Open access

Seeding rate and sulfur drive field pea yields in the Maritime region of Canada

Publication: Canadian Journal of Plant Science
9 February 2024

Abstract

The inclusion of pulse crops in Canadian rotations has the potential to improve cropping system efficiencies, reduce the overall amount of applied nitrogen, provide economic opportunities for producers, and reduce the overall carbon footprint of the cropping system. Although primarily grown in western Canada, many pulse species—field pea in particular, are well suited to temperate growing conditions in the Maritime region of Canada. A study was conducted over 2 years at Harrington, Prince Edward Island, and consisted of four field pea varieties including two yellow varieties (AAC Lacombe and CDC Saffron) and two green varieties (CDC Limerick and CDC Raezer) planted at three plant population densities: 75, 100, and 125 plants m−2. The study also measured the effects of nitrogen fertilizer applied pre-plant (0 kg ha−1 vs. 15 kg ha−1) and applied plant available sulfur (0 kg ha−1 vs. 25 kg ha−1). Overall, yellow pea varieties were higher yielding than green pea varieties, and there was a linear increase in yield with increased seeding rate. There were no significant effects of pre-plant nitrogen fertilizer on yield, although it did slightly increase seed protein. Applied sulfur had a positive effect on yield and a slightly negative effect on thousand seed weight. This experiment provides a recommendation for the optimal seeding rate (100 plants m−2) and fertility recommendations to achieve profitable yields growing field pea in the Maritime region of Canada.

Introduction

Pulse crop production is an integral part of the agricultural landscape in Canada. The inclusion of pulse crops in a rotation holds benefits for overall system productivity in terms of the effects on soil biodiversity (Bainard et al. 2017) and greenhouse gas (GHG) mitigation (Lemke et al. 2007), as well as productivity and quality of succeeding crops (Irvine et al. 2013). Field pea (Pisum sativum L.), in particular, has shown growth in the past decade, but that growth has primarily been limited to western Canada (Bekkering 2014; Statistics Canada 2023). Recent increased interest in plant-based protein for feed and food is helping to expand land dedicated to pea production in all agricultural regions of Canada. Although field pea was traditionally grown as a component of a mixed grain production system (oat, pea, and barley) in the Maritime region of Canada (Johnston et al. 1978), improvements in locally adapted varieties and pest and disease control options have allowed for this crop to perform well in the current climate and soils of Prince Edward Island.
Since 2010, SO2 emissions have dropped from approximately 1.0 × 106 tons released to 0.5 × 106 tons recorded in 2019 (Environment and Climate Change Canada 2019). A study by Pötzsch et al. (2019) reported that when background sulfur in northern Europe was sufficient (atmospherically deposited) there were limited effects of sulfur application on yield, especially when applied in the elemental form. A recent study in the Maritimes has shown a steady decrease in soil sulfur over the past two decades where the majority of fields tested moved from a high soil sulfur concentration (>25 mg kg−1) to a low soil sulfur concentration (7–18.5 mg kg−1) (Nyiraneza et al. 2019).
The Maritimes have a high annual rate of precipitation with the 30-year average at approximately 1500 mm year−1 (Vaughan et al. 2001). The soils of Prince Edward Island are exceptionally light and sandy with declining amounts of organic matter content (Nyiraneza et al. 2017). Both nitrogen and sulfur have a high propensity for leaching from Maritime soils and there is a need for a deeper understanding of plant response to added nitrogen and sulfur, particularly for leguminous crops grown in the region. Low soil organic matter combined with low soil pH may also affect plant nodulation in legumes (Bollman and Vessey 2006) and limit the capacity for these crops to fix atmospheric nitrogen for growth. For Prince Edward Island specifically, previous work has shown that a combination of various edaphic factors (low organic matter, high precipitation, and extensive use of primary tillage) has resulted in an overall reduction in the amount of plant available sulfur in the soil (Nyiraneza et al. 2019). Other studies in the region have shown a positive yield response from sulfur fertilizer addition in forage (Zheljazkov et al. 2006), wheat, and soybean (Parsons et al. 2007).
There is increased interest to include pulses in local cropping systems to increase both crop rotation diversity and diversity of income streams. Therefore, it is now important to have local data on which to base management decisions in the region. Information on the appropriate seeding rate and nitrogen and sulfur fertilizer application rates are needed for the development of a field pea production strategy in the region. Previous work conducted in other pulse growing areas of the world has shown a positive response to increasing seeding rate (Gan et al. 2003; Johnston and Stevenson 2001; Nleya and Rickertsen 2011). The interaction between applied fertilizer and adapted cultural practices provides essential information for producers to manage pulse crops for sustainability and profitability in the Maritimes.
The objective of the study was to evaluate common cultural practices (seeding rate, nitrogen, and sulfur fertilizer treatments) that could help to improve pea yields in the Maritimes. The results from this experiment will provide seeding rate and fertility recommendations for producers in the region. This information will help to mitigate the risk during the early adoption phases of the introduction of a new crop.

