Can plant productivity be increased by inoculation of tree roots with soil microorganisms?

Fostering symbiotic associations between appropriate soil microorganisms and their compatible hosts lies within the management capabilities of agriculturalists and foresters. Using knowledge of the fundamental scientific bases for these associations, one can facilitate the development of beneficial symbioses by inoculation of seeds, seedlings, or growing plants with selected microorganisms to establish and perpetuate effective symbioses leading to increased productivity. Of particular interest to the forester are four major groups of symbiotic associations: Rhizobium or Bradyrhizobium–leguminous trees; Frankia–actinorhizal plants; ectomycorrhizae–host trees; and endomycorrhizae–host trees, including vesicular–arbuscular mycorrhizae. Summarized here are the isolation, characterization, and culture of the microbial symbionts; the demonstrated specificity for infection and effectivity for facilitating nutrient uptake in each case; and the development of the technology for field inoculation to achieve effective symbioses in forest plantations. The factors involved in successful inoculation procedures are reviewed, and recommendations are made as to some of the necessary steps to further the development of this biotechnology.