Soil Acidity Tolerance of Symbiotic and Nitrogen-Fertilized Soybeans1

In lIOybean (Glycine max L.), unlike other legumes, poor growth in add 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 (N~ependent) plants as compared with control plants fertilized with NH,NO. There were two lime x N factorial trials with acid soils: a greenhouse trial in a Typic Haploxerult, with two cultivars and 13 single-strain Ibizobial 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 AI. In both trials, liming from pH 4.4 to 6.0 (aqueous paste) doubled growth, regardless of N-tource, cuftivar, 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 lIOybean growth in the acid soils was limited by Al toxicity to the host plant were confirmed in solution culture experiments with pH, AI, and Ca controlled at levels resembling those found in extracts of the soil solutions. Growth was unaffected by low Ca (200 J.lM) or low pH (4.5), but was depressed at 30 and 65 p.M Al. The data suggest that efforts to improve acid tolerance in soybean should center on plants, not rhizobia. Additional index words: Legume nutrition, Rhizobium, Root nodulation, Nitrogen fixation, Aluminum, Calcium, Strain and cultivar selection. I MPAIRMENT of legume nodulation and nodule function by an environmental stress can induce N starvation and limit growth of the host plant. Conversely, impairment of host plant growth by an environmental stress can limit nodulation and N2 fixation (8, 10). The distinction between these two cases is still ignored to the detriment of otherwise useful research (3, 4). Yet it is a simple distinction to make. In both cases there will be less plant growth, nodulation, N yield, and acetylene reduction. But if slow N2 fixation is limiting growth there will also be evidence of N-<leficiency; e.g., low N, yellow leaves, and positive response to fertilizer N. Nitrogen-fertilized control plants will be less affected than symbiotic plants if the stress primarily impairs fixation. Soil acidity has long been known to induce N-deficiency in legumes by preventing normal nodulation. Acidity, Al toxicity, and Ca deficiency inhibit Rhizobium growth, root infection, and nodule activity enough to account for symbiotic failure. This information agrees wi th more direct evidence of N deficiency, to support the general conclusion that legumes become more sensitive to soil acidity if they depend on symbiotic N2 fixation (9). But the conclusion might not be correct for aU legumes. 'Contribution from the Dep. of Land. Air, and Water Re· sources, Univ. of California, Davis, CA 95616. Supported by a USDA s~cial grant for soybean research and by US Agency for Internauonal Development through the Univ. of Hawaii NifTAL Project. Received 12 June 1980. • Professor of lIOiI science and graduate research asaistants. 407 An important exception mightbe soybean (Glycine max L. Merr.). This paper presents evidence that soil acidity can impair growth of soybeans equally, whether they are N-fertilized or "symbiotic" (dependent on N2 fixation), because the effective limitation of growth and N2 fixation is the host's susceptibility to Al toxicity. The evidence came from simple experiments comparing responses of N-supplied and symbiotic plants to acidity in two soils, with 13 strains of Rhizobium japonicum and nine currently commercial varieties of soybean. The importance of Al was clarified by solution culture experiments comparing the factors pH, AI, and Ca controlled at levels representing those measured in the soil solutions. MATERIALS AND METHODS Soybean seeds were supplied by Dr. B. H. Beard from the USDA soybean breeding project at Davis. The varieties were 'Ada', 'Amsoy', 'Clark', 'Evans', 'Grande', 'Kanrich', ·Lee'. PI297550, and ·Williams·. They represent maturity groups 00. r. III, IV, and V. Strains of Rhizobium were the most effective recently tested on soybean by the University of Hawaii NiITAL Project (strains CBl809, CC709, USDAllO. TALl02, and the O. N. Allen strains 511, 519, 527, and 542) and by the Nitragin Co., Milwaukee (6IA76. 61AIOI, 61AI24, 61A144, and 6IAI50). Rhizobia were maintained on slants of yeast mannitol agar. Inoculants were made from yeast mannitol liquid cultures (barely turbid at 5 X 10' cellsjml), by diluting with 0.3 roM CaCI. + 0.3 mM MgSO,. Th dilutions were pipetted onto seed at planting immediately before covering, and subsamples were plate.counted. Soils and soil treatments are outlined in Table 1. We used subsoils from two California Ultisols chosen for ability to give large response to lime, N. and inoculation. Soil A. Josephine series (Typic Haploxerult), came from the foothills of the Sierra Nevada east of Sacramento. Soil B, Hugo-Josephine intergrade (Typic Hapludult), came from the Coast Range west of Sacramento. Both soils had kaolinite and oxide clay mineralogy. less than 0.1'70 organic matter. and no rhizobia capable of nodulating soybean. All treatments received I millimole of K.SO,jkg soil, and either 4 millimole KH.PO, (Soil A) or 4 millimole Ca(H.PO,). (Soil B). Zinc and Mo were supplied at 5 and 0.1 ppm, respectively. These basal fertilizer treatments and the pH amendments (Table I) were mixed with the soil, followed by watering and a reaction period of 3 weeks (Experiment I) or 13 weeks (Experiment 2).