Materials and methods

A study was conducted to evaluate the effects of seeding rate and pre-plant fertilizer on four field pea varieties in 2017 and 2018 at the Harrington Research Farm, located North of Charlottetown in Prince Edward Island, Canada. Soil at the site is an orthic humic ferro podzol (Soil Landscapes of Canada Working Group 2021) categorized as Charlottetown series (moderately well-drained light sandy loam). During both years of the study, plots were seeded following a barley (Hordeum vulgare L.).
The experimental design consisted of a Latinized split, split plot with three replicates per treatment (4 varieties × 3 seeding rates × 2 sulfur treatments × 2 nitrogen treatments × 3 replicates = 144 experimental units). The Latinized model was balanced to permit the estimation of both intrablock and interblock variation. This allows estimation of variation in two directions. The interblock variation estimation reduces the error term resulting in more precision from predictions. Latinized designs are the preferred model to allow for the maximum estimation of experimental variation (John and Williams 1995).
Soil was sampled to a depth of 15 cm from a representative area of the study site. Soil was bulked and a sub sample was removed and sent to a commercial lab for analysis (PEI Soil and Feed Lab, Charlottetown, PE). For analysis, soils were extracted using a Mehlich-3 solution (Mehlich 1984), and nutrients were quantified using inductively coupled plasma emission spectroscopy using a Radial ICAP 6500 (ThermoFisher Scientific, Waltham, MA). Field pea seed consisted of two green pea varieties (CDC Limerick and CDC Raezer) and two yellow pea varieties (AAC Lacombe and CDC Saffron). CDC Limerick and CDC Raezer are both semi-leafless green pea varieties with good lodging and powdery mildew resistance. Both varieties have high protein and good yield potential, and both were developed at the Crop Development Centre in Saskatoon, SK (Warkentin et al. 2014a, 2014b). CDC Saffron and AAC Lacombe are both semi-leafless yellow pea varieties with good lodging and powdery mildew resistance. CDC Saffron was developed at the Crop Development Centre in Saskatoon, SK (Warkentin et al. 2014c); AAC Lacombe was developed by Agriculture and Agri-Food Canada at the Lacombe Research and Development Centre (Bing et al. 2014). Seeding rate as a factor consisted of three targeted plant populations (75, 100, and 125 viable seeds m−2). Germination tests were performed on all seed lots before planting to ensure the seeding rate was calculated based on the desired number of living plants m−2. This approach considers the percent germination of each variety as well as seed size and weight. Seed was inoculated immediately before planting using a peat-based pea inoculum (Cell-Tech®, Novozymes, Ottawa, ON) and was planted using a Wintersteiger Plotseed XL belted cone research plot seeder (Wintersteiger, Salt Lake City, UT). Plot size was 1.8 m × 3.5 m (6.3 m2) with 12 plant rows at 15 cm spacing. Fertilizer treatments included either nitrogen (15 kg ha−1) applied as ammonium nitrate (47 kg ha−1 of 32–0–0), sulfate sulfur (30 kg ha−1) applied as K-MagTM (136 kg ha−1 of 0–0–22–10 Mn-22 S, The Mosaic Company, Regina, SK), or both. An untreated control for all measured factors was included in the design. Fertilizer treatments were applied pre-plant using a Valmar 1255 pneumatic spreader (Salford Group, Norwich, ON) and incorporated using triple “K” harrows before planting. This method is the standard practice for broadcast fertilizer placement in the region. Soil tests showed sufficiency for all plant available nutrients outside of those being evaluated; therefore, no supplemental fertilizer was required. Within 2 days of planting, a pre-emergent herbicide (S-Metolachlor (915 g L−1 a.i.)) was applied to the soil at a rate of 1.75 L ha−1 in 200 L ha−1 of water. Fungicide was applied at 20% bloom each year and consisted of a single application of boscalid (LanceTM) applied at 420 g ha−1 (70% a.i.) in 100 L ha−1 of water. To facilitate harvesting during the 2017 growing season, diaquat (240 g a.i. L−1, RegloneTM) was applied as a harvest aid at a rate of 1.7 L ha−1. A desiccant was not required for the 2018 harvest season.
Plant count data were collected from all treatments, both years, between V3 and V5 growth stages each year. Counts were performed at two distinct locations per plot on a randomly selected section of one linear meter. Seeds were harvested by mid-August for both 2017 and 2018 using a Wintersteiger plot combine (Winterstieger, Salt Lake City, UT) when seed moisture was approximately 16%. Harvested samples were further air dried, and yields were corrected to 12% moisture before analysis. Seed yield and thousand seed weight (TSW) were determined postharvest, and both protein and moisture were determined using near infrared spectroscopy (NIRS) (SpectraStar 2500, Unity Scientific, Brookfield, CT). NIRS calibrations were built on factory settings and validated by comparing instrument results with results obtained from a commercial lab every 3 years (Prince Edward Island Soil and Food Lab, Charlottetown, PEI).

