4R Phosphorus Management in Pulses for Agronomists

PULSE KNOWLEDGE

Phosphorus is also a component of DNA and RNA, chromosomes, 4R Phosphorus Management in Pulses genes, coenzymes, phosphoproteins, and phospholipids, all of which are necessary for cell function and plant growth and By Alexis Adams, M.Sc., P.Ag. development (Grant and Flaten, 2019). Phosphorus is an essential nutrient for pulse crop productivity, and an adequate amount of phosphorus must be supplied in a plant-available Phosphorus is required in greater amounts for pulse crops than form at the critical demand periods, to maximize crop production. many other crops due to its high demand in the energy transfer Phosphorus management practices that minimize environmental molecules used in nitrogen fixation (Rotaru and Sinclair, 2009). As damage and maximize economic returns for farmers are essential to phosphorus is a key component for many aspects of legume maintain sustainable agroecosystems. The practices of using the right growth, agronomists and growers must take care to supply the rate, right source, right timing, and right placement are known as 4R crop’s phosphorus needs to maximize crop production. nutrient stewardship. 4R nutrient stewardship practices will promote  Soil Phosphorus Cycling and Plant Uptake long-term environmental, economic, and social sustainability. 4R Phosphorus exists in the soil in several organic and inorganic forms phosphorus use practices in pulse crops give farmers and agronomists in soil organic matter, minerals, and in the soil solution (Figure 1). strategies to make best use of resources and maximize pulse crop - Plants take up phosphorus as orthophosphate ions (H2PO4 or production. 2- HPO4 ) from the soil solution. To maintain equilibrium, phosphorus moves from more available organic and inorganic Understanding Phosphorus pools to the soil solution (Grant and Flaten, 2019).  Role of Phosphorus in Pulse Crops Phosphorus is one of 16 essential plant nutrients and one of four This more available, or labile phosphorus includes mineralizable macronutrients required in large amounts to sustain pulse crop organic phosphorus, readily exchangeable adsorbed phosphorus, growth. Phosphorus is a key component of molecules necessary and soluble forms of precipitated phosphorus. Phosphorus then for root growth and development, respiration, photosynthesis, moves from less available, or non-labile phosphorus pools, which nucleic acid synthesis, nitrogen fixation, plant maturity, and seed include stable organic phosphorus and is strongly adsorbed to soil production (Zhang et al., 2014; Niu et al., 2013; Grant and Flaten, minerals and compounds, into the labile pool to maintain 2019). equilibrium (Figure 2) (Grant and Flaten, 2019). Phosphorus reacts strongly with calcium at higher soil pH levels and with iron and Phosphorus is a component of several energy transfer molecules, aluminum at lower pH levels. Because of the many ways that including ADP, ATP, NADP, and NADPH, which provide energy for phosphorus interacts with soil particles and compounds, it moves photosynthesis and other metabolic processes (Grant and Flaten, extremely slowly through the soil. 2019, Zhang et al., 2014).

Figure 1. Phosphorus Cycle (Wikipedia, adapted from Mississippi State University Extension)

