Serena Klippenstein, Department of Soil Science, University of Saskatchewan
FABA BEANS ARE grown both for grain and as a green manure rotational crop in Saskatchewan, however there is little recent information on nutrient requirements of modern faba bean cultivars grown under prairie conditions. As legumes can obtain much of the nitrogen (N) they need from the air through biological nitrogen fixation (BNF) in the nodules on their roots, knowing the contribution of BNF in pulse crops like faba beans is important when selecting crops to use in rotation that can help maintain soil N fertility. Not only must nutrient requirements be met for a target faba bean crop yield, but nutrients removed by faba bean harvest should be accounted for to maintain fertility for successive cropping years. To address this need, a recent collaborative study at the University of Saskatchewan in the Soil Science Department and the Crop Development Centre looked at faba bean yield, nutrient uptake and removal over two years at four sites in the Dark Brown, Black and Gray soil zones. The study was funded by the Saskatchewan Pulse Growers and the Saskatchewan Agriculture Development Fund.
Four zero-tannin faba bean cultivars, two small-seed (CDC Snowdrop, 219-16) and two large-seed (Snowbird, Tabasco), were grown and analyzed in a two-year field study (2016 and 2017) in Saskatchewan. The effect of different environmental conditions and soil types were considered by utilizing four different field site locations each year (Meath Park, Rosthern, Saskatoon and Outlook, SK) (Figure 1) in different soil-climatic zones and in conditions where faba beans are normally grown in Saskatchewan. Two different fertilizer treatments, unfertilized and fertilized with potassium sulfate (K2SO4) (0-0-44-17) and monoammonium phosphate (MAP) (11-52-0), were applied to each of the four site locations. Analysis of grain and straw biomass and nutrient concentration was used to determine yield, uptake and removal of soil macro- and micro-nutrients, and to estimate BNF by different cultivars.
Faba bean showed good yield potential. Average unfertilized and fertilized faba bean grain yields for the four cultivars were between 2,700 lb/ ac and 6,200 lb/ ac (3,000 to 7,000 kg/ha) at the four field site locations in 2016 and the three field site locations in 2017 (Figure 2). Overall average faba bean grain yield in the two site-years was 4,707 lb/ ac (5,283 kg/ha) and ranged from 872–8,720 lb/ ac (979–9,787 kg/ ha).
Note: Yields in this study were determined by hand harvesting, which typically results in higher yield values than harvest of the same site with a combine harvester.
Compared to other site locations, yields were lowest at Outlook due to adverse growing conditions in 2016, and the site was lost to hail in 2017. Compared to other cultivars, Snowbird had the lowest mean total (grain + straw) yield in 2016 and 2017 and the lowest average grain yield in 2017, but the greatest grain yield of all the cultivars in 2016. It was clear from the field results that soil and environmental factors, as well as cultivar, had a profound influence on the faba bean yield components.
Partitioning of yield between faba bean grain and straw, and therefore harvest index (grain yield/grain+straw yield), differed with site location and cultivar each year. However, grain yield was greater than straw yield in both unfertilized and fertilized treatments in 2016 and 2017. Average harvest index (HI) of faba bean ranged from 45–56% in 2016 and 2017, with the highest average HI at Rosthern (56% in 2016; 53% in 2017) and the lowest average HI at Saskatoon (45% in 2016; 50% in 2017) in both years. Lower HI at Saskatoon and Outlook likely reflected impacts of drier summer conditions. The Snowbird cultivar had a significantly greater HI than the other cultivars at most site locations in 2016 and, along with 219-16 and Tabasco, had a significantly greater HI than CDC Snowdrop at Rosthern and Saskatoon in 2017.
NITROGEN AND PHOSPHORUS UPTAKE
In 2016 and 2017, average faba bean total (grain+straw) nitrogen (N) uptake was 205 lb N/ ac (230 kg N/ha), with grain N uptake and removal in harvest ranging from 104–267 lb N/ac (117–300 kg N/ha). The high yield potential of faba bean is associated with significant external contribution of N from BNF. Average faba bean total (grain+straw) phosphorus (P) uptake was 24 lb P/ ac (27 kg P/ha), with grain P uptake and removal in harvest ranging from ~13–31 lb P/ ac (~15–35 kg P/ha) or 30–71 lb P2O5/ ac (34–80 kg P2O5/ha). Grain P removal was similar to the reported P removal by faba bean grain according to the Canadian Fertilizer Institute (2001) and Saskatchewan Pulse Growers (SPG; 2018), for a 50 bu/ ac (3,800 kg/ha) faba bean yield, which falls within the range of the current study. As expected, the majority of above-ground N and P uptake was found in faba bean grain for both fertilizer treatments in all cultivars across all site locations in both field study years as revealed in the uptake harvest indices.
Yield – Faba bean has high yield potential, with significant external contribution of N coming from biological nitrogen fixation, which can help maintain soil N fertility.
Nutrient partitioning – Nutrient partitioning among grain and straw greatly favours grain, with large amounts of N and P being removed in the seed at harvest.
Fertility – Faba bean yield components showed limited response to fertilization at the four sites in this study, but fertility management in rotations with faba bean will need to consider drawdown over the long-term, as most of the P taken up was found in grain.
Controlling factors – Soil and environment are major controlling factors and faba bean yield and N and P uptake vary with location and conditions.