Influence of different iron mineral sources on the growth of iron efficient and inefficient sorghum (Sorghum Bicolor) plants

Abstract

Two crops of Fe efficient (RTx2536) and inefficient (BTx378) sorghum were grown in a greenhouse study both with and without added CaCO3 in sand which was crushed and separated by sedimentation techniques to obtain particle size fractions of <2 μm, 2-5 μm and 5-50 μm for each sample. Iron was supplied in the mineral form as goethite, biotite and hornblende. Essential nutrients other than Fe were supplied at a constant rate in solution mixed with the sand and mineral medium. Leaf chlorophyll content determined in the laboratory was not correlated with a visual rating system for the first crop; however, a high correlation was obtained during the second crop (r^2 = 0.86). Shoot weights, root weights and Fe uptake for the first crop were greater for the Fe inefficient sorghum variety compared to the Fe efficient variety. The opposite effects were observed in the second crop when more Fe stress was placed on the plants. In pots with 20g/kg CaCO3 added, Fe and P uptake increased in both the first and second crops according to the following mineral order: biotite > hornblende > goethite. Iron uptake and plant growth increased as mineral particle size decreased. Iron chlorosis of the plant was not seen when biotite and CaCO3 were added in the first and second crops. However, chlorosis was observed when the 5-50 μm and 2-5 μm particle size fractions of goethite were applied as a treatment with both CaCO3 levels. The 5-50 μm hornblende particle size fraction also produced chlorotic plants. These results may be due to a constant pH which made Fe availability a function of mineral solubility and particle size. The relationship between Fe and P uptake was highly correlated for the 2% CaCO3 treatment. Structural mineral changes were observed, using x-ray diffraction (XRD) analysis in biotite when 2% CaCO3 was added as a treatment. Biotite weathered to interstratified biotite-vermiculite and finally to vermiculite. This was apparently due to K replacement by Ca and to Fe oxidation during or following the opening of the interlayer. Similar changes took place with a mica contaminant in the hornblende sample. In the latter case, mica weathered to high charge smectite and in some cases to vermiculite. Only minor morphological changes were observed with scanning electron microscopy.