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.
Is inoculation necessary for peas?
In field studies conducted on land with no history of peas in Alberta, peas responded to inoculant 45% of the time.1 The magnitude of yield response to inoculant in these studies was, on average, 14% higher than the uninoculated control. In contrast, 38% of fields with a history of peas responded to inoculant but the magnitude of yield response was much lower (5%).1 With recommendations to increase the break period between pea crops and the uncertainty of levels and efficiency of native rhizobia populations, supplementary inoculant is a cost-effective means of ensuring maximum N2 fixation and yield.
The most effective inoculant formulation to increase nodule number, N2 fixation and seed yield is soil-applied granular followed by seed-applied powder followed by seed-applied liquid inoculant (Figure 1).2,3 However, peat and granular formulations may provide some protection to rhizobia against environmental stress, such as desiccation, compared to liquid formulations.
Peas will take up a portion of their required N from soil reserves. However, 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 pea yields increased by an average of only 11%.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 high P soils.4 Maximum seed-row safe rates are 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.