Solubility and iron release characteristics of iron chelates and sludge products

Iron deficiency is an important nutritional problem in the Florida citrus industry particularly under calcareous soils, and to some extent in acid soils. This study reports on iron supplying characteristics for LibFer SP (an alternative EDDHA chelated Fe) and Fe sludge‐unamended and modified‐products (industrial by‐products) in 4 Florida soils. In a batch‐ equilibration technique, the recovery of Fe from LibFer SP applied at rates varying from 15 to 120 mg Fe/kg soil, decreased from 95 to 43% and 95 to 63% in Holopaw (pH = 7.6) and Myakka (pH = 5.8) sand, respectively. Iron recovery was independent of rate of Fe application in Oldsmar sands sampled from a grove (pH = 6.2) or from an uncultivated natural vegetation site (pH = 4.2). However, Fe recovery was greater by 20% in the Oldsmar‐Grove than in the Oldsmar‐Natural sand. The recovery of Fe was negligible from unamended Fe sludge but improved slightly in the modified Fe sludge. In a moist soil incubation study, over 70 d, the recovery of Fe (by Mehlich 3 extraction) from LibFer SP treated Holopaw sand increased by 35% and decreased by 33% at 100 and 200 mg/kg Ferates, respectively. Recovery of Fe also increased by 8 to 13% during 70 d incubation at varying rates of Fe application as Fe sludge. The recovery of Fe from Fe‐sludge amendment was much greater in the Myakka sand than in the Holopaw sand. A slow but increasing recovery of Fe from Fe‐sludge products suggests that these products may provide available Fe for crop uptake slowly over an extended period following their application to soil.