Distribution of zinc forms in highly calcareous soils as influenced by soil physical and chemical properties and application of zinc sulfate

Water-soluble zinc (Zn) fertilizers are rapidly converted to insoluble forms in calcareous soils resulting in lower efficiency of such fertilizers. A knowledge of distribution of native and applied Zn in such soils is necessary for understanding the fate of applied Zn fertilizers and finding ways to increase their efficiency. This experiment was conducted to obtain such information in selected highly calcareous soils of Iran. A sequential extraction method was used to fractionate the Zn forms of surface horizons (0-20 cm) of 20 highly calcareous soils [16 to 58% calcium carbonate (CaCO3) equivalent; pH 7.9 to 8.5] which had previously received 0, 10, or 20 mg Zn/kg as zinc sulfate (ZnSO4) and had been under one corn (Zea mays L.) crop in the greenhouse. The forms determined were exchangeable (EXZN), sorbed (SRZN), organic (ORZN), carbonate (CRZN), residual (RSZN), and sum of forms (SMZN). The native SMZN ranged from 32.4 to 66.7 mg/kg with a mean of 49.9 mg/kg. Application of 10 and 20 mg Zn/kg as ZNSO4 increased the mean to 57.7 and 62.7 mg/kg, respectively. Concentration of different forms of Zn in the soils was determined to be in the following order: RSZN >>> CRZN > SRZN > EXZN > ORZN. The concentration of native EXZN + SRZN + ORZN forms constituted less than 5% of SMZN, while concentration of CRZN alone ranged from 4.37 to 16.05% with a mean of 8.36%. Application of ZnSO4, while significantly increased the concentration of all forms of Zn, had a pronounced effect on CRZN. Averaged over all soils, 58 and 60% of the applied ZnSO4 was converted to CRZN for the 10 and 20 mg Zn/kg, respectively. Regression equations relating different Zn forms to soil physical and chemical properties indicated that the Zn forms are significantly influenced by soil properties. It was concluded that conversion of applied ZnSO4 to CRZN was mainly responsible for retention of this fertilizer in highly calcareous soils, making it temporarily unavailable to plants, and therefore decreasing its apparent recovery by the first crop.