Growth response of maize roots to Azospirillum inoculation: Effect of soil organic matter content, number of rhizosphere bacteria and timing of inoculation

Root surface area of maize seedlings was the most reliable criterion in evaluation and measurement of the growth responses of maize inoculated with Azospirillum. The optimal concentration of Azospirillum inoculum for increasing root surface areas was 10⁷ colony forming units (cfu) per plant. A rhizosphere-inhabiting fluorescent Pseudomonas that was used for comparison did not cause any change in root surface area. No differences were observed when maize was inoculated either at pre-emergence, post-emergence or by combining both treatments. A high concentration of Pseudomonas (10⁸ cfu g^-1 sand) diminished the influence of Azospirillum on root surface area. Azospirillum did not contribute to the increase of root surface area once the organic matter mixed with sand exceeded 1% by weight.