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Previous work had suggested microbially‐produced chelates as potential sources of plant available iron for crops grown on alkaline‐calcareous soils. A “hot spot”; soil (Pernitas fine sandy loam) which historically produced severe iron stress on field grown grain sorghum (Sorghum bicolor L. Moench) was used in this greenhouse study. Iron utilization by grain sorghum was examined where Fe was supplied as three types of ferrated microbially‐produced hydroxamate siderophores; ferrioxamine B (FOB), rhodotorulic acid (RA) and a mixture of ferrichrome‐ferrichrome A (1: 3; Fc‐FcA). Also included in the experiment were ferrated synthetic chelate ethylenediamine [di(o‐hydroxyphenylacetic)acid] (Fe‐EDDHA) and Amaranthus spp plant complexed iron (Fe‐AR). Soil applications were compared with foliar spray treatments using ferrous sulfate, FOB and the Fc‐FcA mixture. Each siderophore source was applied at rates of 1.1 and 3.3 kg Fe ha‐1 soil. The synthetic chelate and Fe‐AR sources were applied at 3.3 kg Fe ha‐l soil.
Untreated soil produced severely chlorotic and stunted sorghum plants, some of which died prior to harvest. However, sorghum receiving soil applications of Fe‐EDDHA and Fe‐AR exhibited only very slight and moderate Fe chlorosis symptoms, respectively. More severe chlorosis was evident on plants treated with ferrated siderophores regardless of iron rates. Sorghum responded to foliar sprays of ferrous sulfate whereas little response to foliarly applied Fc‐FcA mix and FOB was observed.
Dry matter yields were highly variable with Fe source and ranged in the following order for soil applications: Fe‐EDDHA ≫ Fe‐AR > Fc‐FcA = Fe‐FOB = Fe‐RA. Iron uptake generally followed similar trends with the exception of FOB, from which there was slightly higher uptake than other siderophore carriers. Response to foliar sprays were highest for the ferrous sulfate source.