Statistical analysis

Data were analyzed using the REML package of GENSTAT (VSN International 2022) and the ggpubr package (Kassambara 2023) of R (R Core Team 2023). A mixed models Analysis of variance approach was used to measure the influence of treatment factors on the yield and yield components of all harvested material. “Year” and “block (rep)” were included as random factors; “variety”, “seeding rate”, “nitrogen application”, and “sulfur application” were included as fixed factors. A correlation matrix was constructed from mixed models analysis outputs to conduct principal components analysis (PCA). PCA biplots were used to visualize relationships between factors and variates in Euclidian space under low, medium, and high plant population conditions.

Results

Weather and soil

The experiment was conducted at Harrington research farm located North of Charlottetown in Prince Edward Island, Canada. The soil is Charlottetown series (moderately to well-drained sandy loam till) Orthic Podzol (Soil Landscapes of Canada Working Group 2010). A pre-plant soil analysis was performed to assess soil chemical status for each year of the study (Table 1). According to the soil tests, there was approximately 15 kg ha−1 of plant available sulfur present in the soil for both years. Other soil nutrients were sufficient for plant growth including potassium, phosphorus, and magnesium. Weather conditions were close to the 30-year averages for temperature and rainfall for both years (Fig. 1). During early spring of the first year of the study, the site received higher rainfall than normal; during late spring the second year of the study, the site received higher rainfall than normal (Fig. 1a). Overall, temperatures for both years were similar to the 30-year averages (Fig. 1b).
Fig. 1.
Fig. 1. (a, b) Monthly (a) accumulated rainfall and (b) average temperature for Harrington, PEI, for the 2-year study period, as compared to the 30-year average for the location. Error bars represent standard errors of the mean.
Table 1.
Table 1. Soil test results for two different fields over the 2 years of the study.

