Pea Nutrient Requirements in Alberta

With the introduction of new pea cultivars that featured improved agronomic traits, Alberta pea acreage rose from less than 10,000 acres in the mid 1970's to more than 640,000 acres in 2000.

As acreage grew, so did the demand for information on the nutrient requirements for pea being grown in Alberta. Much of this interest focused on whether inoculant, nitrogen and phosphate fertilizers were needed for this crop under Alberta conditions.

Alberta Agriculture, Food and Rural Development led a study from 1995 to 1998 to find out what effects rhizobia inoculation, starter nitrogen (N), phosphorus (P), potassium (K) and sulfur (S) fertilizers had on the yield and protein content of four semi-leafless pea cultivars. Trials were conducted at 58 sites across Alberta over a wide range of agro-eco zones.

The study was done in partnership with Agriculture and Agri-Food Canada and Westco, with funding from the Alberta Agricultural Research Institute, Alberta Pulse Growers Commission, Agrium and Westco.

The study had four objectives:

• to examine the responsiveness of different types of pea cultivars to rhizobium bacteria.
• to determine the need for additional nitrogen fertilizer for optimum crop production.
• to correlate pea response to phosphate fertilizer with plant-available soil phosphorus.
• to examine the need for K and S fertilizers.

• Baroness (large yellow)
• Carneval (small yellow)
• Majoret (medium green)
• Radley (small green)

• are semi-leafless with shorter vine length and upright growth habit, both of which increase the ease of harvest.
• have higher yield potential and are more suitable for human consumption than previous cultivars.

• different pea cultivars differ in their genetic yield and protein potential. Pea cultivars varied significantly in yield at 71 per cent of the conducted trials, while protein differed at 85 per cent.
• nutrient levels required by higher-yielding pea cultivars will generally be proportionally higher than cultivars with lower yield potential.
• Radley pea consistently had the lowest yield and the highest protein concentration.
• Carneval and Majoret pea frequently had the highest yields, while Carneval pea had the lowest protein concentration.
• Baroness pea produced yields similar to Carneval pea and Majoret pea, except in the Black soil zone, where it was lower.
• Majoret pea consistently had the highest seed N yield (total nitrogen in the seed), which was 9 to 27 per cent higher than Radley pea. Radley had the lowest seed N yield simply due to lower yield.

• rhizobia inoculation increased pea seed yield in 41 per cent of trials across Alberta by an average of 14 per cent. Pea yield was unaffected by inoculation in the remaining trials.
• in inoculated treatments on land with no history of legumes, the yield rose only slightly more often than on inoculated land that previously had a legume crop (45 per cent versus 38 per cent). However, the yield increase after inoculation was significantly larger (19 per cent vs. 5 per cent) on land with no history of legumes.
• inoculation had a major effect on pea cultivars in 6 of 22 trials. Majoret pea responded the best to inoculation, while Baroness pea was the least responsive. From this study, it was estimated that approximately 80 per cent of N in the pea plant comes from nitrogen fixation.
• application of N fertilizer improved pea yield in 24 per cent of the 58 trials conducted with an average increase of 9 per cent. When spring soil test N to 12 inches (30 cm) was less than 18 lb/ac (20 kg N/ha), application of N fertilizer increased pea yield in one third of the trials by an average of 11 per cent. The modest benefits of rhizobia inoculation or starter N fertilizer in most trials, despite appreciable deficits between spring soil test N and accumulation of N by pea, was likely due to adequate nodulation of pea by naturally occurring rhizobia.
• generally, the effects of applied N fertilizer on protein concentration were small. Increases in protein concentration due to addition of N fertilizer were most frequent when soil test nitrate nitrogen (NO₃-N) was less than 18 lb N/ac (20 kg N/ha). In trials that showed significant benefit from applied N fertilizer, the average protein concentration increased significantly with the rate of N application from a 0.9% increase at 18 lb N/ac (20 kg N/ha) to a 2.9% increase at 54 lb N/ac (60 kg N/ha)

• Pea benefits little from starter N. Starter N is only recommended when soil test N levels are less than 18 lb/ac in the 0 - 12 inch soil depth, in the Brown, Dark Brown and Thin Black Soil areas. Generally, nitrogen fertilizer is not required for pea grown on Black or Gray Wooded soils in central and northern Alberta. Frequently, most irrigated fields in southern Alberta have soil N levels greater than 40 lb N/ac and therefore do not require N.
• If soil test N levels are less than 18 lb N/ac (20 kg N/ha), a small amount of N fertilizer should be either seed-placed or side banded at the time of seeding. N fertilizer could also be banded or broadcast and incorporated before seeding. Small amounts (< 20 lb N/ac) of urea (46-0-0) or ammonium nitrate (34-0-0) can be safely seed-placed, when seed bed moisture is good.
• Soil tests are important when choosing whether to apply fertilizer. Generally speaking, with the exception of beans, little or no additional N fertilizer is needed by most pulse crops. In fact, excess N fertilizer may reduce the amount of N fixed by a pulse crop and delay crop maturity. Mid-season N applications are normally not recommended except when inoculation has failed and there is an obvious N deficiency.

• applying phosphate significantly increased pea seed yield at 19 of 52 sites. The average increase in seed yield from a phosphate application was 7 per cent.
• the average yield benefit was similar in the Thin Black, Black and Gray Wooded soil zones, but was negligible in the Dark Brown soil zone and in irrigated trials.
• of the 31 trials with soil test P (modified Kelowna method) levels of less than 27 lb P/ac (30 kg P/ha) to 6 inches (15 cm), 52 per cent had a significant yield increase due to the application of phosphate, while only 1 of 17 trials with soil test P levels of more than 27 lb P/ac (30 kg P/ha) had a significant yield increase. Application of 27 lb P2O5/ac (30 kg P2O5/ha) was enough to get close to maximum yields in trials with soil test P levels of less than 27 lb P/ac.
• Phosphate placement had no effect on pea yields. There was no difference in pea yield between seed-placed and banded P. The minimal impact of seed placement on yield in this study was likely due to the less damaging effects to seedling growth of 0-45-0 than of the more commonly used source of P, monoammonium phosphate (MAP). Seed protein and P concentrations were not strongly affected by phosphate fertilizer application.

• given the uncertainty of naturally occurring levels of rhizobia in soil and the responsiveness of the many pea cultivars available to growers, rhizobia inoculation is strongly recommended. Rhizobia inoculation represents a cost-effective means of ensuring maximum pea yields. However, the frequency and magnitude of inoculation benefits are likely to be less than 50 per cent, especially in fields that have regularly grown a pea crop.
• the benefits from applying starter N with pea are infrequent and usually small. Generally, starter N is only recommended when soil test N levels are less than 18 lb N /ac in the 0 to 12 inch soil depth in the Brown, Dark Brown and Thin Black soil areas of Alberta. Starter N is normally not recommended in the Black and Gray Wooded soil areas of Alberta. Nitrogen fertilizer recommendations are summarized in Table 2.
• P fertilizer is not particularly effective on pea. Results from this project suggest that P fertilizer works best when soil P levels are <27 lb P/ac. Above this level, it is unlikely pea will respond to P fertilizer. Phosphate fertilizer recommendations are summarized in Table 4.
• potassium and sulfur were not found to be major factors on pea yield. Adding K and S fertilizer resulted in significant yield increases in only 3 of 44 trials. No relationship was seen between the response to K and S additions and soil test levels. General recommendations and guidelines were developed for pea with K in Table 5 and S fertilizers in Table 6.

Source: Agdex 142/532-2. April 2002.