Phosphorus will not readily move through the soil towards plants, Generally, phosphorus deficiency symptoms in pulse crops thus plant uptake is dependent on the presence of phosphorus in the include delayed emergence, delayed development of plants or soil solution, and roots in close proximity to it (Grant and Flaten, leaves, stunted plants, thinner stems and roots, increased root 2019; Syers et al., 2008). For continued phosphorus uptake, the soil to shoot ratio, reduced nodulation, lower yield, leaf colour solution of the root zone must be continually replenished with it. change (dark green, yellow, purple, or brown), and senescence of oldest leaves (Hopkins, 2015; Hoppo et al., 1999, Sarker et al., When phosphorus fertilizer is added to the soil, the fertilizer granule 2015; Nielsen et al., 2001). Darkened green leaves are a result of attracts water, which dissolves the granule releasing orthophosphates accumulation of carbohydrates from decreased plant respiration. into the soil solution for plant phosphorus uptake. Excess phosphorus Leaf purpling is a result of anthocyanin accumulation due to a not taken up by plants will move from the soil solution to labile and blockage of metabolic pathways from a lack of the necessary non-labile pools through several processes including precipitation, phosphorus-containing molecules (Close and Beadle, 2003). adsorption, and microbial immobilization. Phosphorus is mobile in plants, thus leaf colour change will appear first in the oldest plant tissue, as phosphorus is Because of the many soil reactions phosphorus may undergo, it is transported to growing points. As phosphorus deficiency relatively immobile in the soil, and a crop will only use about 25% of progresses, metabolic activities in the oldest leaves shut down, applied fertilizer phosphorus in the year of application. This may be and the leaves prematurely senesce and fall off of the plant. discouraging to growers, however the applied phosphorus is retained in the soil and will be available to subsequent crops. When In peas, phosphorus deficiency may result in reduced branching phosphorus applications are lower than crop requirements, the non- as well as darkened plants with red stems, tendrils, petioles, or labile and labile phosphorus soil pools will be drawn on for plant leaf margins. In faba beans and lentils, the oldest leaves of uptake (Liu et al., 2015; McKenzie et al., 1992a; McKenzie et al., phosphorus-deficient plants may have mildly mottled yellowing, 1992b; Syers et al., 2008). With continually low phosphorus additions, along with purpling within dark green spots on chlorotic leaves. however, soil reserves will be depleted, with limited phosphorus Chickpeas under phosphorus-deficient conditions will also be available for crop growth. stunted and turn dark green, followed by interveinal leaf chlorosis, with leaf purpling in some cases. Phosphorus  Plant Phosphorus Movement, Acquisition, and Deficiency deficiency in dry beans, similarly to other pulse crops, results in Plants live off of phosphorus stored within the seed during the slow growing, stunted plants with thin stems. Younger dry bean process of germination and the first few days of plant growth. leaves are small with darker green pigmentation, and older Once plant roots have begun to grow, phosphorus uptake leaves are chlorotic and browned and eventually die off. In begins. A small portion of plant phosphorus is metabolically soybeans, phosphorus deficiency causes stunted plant growth active in energy cycles involving ATP and ADP. The rest of the and appears as dark green or bluish leaf coloration, with plant phosphorus is stored in the plant as phytic acid or complete or mottled interveinal chlorosis and purpling or polyphosphate or stored as orthophosphate in cell vacuoles. browning within chlorotic regions. When soil phosphorus availability is low, the plant will draw on this stored plant inorganic phosphorus (Ozanne, 1980).

When soil phosphorus is limited, plants attain more soil phosphorus through using acquisition strategies such as increasing root surface area through increased root hairs, release of root exudates to acidify and solubilize phosphorus, recruiting phosphorus solubilizing arbuscular mycorrhizal fungi (AMF) to colonize roots, and/or increasing the activity of phosphorus transporters (Sandana and Pinochet, 2014).

Figure 3. Phosphorus deficiency (left) vs. sufficient levels (right) in lentils. Source: Andrew Reddekopp

Figure 2. Soil phosphorus pools and relative accessibility, extractability, and availability (adapted from Johnston et al., 2014).