Yield and field measurements

Seeding rate

There were strong effects of seeding rate on both plant population and yield (Tables 2 and 3). Although the aim was to achieve as homogeneous plant stand as possible, there was variability between the intended plant population and the actual population. The actual plant number was approximately 60.6 plants m−2 (19% lower) in the low population; 74.5 and 87.7 plants m−2 (25% and 30% lower) for the medium and high population treatments, respectively. Total yield was different at different seeding rates, with increased yield associated with increased seeding rates in all varieties (Fig. 2). Yield was highest in the highest seeding rate (5138 kg ha−1) and lowest at the low seeding rate (4401 kg ha−1).
Fig. 2.
Fig. 2. Regression analysis showing the linear relationship between yield and plant population for each variety.
Table 2.
Table 2. Mixed models (REML) outputs for all variates in the study; p values are listed where significant effects occur.
Table 3.
Table 3. Least square means of emergence, yield, thousand seed weight (TSW), seed moisture, and protein grown with or without nitrogen and sulfur for four varieties of field pea.

Cultivar

There were highly significant differences between the cultivars for all measured factors except yield (Tables 2 and 3). Emergence, TSW, and protein concentration was different between cultivars. Live plant population was highest in CDC Saffron (80.2 plants m−2) and lowest in AAC Lacombe (61.4 plants m−2) with intermediate values for CDC Raezer and CDC Limerick (79.8 and 76.4 plants m−2, respectively). Yellow pea varieties CDC Saffron and AAC Lacombe had numerically higher yields than the green pea varieties CDC Raezer and CDC Limerick by approximately 250 kg ha−1. Conversely, the green field pea varieties had higher protein concentrations (333 and 331 g kg−1 for CDC Limerick and CDC Raezer, respectively) than the yellow pea varieties (317 and 315 g kg−1 for CDC Saffron and AAC Lacombe, respectively). There was a significant interaction between cultivar and applied nitrogen for TSW where slight numerical reductions were observed in CDC Saffron and AAC Lacombe with added nitrogen.

Nitrogen and sulfur

Applied nitrogen fertilizer resulted in an increase in seed protein concentration over the control (326 g kg−1 vs. 322 g kg−1) but did not affect any of the other measured factors (Tables 2 and 3). Overall plant response to nitrogen fertilizer application was minimal with observed cultivar × nitrogen interactions on TSW and protein concentration.
Sulfur had a significant effect on yield and TSW. Sulfur treatments on average yielded approximately 140 kg ha−1 more than untreated controls (4889 kg ha−1 vs. 4742 kg ha−1). Conversely, the overall TSW was lower in sulfur fertilizer treatments (227.1 g vs. 224.3 g). There was a significant interaction between sulfur × seeding rate where there was a greater yield response at higher seeding rates with applied sulfur fertilizer (Fig. 3).
Fig. 3.
Fig. 3. Effect of sulfur × seeding rate on yield. Different colored bars represent rates of applied sulfur (blue = 0 kg ha−1; red = 25 kg ha−1). The separate groups of bars represent low, medium, and high seeding rate conditions (75, 100, and 125 seeds m−2). Error bars represent the standard error or the mean.

