Soil Acidity Tolerance of Symbiotic and Nitrogen‐Fertilized Soybeans1

Abstract

AbstractIn soybean (Glycine max L.), unlike other legumes, poor growth in acid soil might not be due to nodulation failure. This possibility was tested by observing lime effects on nodulation, early growth, and N concentrations in symbiotic (N2‐dependent) plants as compared with control plants fertilized with NH4NO3. There were two lime ✕ N factorial trials with acid soils: a greenhouse trial in a Typic Haploxerult, with two cultivars and 13 single‐strain rhizobial inoculants; and an outdoor container trial in a Typic Hapludult, with nine cultivars and three rates of inoculation that produced large differences in nodule weight and number. The soils were high in exchangeable and soluble Al. In both trials, liming from pH 4.4 to 6.0 (aqueous paste) doubled growth, regardless of N‐source, cultivar, or Rhizobium strain or numbers; and inoculated plants were nodulated, green, and high in N even when their growth was severely acid‐affected.Symptomatic indications that soybean growth in the acid soils was limited by Al toxicity to the host plant were confirmed in solution culture experiments with pH, Al, and Ca controlled at levels resembling those found in extracts of the soil solutions. Growth was unaffected by low Ca (200 µM) or low pH (4.5), but was depressed at 30 and 65 µM Al.The data suggest that efforts to improve acid tolerance in soybean should center on plants, not rhizobia.