Soil factors affecting iron chlorosis of soybean in the Red River Valley of North Dakota and Minnesota

Iron chlorosis tolerant soybeans exhibit chlorosis symptoms in the Red River Valley of North Dakota and Minnesota. Previous research suggests that chlorosis was generally related to high calcium (Ca) carbonate levels in the soil, so a relationship of chlorosis with the presence of Calciaquoll (carcareous at the surface, with very high levels of calcite in the subsurface) and non-Calciaquoll soil types would be expected. Six sites each in 1996, 1997, and 1998 were studied to identify soil factors correlated with the incidence and degree of chlorosis. Gradients between green soybeans and severely chlorotic plants were established, soil type was determined at the green and chlorotic gradient endpoints, and soil samples were taken at the 0-15 cm depth at each gradient location. Iron chlorosis symptoms were filmed using a video recorder. The images recorded were then analyzed using an image analyzer to give a digital chroma number. The digital values were correlated with soil factors to determine the degree of relationship of symptoms with each factor. Soil type differences were only associated with chlorosis at four of twelve sites. Both Ca carbonate equivalence and soluble salts were most often correlated with chlorosis symptoms. The relationship of iron chlorosis with soluble salts, along with soil carbonate level, appears to be a factor that should be considered in soybean chlorosis resistance breeding for the Red River Valley. A greenhouse experiment was conducted which was unsuccessful in duplicating field chlorosis symptoms when gypsum and Ca carbonate was added to soil obtained from non-chlorotic areas. The study was able to show a decrease in soybean nodule number with increasing Ca carbonate and gypsum levels.