Interactions

A linear interaction between cultivar and seeding rate was observed where all varieties yielded higher with increasing seeding rate (Table 2). There was also a greater yield increase response observed in the yellow pea varieties AAC Lacombe and CDC Saffron, compared to the green pea varieties CDC Limerick and CDC Raezer (Fig. 2). Interactions between applied nitrogen fertilizer and most other factors were limited; however, the nitrogen × cultivar interaction resulted in differences between cultivars for both TSW and protein concentration (Table 2). This interaction had lower TSW associated with nitrogen application for all varieties except CDC Raezer. There was a slight increase in protein concentration (6.41–9.82 g kg−1) associated with applied nitrogen in all varieties except CDC Limerick, which decreased very slightly with the nitrogen fertilizer treatment (2.52 g kg−1). Applied sulfur fertilizer caused a more dramatic response than applied nitrogen fertilizer including interactions between sulfur × seeding rate (Fig. 3). At lower seeding rates, yield response to sulfur application was neutral to negative, at medium seeding rate levels yield response was neutral to positive, and at high seeding rates yield response was positive.
Despite the variability within intended seeding rate treatments, there was an effect of increased plant population on yield ranging from 4401 to 5138 kg ha−1. Interactions were observed between seeding rate and cultivar with each cultivar showing an increase in yield with increased seeding rate (Fig. 2). There were no effects of starter nitrogen fertilizer on any of the measured factors; however, there were interactions between nitrogen × cultivar for plant emergence and seed size with plant emergence improving with added nitrogen. Seed size decreased slightly with applied nitrogen fertilizer. Sulfur addition resulted in increased yield and also a slight decrease in seed size. The interaction between seeding rate and sulfur application was significant, and interestingly this response was not as pronounced at the lower seeding rates as it was at higher seeding rates (Fig. 3). Finally, there was an interaction in the quadratic model observed between seeding rate, cultivar, and sulfur application on yield where different varieties responded differently to the increased seeding rate and application of sulfur.

Multivariate analysis

PCA biplots were created for low, medium, and high seeding rate conditions (Figs. 4a4c). Under a low seeding rate condition (targeted plant population of 75 plants m−2) (Fig. 4a), 45% of the variation was explained under the first score. This score was loaded (eigenvector length > ±0.2) by the qualitative parameters of TSW on the positive side, and protein concentration on the negative side. The zero nitrogen treatments of AAC Lacombe correlated with higher TSW values, and both CDC Raezer and CDC Limerick correlated with increasing plant protein concentration. For Score 2, 39% of the variation was explained by both plant population and yield, which loaded on the positive side of the second axis (eigenvector length > ±0.2) and were highly correlated. CDC Saffron and all sub treatments correlated with increased plant population, yield, and larger seed size.
Fig. 4.
Fig. 4. (ac). Principal components analysis biplot constructed from mixed models outputs for (a) low plant population condition (84% of variability explained); (b) medium plant population condition (87% of the variability explained); and (c) high plant population condition (85% of the variability explained). Score one is the proportion of variability explained under the first axis and Score two is the proportion of variability explained by the second axis. Colors represent pea cultivars (gray is for CDC Raezer (Rz), blue is AAC Lacombe (Lc), yellow is for CDC Limerick (Lm), and red is for CDC Saffron (Sf)). Abbreviated variety names are listed followed by associated treatments for sulfur and nitrogen (0, 15 for nitrogen; 0, 25 for sulfur). TSW, thousand seed weight.
The medium plant population condition (targeted plant population of 100 seeds m−2) (Fig. 4b) explained 67% of the variation in the data by the first axis, and 20% of the variation was explained by the second axis. Eigenvectors for plant population and protein heavily loaded on the positive side of the first axis (eigenvector length > ±0.4), while yield and seed size loaded on the negative side of the first axis. The second axis was loaded heavily on the positive side by both yield and plant population. Higher fertility treatments with CDC Saffron correlated with yield; CDC Raezer and CDC Limerick correlated with the eigenvector for increasing protein, as well as increasing plant population, but to a lesser extent. AAC Lacombe main treatments were correlated with increasing seed size.
The high plant population condition (targeted plant population of 125 seeds m−2) explained 71% of the variation in the data on the first axis, and 14% of the variation on the second axis (Fig. 4c). Eigenvectors for protein concentration loaded positively on the first axis and eigenvectors for TSW and yield loaded negatively. On the second axis, the eigenvector for plant population loaded heavily on the positive side. Higher fertility treatments for CDC Limerick and the zero-fertility treatment for CDC Raezer correlated with higher protein concentration. Most CDC Saffron treatment combinations correlated with seed size apart from the zero-fertility treatment, which in addition to AAC Lacombe treatments, correlated with yield.
Multivariate analysis shows the shift in the relationship between plant population and yield responses. Under lower seeding rate conditions, yield is highly correlated with plant population; under higher seeding rate conditions, yield correlates more with seed size than with plant population per se (Figs. 4a4c).