B) Reclaimed and By-Product Phosphorus Sources 4R Management Principles for By-product and reclaimed sources of phosphorus such as manure and Phosphorus struvite have potential to improve farm sustainability through recycling nutrients. Animal manure contains both inorganic and 1) Right Source organic sources of phosphorus, and when available in a reasonable distance from a field, is an economical source. Manure, however, is a Phosphorus can be applied in several forms, from several sources. source of nitrogen, thus growing pulse crops in the same year as The most accessible and commonly applied source is commercial manure addition may inhibit nitrogen fixation. Manure applied at fertilizer, of which there are several products and some special rates matching nitrogen uptake of Western Canadian crops will formulations. Other sources include rock phosphate, animal manure, typically leave phosphorus in the soil to benefit pulse crops grown and reclaimed waste products. As well, fertilizer additives, polymer later on in the rotation. coatings, and microbial products have been developed with hopes of improving phosphorus availability and uptake. Struvite is a phosphorus source derived from municipal wastewater and/or animal manure, which shows potential for nutrient recycling A) Mineral and Commercial Sources of Phosphorus Fertilizer and provides an alternative source to non-renewable phosphate rock Commercial phosphorus fertilizer is derived from phosphate rock, (Ackerman et al., 2013). Struvite is formed through the addition of which is also available for application as an unprocessed product. The magnesium to wastewater, which causes precipitation of nitrogen phosphorus in phosphate rock is present in its mineral form, mainly and phosphorus into pearls. In this process approximately 50% of bound to calcium, and has low plant availability. The mineral wastewater nitrogen and 90% of wastewater phosphorus contents structure, particle size, and source of the rock phosphate being are recovered (Rahman et al., 2014). Struvite has been found in applied impacts the ability of the phosphorus to dissolve into the soil research to perform well in soils with high and low pH (Massey et al., solution (Grant and Flaten, 2019). Solubility of rock phosphate also 2007). Struvite is reported to have similar phosphorus uptake and dry decreases in basic soils and with increased soil calcium content. matter yield to commercial fertilizers in some research (Gonzalez Saskatchewan soils are generally both basic and calcareous, thus rock Poneer and Garcialopez, 2007; Johnston and Richards, 2003), but has phosphate is not recommended as an immediate source, however it received limited field evaluation in pulse crops in Western Canada. may be suitable as a long-term source for building soil phosphorus. Pulse crops acidify their rhizosphere to solubilize phosphorus and C) Phosphorus Additives and Coatings thus may do a better job of accessing rock phosphate than other Maleic-Itaconic Co-Polymer additives, such as Avail, are promoted to crops (Bekele et al., 1983). form complexes with calcium, iron, and aluminum surrounding the phosphorus fertilizer granule. Complexed calcium, iron, and Rock phosphate is processed to form commercial fertilizer, which is a aluminum are then unable to react with phosphates, allowing more pure and readily available source. The most commonly used dry phosphorus to remain in the soil solution (Grant and Flaten, 2019). commercial phosphorus fertilizer in Western Canada is The most benefit for this co-polymer appeared to be where soil test monoammonium phosphate (MAP, 11-52-0, NH4H2PO4). MAP phosphorus was low, rate of applied phosphorus was low, and at + 3- dissolves readily into ammonium (NH4 ) and phosphate (PO4 ) in extreme pH levels (Hopkins et al., 2018). + + water. As plants take up NH4 ions, H ions are released to the soil solution, which decreases soil pH, preventing precipitation of the Phosphorus may also be sold as a polymer-coated granule. The idea phosphate ions, which takes place at higher pH levels (Miller et al., behind polymer coatings is to slow the release of phosphorus 1970). The most common liquid source of phosphorus is ammonium fertilizer by slowing the rate of dissolution and providing available phosphorus throughout the growing season. Polymer breakdown polyphosphate (APP, 10-34-0, (NH4H2PO3)n(OH)2). As with MAP, the + rates in the soil increase with increasing temperature and moisture ammonium (NH4 ) ions in this product lower pH and improve phosphorus uptake (Grant and Flaten, 2019). A less common source (Du et al., 2006). Field research in Manitoba found polymer-coated of phosphorus fertilizer is diammonium phosphate (DAP, 18-46-0, MAP did not improve yield over regular MAP (Grant, 2002), however, - polymer-coated phosphorus was found to improve seed safety of (NH4)2HPO4 ). DAP is less suitable for Western Canadian pulse crops as it has lower seedling safety especially in calcareous soils, and is less MAP in a plot study of 10 different crops (Schoenau et al., 2007). acidifying than MAP and thus precipitates more readily in high pH D) Microbial Products soils (Bouldin and Sample, 1959). Microbial products such as Penicillium bilaiae and Arbuscular Other specialty formulations of phosphorus such as Mosaic’s MES-15 Mycorrhizal fungi (AMF) have also been marketed to improve soil (13-33-0-15), Koch’s 12-45-0-5, and Simplot’s 16-20-0-14 combine phosphorus availability and plant uptake. P. bilaiae is sold nitrogen, phosphorus, and sulphur in one granule. MES-15, a commercially as an inoculant and seed treatment. P. bilaiae has combination of MAP, ammonium sulphate (AS), and elemental potential to improve availability and uptake by secreting phosphorus- sulphur is thought to improve phosphorus uptake because the solubilizing organic acids that decrease pH and bind with calcium + phosphate minerals to release phosphorus (Knight et al., 2011). ammonium (NH4 ) in the AS reduces soil pH, and the sulphate ions precipitate with calcium in place of phosphate. Growth chamber and Western Canadian research has shown mixed results across several field research in canola, however, found no difference in phosphorus annual crops from no response, to increased root growth and tissue uptake between AS applied with MAP and MES-15. MES-15 was, phosphorus concentration, with some yield responses (Grant et al., however, found to have better seedling safety than AS+MAP at the 2002; Karamanos et al., 2010; Mohr et al., 2013; Vessey and same rates (Grenkow, 2013). Alpine is a company that produces Heisinger, 2001, Gan et al., 2003). Crops grown on soil with higher pH several liquid nitrogen, phosphorus, potassium fertilizer blends. In levels and calcium phosphate content will likely show a greater comparison to liquid APP, a larger percentage of the phosphorus in benefit from P. bilaiae application. Alpine is in the orthophosphate form than the polyphosphate form, Arbuscular mycorrhizal fungi (AMF) is a naturally occurring soil and as such is thought to be more readily plant available. microbial organism that has been researched as a soil amendment to improve plant uptake of phosphorus.

