Synthetic ferrihydrite as a potential iron amendment in calcareous soils

Abstract

A greenhouse study was conducted to evaluate the use of siliceous ferrihydrite as a slow release Fe fertilizer for a sandy calcareous soil from the Coastal Bend area of Texas. This soil is low in amorphous Fe oxide and has a history of severe Fe chlorosis. Siliceous ferrihydrite was prepared in the laboratory by reaction of ferric sulfate with NaOH in the presence of varying amounts of sodium silicate. Silicate was used during the synthesis of ferrihydrite structure to: (1) promote the formation of smaller crystals with higher reactive surface areas, and (2) inhibit the transformation of this material to more crystalline phases. Cumulative amount of Fe released from each of the materials after six successive DTPA extractions was related to the Si/Fe ratio of the ferrihydrite. The greenhouse results indicated that at the level of 100 mg Fe per kg, Fe‐.75 Si and Fe(III) were equally effective in reducing chlorosis; however, Fe(II) was a much less effective source of Fe. The Fe supplying abilities of the Fe sources decreased in the following order: Fe‐.75 Si = Fe(III) > Fe‐.25 Si > Fe‐.50 Si > Fe‐.036 Si = Fe (II). The ammonium oxalate extractable Fe (poorly crystalline Fe‐oxide) of the soil following harvest was approximately equal to the sum of Fe extracted from the control plus the amount of Fe added as siliceous ferrihydrite or Fe(III). These results indicate that Fe(III) reacted with calcium carbonate to form a poorly crystalline phase and that there was little or no transformation of siliceous ferrihydrite or the Fe(III) reaction product to more crystalline phases. For the Fe(II) treated soils the quantity of Fe extracted by ammonium oxalate was lower, which suggests that the Fe(II) reacted with calcium carbonate to form a more crystalline phase. Therefore, it was concluded that differences in the effectiveness of soil applied Fe(III) and Fe(II) salts in the mitigation of Fe chlorosis was related to the chemistry of reaction with CaCO₃ in the soil. Also, the siliceous ferrihydrite effectively reduced chlorosis in this sandy calcareous soil.

Key words:

• CaCO₃
• Ferric
• Ferrihydrite
• Ferrous
• Iron
• Hot spot
• Chlorosis
• Silicate
• Siliceous ferrihydrite