Discussion

Although land dedicated to field pea production has been increasing globally, the growth in pulse production has been limited in the Maritime region of Canada (Statistics Canada 2023). Despite the widespread availability of well-adapted modern varieties, both the distance to market and the lack of local processing capacity continue to be the main factors for the limited expansion of field pea production in the Maritime region of Canada (ECODA 2023). Additionally, more information on basic production practices is needed to foster the growth of this crop in the region. The purpose of this study was to evaluate basic, region specific cultural practices associated with pea production to foster the establishment of field pea in the Maritimes.
Plant emergence was reduced regardless of seeding rate and the targeted plant population. The targeted plant population was based on the number of pure live seeds (as determined by germination tests) and seed size. One reason for variation or a reduction in emergence can be attributed to a lack of consistency in seeding depth. Seeding depth will influence emergence due to insufficient seed to soil contact, access to moisture, or potential damage from pre-emergent herbicides (in the case of shallow seeding). Although recommended seeding depths vary by region (Stephanovic et al. 2018), seeding depth greater than 76 mm could result in significant reductions in seedling emergence and overall yield (Johnston and Stevenson 2001). The ideal depth of planting for field peas in the Midwestern United States ranges between 20 and 75 mm (Stephanovic et al. 2018). Our study was seeded to a targeted depth of 50 mm; however, lighter soils in the Maritime region of Canada make it difficult to seed to a consistent depth, and many producers will mechanically roll to compact the soil following planting to improve seed-to-soil contact.
A comprehensive evaluation of field pea seeding rate and planting date performed in the Midwestern US reported a rate between 70 and 109 seeds m−2 to be the range necessary to achieve an economically optimal plant population across three planting dates (Koeshall et al. 2021). That study supported previous findings where a positive relationship between plant population and yield was also reported (Gan et al. 2003; Johnston and Stevenson 2001; Nleya and Rickertsen 2011). Gan et al. (2003) recommended a rate of 60–70 plants m−2 in western Canada, but even higher targeted seed populations have been shown to increase yield response (Pageau et al. 2007), with a reported seeding rate of 100 seeds m−2 as the ideal plant population for pure stands of field pea seeded in Quebec. The authors of that study noted lodging as a potential risk to achieving peak yields. Modern, shorter-statured field pea varieties used in the present study were not susceptible to lodging, and plant height was homogeneous throughout the study and among the selected varieties. The highest yields in our study were observed at the highest seeding rates, particularly with added sulfur fertilizer, when a plant population of approximately 88 live plants m−2 was achieved. Our study shows a linear increase in yield with increasing seeding rate, despite lower emergence. It has been previously reported that when increasing seeding rates, emergence may decrease (Gan et al. 2003); however, the reduced emergence in that study did not result in lower yields in that study, nor with ours. Planting date has been shown to influence seedling emergence, and later planting can result in both higher plant emergence and yield if season growing temperatures are low (Koeshall et al. 2021). Typically, an early planted crop avoids prolonged exposure of pea plants to higher air temperatures during flowering, which may cause greater yield losses than seedling exposure to cold when planted early (Tawaha and Turk 2004; Koeshall et al. 2021). With the shorter growing season in the Maritimes, a higher seeding rate (100 seeds m−2) is recommended to achieve economically optimum yields. This rate is slightly higher than what we have seen to be the optimal plant population; however, additional seed serves as a buffer in case of limited seed emergence when seeded early when there is a risk of frost injury.
Most studies on field pea plant population density report that a higher seeding rate resulted a negative response of plant yield components per se (pods per plant, seeds per pod, and seeds per plant); despite this, yields were generally reported to be higher (Gan et al. 2003; Tawaha and Turk 2004; Pageau et al. 2007; Nleya and Rickertsen 2011; Koeshall et al. 2021). We have shown that at a lower plant density, yield shows a stronger correlation with seeding rate; however, this relationship changes as the seeding rate gets higher (Figs. 4a4c). These results highlight the complexity of the responses of plant resource allocation based on intraspecific plant competition and substantiate previous findings of the studies listed above.

