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

Greenhouse and field experiments were conducted with field peas (Pisum sati vum, L.) in soils of pH 3.4 to 6.8 to determine the best rate of inoculatio n with rhizobium and to evaluate pre-inoculated (coated) seeds as an altern ative to the traditional seed inoculation method of using sticking agents. Inoculation rates higher than 10(5) cells seed(-1) were usually required fo r high nodulation, nitrogen fixation and grain yields. Therefore, Canadian standards, which require that 10(5) nodulating rhizobia be delivered per se ed for large-seed legumes like peas, may need to be increased. Counts of rh izobia on coated seeds were about 3 log units lower than those on freshly i noculated 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 g reater nodulation and yield, but the differences were not always significan t. 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 op tion. Use of coated seeds on other soils will depend on the trade-off betwe en the time and money saved in inoculation in, order to seed early and a po ssible reduction in yield due to insufficient numbers of rhizobia being app lied.