Crop roots form symbiotic relationships with AMF in which the plant Table 1 summarizes the pH levels and high and low values for the provides photosynthates to the AMF and the AMF draw in nutrients common soil phosphorus tests available in Western Canada. Tests beyond the root depletion zone through fungal hyphae (Grant et al., using an anion exchange resin, such as the Plant Root Simulator (PRS) 2005). Pulse crops are dependent on AMF associations for nutrient probes, can also be used to measure soil phosphorus available to acquisition, however, research has shown mixed response to plant roots (Ashworth and Mrazek, 1995; McKenzie et al.,1995; Qian additions of AMF amendments, especially under field conditions. et al., 1994; Karamanos, 2007). Soil test phosphorus concentration is (Xavier and Germida, 1997; Holzapfel et al., 2015; Knight et al., 2011). usually given in parts per million (ppm), and can be converted to -1 AMF amendment is most likely to provide a yield benefit in pulse pounds acres (lb ac ) in 15 centimetre (cm) depth soil increments by crops grown after non-AMF host crops such as canola (Grant and multiplying by 2. Another important conversion factor to note, Flaten, 2019; Holzapfel et al., 2015), under conventional tillage especially when reviewing literature, is that fertilizer rates may be (Monreal et al., 2011; Grant and Flaten, 2019), or in low phosphorus given as P or as P2O5. Typically, farmers are given fertilizer rates in lb -1 -1 soils (Grant et al., 2005). There may be potential for microbial P2O5 ac , whereas in research, rates are often given in kg P ha . To -1 -1 products such as P. bilaiae and AMF to improve phosphorus convert lb P ac to lb P2O5 ac , multiply by 2.3. One final note on acquisition, however research results have been mixed, and interpreting soil tests is that some soil tests will provide multiple effectiveness is likely highly dependent on soil conditions. available phosphorus tests. Look at the pH, or the buffer pH if provided, to determine the appropriate soil test to use.