Sulfur fertilizer in field pea production

A study by Pötzsch et al. (2019) showed that when background sulfur in northern Europe was sufficient (atmospherically deposited), there were limited effects of sulfur application on yield, especially when applied in the elemental form. Although SO4 uptake and concentration within the plant increased with sulfur application, the uptake did not translate into yield in that particular study. In our study, there was a clear effect of sulfur application on improved yield and there was also an effect on TSW, where sulfur application resulted in lower mass per count values. A greenhouse study examining sulfur nutrition through time did not see an increase in single seed weight; however, they did report a significant increase in yield, number of pods per plant, and the number of seeds per plant, at the highest level of SO4 sulfur in the soil (Zhao et al. 1999). This helps to explain why we saw increased yield response with seeding rate under the sulfur treatment versus the control. A higher number of plants m−2 would result in a greater number of seeds with only a slight reduction in single seed weight, with added sulfur fertilizer.
Sulfur in our study was applied as part of a commercial fertilizer blend, K-MagTM (0-0-22-10.8 Mg-22 SO4, Mosaic Crop Nutrition, Tampa, US), and there may be a possibility that the observed responses were synergistic based on nutrients in the blend other than sulfur. However, previous reports examining the effects of applied chemical fertilizer (N, P, and K) on field pea showed limited responses when soil levels were sufficient in both potassium and phosphorus (Wilson et al. 1999). Typically with field pea, there will not be a potassium response unless soil levels are extremely low (Wilson et al. 1999). Soil potassium levels in our study were high during both years of the study (114 and 201 mg kg−1 for 2017 and 2018, respectively). Although there is a chance for a minor yield response from additional applied magnesium contained within the commercial blend, there is a lack of evidence in the literature showing a measurable plant response to applied magnesium. Our soil tests showed sufficiency of all other nutrients required for optimal field pea growth (Table 2). The critical values for plant available sulfur for field pea production in the region are not yet established, and a recent characterization of soil sulfur in the region has reported a dramatic decline through time (Nyiraneza et al. 2019).
Applied sulfur fertilizer resulted in an average yield increase of approximately 146 ± 31.7 kg ha−1. The input cost for the commercial sulfur blend at the time of writing ranged from $110 to 140 ha−1. Although yield response was higher than the untreated control, it is not likely that the addition of sulfur in this form, at the chosen rate, would show a positive economic return. There is a positive yield response from added plant available sulfur applied as K-MagTM; however, to break even, the costs of the input would need to be below approximately $70 ha−1, or alternatively, the commodity value of peas would need to be higher. A less expensive form of sulfur could be chosen to improve production margins. For example, the use of ammonium sulfate could be an option; however, if the goal is to limit the carbon footprint of pea production, the addition of sulfur bound to ammonium would be problematic by increasing the carbon footprint. Another potentially economical sulfur option would be to apply calcium sulfate in the form of gypsum. This soil amendment is less expensive and may provide a consistent sulfur supply over a longer period (Ahmed et al. 2017). Sulfur supplied as gypsum also has corollary benefits of reduced leaching, provision of soil calcium and soil buffering capacity. Gypsum is less expensive, and the only drawback would be timing the plant availability of sulfur from the product.
Sulfur is essential for nitrogen metabolism within the plant, and SO4 is commonly added to nitrogen fertilizer blends. In addition to enhancing yield potential of leguminous crops, sulfur and sulfur-containing biomolecules have been shown to be important plant defense compounds (Kunstler et al. 2020). There is increasing evidence that the supplementary application of sulfur has implications for overall plant health beyond simple yield metrics, which has led to the concept of “S-induced resistance” to plant disease (Wang et al. 2022). These recent developments suggest that sulfur application to pulse crops may hold benefits in terms of whole-plant heath in addition to direct responses in terms of seed yield.

