Evaluation of coated seeds as a Rhizobium delivery system for field pea

Greenhouse and field experiments were conducted with field peas (Pisum sativum, L.) in soils of pH 3.4 to 6.8 to determine the best rate of inoculation with rhizobium and to evaluate pre-inoculated (coated) seeds as an alternative to the traditional seed inoculation method of using sticking agents. Inoculation rates higher than 10(5) cells seed(-1) were usually required for high nodulation, nitrogen fixation and grain yields. Therefore, Canadian standards, which require that 10(5) nodulating rhizobia be delivered per seed for large-seed legumes like peas, may need to be increased. Counts of rhizobia on coated seeds were about 3 log units lower than those on freshly inoculated seeds, but coated seeds significantly outperformed standard seed-inoculated seeds in nodulation and crop yield in acid soils (pH 4.4 and 4.7). However, field pea yields were too low to have commercial value at these low pH levels. In soils with higher pH, standard inoculation resulted in greater nodulation and yield, but the differences were not always significant. It is concluded that the use of coated seeds provides a yield advantage for field pea grown on acid soils, but liming would probably be a better option. Use of coated seeds on other soils will depend on the trade-off between the time and money saved in inoculation in, order to seed early and a possible reduction in yield due to insufficient numbers of rhizobia being applied.