2) Right Rate Table 1. Soil pH and critical values for various soil phosphorus A) Phosphorus Rate Strategy tests available in Western Canada (McKenzie et al. 1995) There are two strategies for selecting phosphorus fertilizer rates, the short-term sufficiency strategy and the long-term sustainability Soil pH Low High strategy. The short-term sufficiency strategy involves applying only Soil Test Method Range Soil P Soil P the amount of phosphorus that would elicit a crop response in the year of application. The long-term sustainability strategy, contrarily, ppm involves replacing phosphorus removed by crop harvest over time to maintain soil levels (maintenance) or applying beyond crop Olsen/Bicarb ≥ 7.0 < 10 > 20 requirements to increase soil phosphorus (building) (Malhi et al., Bray 1 (weak) ≤ 7.0 < 15 > 25 1993; McKenzie et al., 2003; McKenzie et al., 2001) (Figure 4). Bray 2 (strong) ≤ 7.0 < 27 > 40 B) Soil Testing Importance and Soil Phosphorus Tests Kelowna/Modified Kelowna Broad range < 15 > 30 Soil testing provides an indication of current soil levels, which allows agronomists and growers to determine the required rate of C) Determining Fertilizer Phosphorus Rates phosphorus to elicit a desired yield response under adequate growing Every bushel produced requires a certain amount of phosphorus conditions (McKenzie et al., 1995). As well, with multiple years of (Table 2). The full amount of phosphorus required per bushel of crop testing, soil testing indicates whether soil phosphorus is being produced is known as the uptake rate. The amount of nutrients taken depleted over time so that a grower can sustainably manage their soil out of the field with harvested grain is known as the crop removal resource (Grant and Flaten, 2019). Soil testing thus is a key part of rate. Actual uptake is dependent on environmental conditions such as sustainable phosphorus management. soil moisture and temperature, and also varies by specific crop variety Laboratories in Western Canada will use one of several soil (Johnston et al., 1999; Heard et al., 2006; Xie et al., 2017). phosphorus tests, with different tests being effective at different soil Environmental conditions are highly variable and research into pH levels. As well, each method has a specific critical level, at which removal rates by variety is limited, thus, soil sampling at least every there is the greatest likelihood of a crop response, and below which, three years is recommended to ensure appropriate rates are being phosphorus is considered low. used to maintain or build soil phosphorus based on the desired outcome Uptake and removal rates can be used to calculate phosphorus rates. Recommended application rates are often provided with soil tests to target specified yields but doing your own calculations of phosphorus balance is a good option to ensure you are targeting the desired outcome based on yield goals. When using the long-term sustainability strategy with the aim of building soil phosphorus, it is difficult to determine an exact build rate, as the rate of build up will vary by growing conditions and with soil characteristics such as texture and pH (Grant et al., 2014; Mohr et al., 2016). A good strategy for determining a phosphorus build rate when soil levels are low would be to add an additional 20-30 lb P2O5 ac-1 above crop removal rates. Build rates must fit within the farm budget and equipment logistics. Continued soil testing is necessary to Figure 4. Diagram illustrating the stages of the long-term sustainability determine if rates are effectively building soil phosphorus. strategy. Phosphorus rate relative to crop removal decreases as soil phosphorus increases in the buildup range. Phosphorus rate is crop removal rate in the maintenance range. Phosphorus rate decreases to the starter level in the drawdown range (Grant and Flaten, 2019).

D) Phosphorus Rate Response Research Results for Pulses zone and experience deficiency. Application of fertilizer phosphorus Pulse crops often do not respond strongly to fertilizer phosphorus, at seeding with early-sown pulse crops is especially important as cold and instead scavenge native soil phosphorus as they can form AMF soil temperature slows root growth and restricts the rate of associations to acquire soil phosphorus, and are also able to release phosphorus-releasing reactions and movement in soil, limiting plant root exudates to acidify the root zone and solubilize phosphorus from root access to new soil phosphorus sources (Sheppard and Racz, calcium phosphate minerals. Although pulse crops do not always 1984; Grant and Flaten, 2019). When seeding pulse crops first, respond strongly to fertilizer phosphorus, fertilizer should be applied application of phosphorus near the seed at seeding is an important to prevent mining of soil phosphorus. Pulse crops will most likely practice to ensure early season availability. respond to applied phosphorus in low phosphorus soils, in cold soils, Because pulse crops are effective scavengers of phosphorus, they will in very wet or dry soil, or in rotation with non-AMF hosting crops such be able to utilize residual phosphorus applied in previous years. as canola (Schoenau presentation). Review of Western Canadian Another timing option is to apply excess phosphorus with the research specific to each pulse crop gives a good indication of what previous crop to cover removal rates for both crops, and to apply only rates and conditions may produce a yield response. the seed safe rate of starter phosphorus with the seed in the pulse year. This strategy would allow growers to achieve removal rates Table 2. Pulse Crop Phosphorus Uptake & Removal Rates per Harvest Unit without compromising seed safety. Phosphorus Uptake Phosphorus Removal Applying phosphorus at the time of seeding is more effective than a