Nitrogen fertilizer in field pea production

As field pea are leguminous crops that can biologically fix nitrogen (BNF) from the atmosphere, there is ongoing debate within the industry on the benefits of applying nitrogenous fertilizer to legumes (Almeida et al. 2023). The N fixation capacity of pea grown in western Canada can range from 50% to 80% (Dhillon et al. 2022), with typical amounts of BNF to be 40–80 kg ha−1 (Beckie and Brandt 1997; Soon and Arshad 2002; Liu et al. 2019). The use of a starter application of nitrogen fertilizer is believed to have a beneficial effect on leguminous crop yields among farmers. However, higher levels of nitrate in the soil have been shown to restrict nodule development and N fixation in peas (Bollman and Vessey 2006). Work done in western Canada has shown that fertilizer N application rates less than 40 kg ha−1 had no effects on biomass N, and rates exceeding that amount resulted in reduced root nodulation (Clayton et al. 2011). Wysokinski and Lozak (2021) compared rates of labeled nitrogen (15 N) from atmospheric, fertilizer, and soil sources and reported the greatest uptake from the atmosphere occurred between the three-internode stage and the first visible flower; the greatest uptake as a fertilizer source was between the four-leaf stage and the three-internode stage. Overall, they reported that atmospherically fixed N accounted for the majority of the nitrogen taken up by the plant. Applied nitrogen fertilizer to legumes rarely results in increased yield. We observed a minor increase in protein concentration in the seed (∼50 g kg−1), but this would not justify the application of nitrogen fertilizer.
A spatially and temporally comprehensive study evaluating nitrogen and sulfur dynamics in soybean grown in the mid-western US reported responses to sulfur, and not to nitrogen most of the time (Almeida et al. 2023). The authors suggest that yield uncertainties associated with fertilizer addition were greater than any resulting positive effects on yield (Almeida et al. 2023). Although this study was based on soybean, it highlights the need to reduce or eliminate the application of nitrogen fertilizer to leguminous crops.

Conclusions

The results of our study show that there is no advantage to applying early nitrogen to field pea grown in the Maritime region of Canada. We did see a very slight increase in protein resulting from nitrogen application; however, the costs of applied fertilizer would not justify the increase in protein concentration. The use of sulfur fertilizer resulted in a positive yield response; however, the use of a specialty blend of fertilizer, as was done in this study, adds to the costs of producing the crop and makes the crop less profitable. The potential for less expensive forms of sulfate needs to be evaluated on field pea in the region. The manipulation of seeding rate is the greatest driver of field pea yield for the region with the ideal seeding rate being approximately 100 seeds m−2 for most varieties in the region.

Acknowledgements

The authors of this study would like to acknowledge the contributions of Sylvia Wyand and Christian Gallant who were responsible for field operations that made this project possible.

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Information & Authors

Information

Published In

cover image Canadian Journal of Plant Science
Canadian Journal of Plant Science
e-First

History

Received: 19 September 2023
Accepted: 24 November 2023
Accepted manuscript online: 20 December 2023
Version of record online: 9 February 2024

Data Availability Statement

Data generated or analyzed during this study are available from the corresponding author upon reasonable request.

Key Words

  1. field pea
  2. seeding rate
  3. nitrogen
  4. sulfur
  5. yield

Authors

Affiliations

Agriculture and Agri-Food Canada, Charlottetown Research and Development Centre, 440 University Ave., Charlottetown, PE C1A 4N6, Canada
Author Contributions: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, and Writing – original draft.
Agriculture and Agri-Food Canada, Kentville Research and Development Centre, 32 Main St., Kentville, NS B4N 1J5, Canada
Author Contributions: Data curation, Formal analysis, Methodology, and Writing – review & editing.

Author Contributions

Conceptualization: AASM
Data curation: SAEF
Formal analysis: AASM, SAEF
Funding acquisition: AASM
Investigation: AASM
Methodology: AASM, SAEF
Project administration: AASM
Writing – original draft: AASM
Writing – review & editing: SAEF

Competing Interests

The authors declare there are no competing interests.

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