Field experiments with rhizobia

In this chapter, we discuss methodologies to select rhizobial strains for their ad¬aptation to field conditions as this is an important trait when developing inocu¬lant-quality strains. Techniques to assess nitrogen fixation per se are described in Chapters 5 and 10, and traits related to manufacturing in Chapter 9. For many legume symbioses, the greatest challenge in developing inoculants is to select strains that confer consistent nodulation upon the legume in the target soil environment. Edaphic stresses after inoculation, such as dry heat and acidity, can be detrimental to rhizobial survival, so selecting strains tolerant of these stresses is very useful. Some legumes regenerate annually from hard (impermeable) seeds in the soil and therefore, like their rhizobia, are only sown once in several dec¬ades. Hence, assessment of the ability of strains to persist in the field environment through several seasons is required. This trait cannot be reliably predicted without field experimentation (Howieson et al. 2000). Many legume evaluation programs over the years have been compromised because inoculants suitable to the field conditions have not been available.