(lb P2O5 per harvest unit) (lb P2O5 per harvest unit) foliar application, due to the importance of early season phosphorus for crop development. However, foliar phosphorus may be helpful as Harvest a top-up when a plant is exhibiting deficiency symptoms or is under Crop Low High Avg. Low High Ave Unit stress with phosphorus limiting. Research in other regions found that foliar phosphorus did prove beneficial where soil levels were low or Peas bushel 0.76 0.92 0.84 0.62 0.76 0.69 the crop was under stress (Benbella and Paulsen, 1998a; Benbella and Lentils bushel 0.73 0.90 0.82 0.60 0.66 0.63 Paulsen, 1998b; Girma et al., 2007; Mosali et al., 2006).

Faba Research on low phosphorus Saskatchewan soils in field peas bushel 0.70 0.85 0.78 0.67 0.78 0.73 Beans compared 100% soil-applied, 100% foliar, and combinations of both (1:4 and 1:1 foliar to soil-applied phosphorus) at the 6-9 node stage. Chickpeas CWT - - 0.83 - - - At a site that was highly phosphorus deficient, a 100% soil application and the combination treatments produced significantly higher yield Soybeans bushel 0.8 1.0 0.90 0.80 1.00 0.90 than the 100% foliar treatment in one year. In the other site year, Dry Beans CWT - - 1.39 - - 1.12 100% seed-placed phosphorus was higher than all other treatments (Froese et al., 2018). This yield response is likely due to the CWT = 100 pounds importance of phosphorus availability early on in the season that is Source: Adapted from Canadian Fertilizer Institute, 2001; Heard et al., 2006; lacking when much or all is applied as a foliar application. and Government of Saskatchewan 4) Right Placement Each pulse crop varies in their responsiveness to applied phosphorus. Due to the importance of early season phosphorus availability and its Faba beans may be highly responsive with some research reporting -1 relatively low mobility in soil, banding is generally the best placement yields increasing at rates even above 80 lb P2O5 ac (Henry et al., method for maximizing plant access and fertilizer use efficiency. 1995; Holzapfel et al., 2017). Peas and lentils are considered -1 Banding puts fertilizer phosphorus into the soil where moisture for intermediate in response with 15-30 lb P2O5 ac most commonly dissolving fertilizer granules is higher in comparison to the soil eliciting a response when soils are low in phosphorus (less than 10 ppm Modified Kelowna phosphorus in the top 6 cm) (Henry et al., surface, and where plant roots can access it early in the season (Grant et al., 2001). Because phosphorus moves so slowly in the soil, only a 1995, Karamanos et al, 2003, McKenzie et al., 2001, Knight et al 2012, limited amount of broadcast phosphorus will likely make it to the Holzapfel et al., 2019). Soybeans, chickpeas, and dry beans respond rooting zone in the year of application. Incorporation of broadcast variably to phosphorus application. Regardless of the responsiveness phosphorus through tillage will move the applied closer to the rooting of the pulse crop to applied phosphorus, the amount used by the crop should eventually be replaced in the rotation to prevent soil zone, but will dry out soil. depletion. Banding of phosphorus fertilizer increases fertilizer use efficiency, as higher broadcast rates are required to elicit the same yield response 3) Right Timing as a lower soil-placed rate (Grant and Flaten, 2019). In a Saskatchewan field study in the Brown soil zone, banding of 18 lb The highest demand for phosphorus in pulses is in the early season -1 P2O5 ac elicited a 30% yield increase for soybeans compared to during the vegetative period and early reproductive phase, thus early -1 season availability of phosphorus is essential to maximize crop yield phosphorus broadcast at up to 71 lb P2O5 ac (Weiseth, 2015). As potential. Saskatchewan research found that in peas and lentils, the well, banding increases crop phosphorus uptake relative to weed highest uptake rate occurred at 28-49 days after emergence, which is uptake, whereas broadcast phosphorus provides nutrients non- around the time plants begin branching (Malhi et al., 2007). Due to selectively to both weeds and the crop (Blackshaw and Brandt, 2009; the importance of early season phosphorus availability, application of Blackshaw et al., 2004). Phosphorus use efficiency is also reduced some fertilizer close to the seed at the time of seeding is imperative. with broadcasting because it can be lost through surface runoff, Newly developing plant root systems have limited surface area to which presents a loss from the cropping system as well as a access residual soil phosphorus. Without added phosphorus near the contaminant to watersheds (Wiens et al., 2019). seed, juvenile roots may quickly deplete the phosphorus in their root

