Symbiotic nitrogen fixation of Rhizobium (Galega) in acid soils,and its survival in soil under acid and cold stress

Summary Goat's rue (Galega orientalis) is a potential perennial forage legume for northern temperate acid soils. Greenhouse experiments were performed to compare symbiotically nitrogen fixing goat's rue with plants receiving mineral nitrogen in five different acid soils. Soil acidity had the same effect on yields of symbiotically grown plants as on plants receiving mineral nitrogen, suggesting that the acid sensitivity of the symbiosis was not limiting plant growth, even under very acidic conditions. The survival of an antibiotic resistant Rhizobium (Galega) strain in acid soil and freezing conditions was also studied. The survival of the bacteria was not affected at 15°C, when the pH of the soil (measured in 0.01M CaCl₂) was 5.2 or 4.9. In pH 3.4, and after freezing to –5°C, the population density decreased from 3×10⁸ to 1×10⁵/g in a few weeks. It is concluded that goat's rue, its symbiotic nitrogen fixation and R. (Galega) are tolerant of moderately acid agricultural soils, but that harsh winters may reduce bacterial numbers in the soil.