INDIGENOUS SOIL FACTORS INFLUENCING IRON CHLOROSIS OF SOYBEAN IN CALCAREOUS SOILS

Iron deficiency is the most prevalent and difficult to control of the plant-nutrient deficiencies encountered on calcareous soils and may be especially severe for soybean [Glycine max (L.) Merr.]. This study was conducted to determine the predominant indigenous soil properties, i.e., those soil properties which are largely unaffected by environmental and management conditions, that influence Fe availability to soybean on calcareous soils. Three varieties of soybeam were grown in a growth room in sand/soil mixtures of 23 calcareous soils that were selected to give wide ranges of clay, Fe oxide, CaCO3, and organic matter contents and exchange ion composition. Iron-deficiency stress was quantified using visual chlorosis ratings in addition to chlorophyll and Fe concentrations of the plant tissue. The concentration of amorphous Fe oxide in the soil was an important factor influencing the uptake of Fe by soybean. Chlorophyll concentration of the plant tissue was negatively correlated with soil-solution ionic strength and reactivity of the soil carbonate phase. The carbonate effect was likely due to the influence of the highly-reactive carbonate phase on rhizosphere pH and its subsequent influence on effectiveness of the Fe-stress response mechanism of soybean. These results differ from results of a previous study with sorghum, in which concentration and reactivity of the carbonate phase of calcareous soils had no influence on the incidence of Fe chlorosis. The different behavior of soybean and sorghum with respect to carbonate reactivity may be attributed to the different predominant Fe-stress response mechanisms of these crops.