One appropriate use for broadcasting of phosphorus is to build up soil Fertilizer source also has an impact on seed safety. DAP (18-46-0) has levels when using the long-term sustainability technique. Larger reduced seed safety in comparison to MAP (11-52-0), as DAP amounts of phosphorus can be applied by broadcasting than with produces higher ammonia toxicity. TSP (0-45-0) has a lower impact on seeding equipment, and as phosphorus is relatively immobile in soil, pulse seeds due to the absence of nitrogen and a lower salt index, but the nutrients will stay in the soil profile and build up soil levels. is not as widely available (McKenzie et al., 2001). Seed safety Incorporation is recommended to prevent losses from runoff (Grant guidelines were developed using MAP fertilizer, thus agronomists and and Flaten, 2019). Based on the above research, in most situations, growers should take into consideration the phosphorus source being phosphorus fertilizer should be banded rather than broadcast due to used when determining the amount to put with the seed. Polymer- maximized plant uptake and use efficiency. coated phosphorus products may also improve seed safety by slowing the dissolution of the fertilizer. When banding phosphorus, depending on seeding equipment, a farmer may be able to apply fertilizer directly with the seed, in a Soil factors impacting phosphorus toxicity include soil moisture, sideband, or in a mid-row band. Fertilizer placed directly with the texture, organic matter content, and pH. Seedling damage from seed provides the seed with early season access to phosphorus, phosphorus fertilizer will be worse on dry, coarse textured soils at a maximizing root growth and crop development. With banded higher pH. In moist soils, fertilizer concentration in the soil solution is fertilizer, the plant must rely on soil phosphorus for initial lower than in dry soils. Coarse textured and low organic matter soils development until roots can reach the applied fertilizer. Banding may have a lower CEC and thus fewer exchange sites to adsorb result in an early season deficiency, especially on low soil phosphorus ammonium, allowing more ammonium in the soil solution to convert and when the band is further from the seed (ex. mid-row band). to ammonia and damage seed (Karamanos et al., 2003). Similarly, higher pH shifts equilibrium towards ammonia rather than Top Dressing or Foliar Applications ammonium increasing ammonia toxicity (Kaiser and Rubin, 2013; A Saskatchewan study looking at foliar KH2PO4 applications compared Rehm and Lamb, 2009). Several factors that vary by farm and soil type to soil-applied phosphorus in peas as well as canola and wheat, found impact the safety of seed-placed fertilizer. These factors must be a better response to soil-applied than foliar because of improved considered to determine whether the government seed safety early season phosphorus availability. Canola, having a larger leaf area guidelines are too high or low for the specific crop to be grown. and the most complete canopy closure at time of spraying, had greater uptake than wheat and peas, as it was able to capture more Seed safe rates of phosphorus are well below crop removal rates for of the applied foliar phosphorus (Froese et al., 2018). In-crop most pulse crops. In order to apply removal rates, phosphorus topdressing of dry phosphorus fertilizer is not an effective placement beyond seed safe levels must be applied in the sideband or mid-row, method, as it has low mobility in the soil and will not move to the broadcast and incorporated, banded ahead of time, or applied with plant’s rooting zone in the year of application. the previous crops. Seed Safety Summary Seed safety is an important component of fertilizer placement, as Phosphorus is an essential nutrient for pulse crop production, as it is a fertilizer can harm seed as a result of salt damage and ammonia component of many plant molecules necessary for growth and toxicity (Grant and Flaten, 2019). Pulse crops can only tolerate a small development, as well as for nitrogen fixation. 4R stewardship of amount of seed-placed phosphorus (Table 3). phosphorus in pulse crops involves using the right source, at the right rate, applied at the right time, in the right place. Understanding the Toxicity of seed-placed phosphorus to pulse crops is impacted by 4Rs for phosphorus, taking into consideration its high reactivity and several factors related to seeding equipment, fertilizer source, and low mobility in the soil, and the importance of early season soil characteristics. The main variable in determining seedling phosphorus nutrition for plants, is essential to maximize pulse crop phosphorus toxicity is the type of seeding equipment being used. production. Seedling safety increases with narrower row spacing and wider The right source of phosphorus for pulse crops is the source that will openers, as the same amount of fertilizer is spread over a larger be available to meet plant demands with the least amount of opener area, in more rows, in comparison to a seeding implement environmental impact. The most readily available form of phosphorus with wider row spacing and narrower openers. Seedbed utilization is commercial fertilizer, with dry monoammonium phosphate (MAP) (SBU) is a calculation that indicates the area of seedbed over which and liquid ammonium polyphosphate (APP) being the most common the fertilizer will be spread. sources available in Western Canada. Using by-product forms of Table 3. Safe Rates of Seed-Placed P2O5 for Pulse Crops phosphorus, such as manure and struvite, is an effective means to a b recycle phosphorus. Fertilizer additives and coatings, and microbial Crop Saskatchewan Manitoba (lb ac-1) (lb ac-1) products have sometimes shown inconsistent results in improving phosphorus uptake and yield in pulses. The right rate of phosphorus Faba Beans 40 20 for pulse crops is crop and field specific and depends on consideration Dry Beans 30 (pinto) 10 of target yields and soil availability. Pulse crops show variable Lentils 20 - responses to phosphorus fertilization in the year of application, with the greatest likelihood of response on soils that test low in supplies of Chickpeas 20 - plant available phosphorus. Peas 15 20 To maintain or build soil phosphorus levels, removal rates should be Soybeans 20 (limited data) 10 considered over time. The greatest demand for phosphorus is in the a Recommendations from Saskatchewan Ministry of Agriculture, based early season, thus the right timing for pulse crops, especially when on 1" openers with 9" row spacing (10-15% SBU) and good-to-excellent soil phosphorus is low, is at seeding. moisture conditions, using MAP b Recommendations from Manitoba Agriculture based on disc or knife openers with 1” spread and 6-7” row spacing (15% SBU)

