True chlorosis is not just a pale yellow color but shades of yellowness from subtle to bright yellow between the veins of the leaf. Many factors contribute to chlorosis, Franzen says. The first is presence of free carbonates, especially bicarbonate, resulting from a combination of water and carbonate minerals in the soil. So in dry periods, chlorosis can be relatively minor, because bicarbonate levels are low. As the soil becomes moister, bicarbonate concentrations increase and the plants become less able to take up iron.
Cool weather slows down plant systems, including iron uptake.
Soluble salts in the soil contribute to chlorosis in supposedly chlorosis tolerant varieties. There are wide differences between soybean varieties in their tolerance to chlorosis. Another concern of soybean growers is the impact of herbicide stress. Studies indicate that herbicides may further stress already stressed soybeans. Soybeans growing on higher salt soils seem to perform most poorly when treated with harsh contact herbicides or herbicides with systemic activity.