Is inoculation necessary for peas?
Field peas can fix atmospheric nitrogen (N2) through a symbiotic relationship with Rhizobium leguminosarum. This bacteria is native to prairie soils, yet farmers typically inoculate peas.
In field studies conducted Alberta, peas responded to inoculant 45% of the timeon land with no history of peas.1 Yields of inoculated peas in these studies were 14% higher than the non-inoculated control, on average. In fields with a history of peas, 38% of fields responded to inoculant, where yields were only 5% greater than non-inoculated peas.1
With a long break period recommended between pea crops, uncertainty over surviving rhizobia levels and the effectiveness of native populations or surviving strains at facilitating N2 fixation, supplementary inoculant is a cost-effective way to ensure maximum N2 fixation and yield.
Inoculant Formulations and Placement
The most effective inoculant formulation at increasing nodule number, N2 fixation and seed yield, is soil-applied granular inoculant, followed by seed-applied powder (peat) and seed-applied liquid (Figure 1).2,3 However, all three formulations can facilitate successful N2 fixation. Peat and granular formulations provide some protection to rhizobia against environmental stress (e.g., dry conditions) compared to liquid formulations.
Currently, there is limited research-based information on double vs. single inoculation in peas. Following similar guidelines as soybeans, consider double-inoculating first-time pea fields. This means two different formulations and placements of inoculant (e.g., liquid on seed plus granular in-furrow). Single inoculate if the field has a history of peas and if previous crops have nodulated well. If you are considering not applying inoculant to your peas, test this practice on your farm first in a replicated, randomized trial to determine the impact on the crop.
Peas will take up a portion of their required N from soil reserves. Proper inoculation generally eliminates the need for starter N fertilizer. McKenzie et al. (2001) found that 33% of trial sites with <20 kg N/ha in the top 12 inches of soil responded to starter N and increased pea yields by 11%, on average.1 High rates of urea (35-71 lbs N/ac) applied at seeding can reduce nodule number and N2 fixation.2
Starter phosphorus (P) fertilizer has been shown to increase pea yields even on soils with high P levels.4 The maximum safe rate of seed-placed P is 20 lbs P2O5/ac with seed bed utilization (SBU) >15%. P-fertilizer should be placed away from the seed-row with lower SBU.
1 McKenzie, R. H., Middleton, A. B., Solberg, E. D., DeMulder, J., Flore, N., Clayton, G. W. and Bremer, E. 2001. Response of pea to rhizobia inoculation and starter nitrogen in Alberta. Can. J. Plant Sci. 81: 637–643.
2 Clayton, G. W., Rice, W. A., Lupwayi, N. Z., Johnston, A. M., Lafond, G. P., Grant, C. A. and Walley, F. 2004a. Inoculant formulation and fertilizer nitrogen effects on field pea: Nodulation, N2 fixation and nitrogen partitioning. Can. J. Plant Sci. 84: 79–88.
3 Clayton, G. W., Rice, W. A., Lupwayi, N. Z., Johnston, A. M., Lafond, G. P., Grant, C. A. and Walley, F. 2004b. Inoculant formulation and fertilizer nitrogen effects on field pea: Crop yield and seed quality. Can J. Plant Sci. 84: 89–96.
4 Johnston A. M. and Stevenson, F. C. 2001. Field pea response to seeding depth and P fertilization. Can. J. Plant Sci. 81: 573–575.