Alternatively, higher rates can be applied in the year prior to a pulse, 4R Management of Phosphorus in Pulses as pulse crops effectively scavenge residual soil phosphorus. The right placement for phosphorus is in-soil, considering maximum safe rates 1. Right Source—in a form that suits the use; for immediate with the seed, and proximity from the seed-row. Soil application of uptake by the crop phosphorus needs to be in a readily phosphorus maximizes plant uptake and nutrient use efficiency in available form; for soil building or longer term sustainability comparison to broadcasting. Seeding equipment, fertilizer source, the source of phosphorus is less critical and soil conditions impact seed safety and are important factors to consider when seed placing phosphorus. As well, seed safe rates are 2. Right Rate— the rate that will supply enough phosphorus often lower than crop removal rates, thus more phosphorus should (along with soil phosphorus) for the crop to reach target be applied through another placement method to maintain or build yields; removal rates are good rates for sustainability over soil levels. 4R stewardship of phosphorus in pulse crops benefits both the long term; soil testing for monitoring soil levels over time the environment, and farm bottom line, as the most agronomically is important beneficial practices of 4R phosphorus management are also the most 3. Right Timing— Phosphorus needs to be available for uptake environmentally sustainable. throughout the season but highest requirements are during rapid vegetative growth and reproductive phases. Applications early in the season (seeding) target availability during growth 4. Right Placement—Phosphorus does not move in the soil and needs to be accessible to the roots. Banding into the soil is the best option and can include banding prior to seeding, placing with the seed, sidebanding, and/or mid-row banding

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