Fe, Cu, Zn, and Mn) in selected seven soil samples revealed that the exchangeable fractions were the smallest, with Zn and Cu below detection limits. Carbonate bound fractions showed the micronutrient concentrations in the order Mn>Fe>Cu>Zn. Oxide and hydroxide bound fraction was higher than the previous two fractions; the order of the metals was Fe>Mn>Zn>Cu. For the organic bound fractions, the order was Fe>Mn>Cu>Zn. The residual fraction was the largest. The order of residual micronutrients was Fe>Mn>Zn>Cu. Overall, and considering the total amount of the four micronutrients within each fraction, the five fractions followed the order: Residual> oxide bound>CO3 bound>organic bound>exchangeable. Based on the data, it is recommended that Fe and Zn applications should be included in balanced fertilization programs. Mn and Cu are sufficiently available in the studied soils, and may not need to be supplied to crops