Iron and Humic Acid Accumulation on Soybean Roots Fertilized with Leonardite Iron Humates under Calcareous Conditions

Iron humates are eco-friendly fertilizers that are less efficient than iron synthetic chelates at correcting iron chlorosis. The aim of this work was to improve the efficiency of a leonardite iron humate (LIH), by studying the relationship among humic acid (HA) accumulation, iron biomineralization on soybean roots, and iron nutrition in soybean plants under calcareous conditions. Two hydroponic experiments were performed: a short-term bioassay (21 days) with several doses (10, 20, 50, and 100 mu mol of Fe pot(-1)) of LIH applied once a week and a long-term bioassay (60 days) with just one application of LIH (250 mu mol of Fe pot(-1)). When LIH was applied several times, it precipitated on the root, blocking the cell wall pores and reducing iron transport in plants, while these effects decreased when LIH was applied just once, thus favoring iron uptake by the plants and avoiding HA accumulation. Jarosite was observed on the surface of soybean roots.