Effect of phosphorus fertilizer form, opener spread and rate of application on biomass yield, P uptake and recovery in a canola-wheat-pea rotation under controlled environment conditions

Abstract The form and opener configuration used to apply phosphorus fertilizer can influence its effectiveness and crop response. A growth chamber study was conducted to examine the response of crop (canola, wheat, and pea grown in rotation) and soil to P fertilizer form (monoammonium phosphate and struvite), opener spread (narrow versus wide), and rate treatments of 0, 20, 40, and 60 kg P2O5 ha−1 added to canola (B. napus) in the seed-row, followed by wheat and pea without P fertilization to examine residual effects. Crop emergence, 30 days above ground biomass yield, uptake of P and apparent recovery of fertilizer P were determined, along with labile soil residual P concentrations at the end of the rotation. Both MAP and struvite produced similar crop biomass yield and P uptake response. The narrow opener spread (1”) performed better in canola yield response to added P and in recovery of P fertilizer compared to the wide (3”) spacing and did not appear to reduce canola emergence even at the highest rates of added P fertilizers.


Introduction
Application of phosphorus fertilizer is critical in maximizing crop performance in western Canada (Grant and Flaten 2019). The supply of P to canola early in the growing season via placement in the seed row is important in providing a 'jump start' effect, which refers to increased root growth and early season vigor due to P being available for uptake by the seedling early on. Generally, the P fertilizer is applied to crops on the prairies as granular monoammonium phosphate (MAP) (11-52-0) in the seed row at low rates, as rates higher than 25 kg P 2 O 5 ha À1 in the seed row can cause damage to sensitive crops like canola and peas (Saskatchewan Ministry of Agriculture 2019).
Unlike N and S that are mobile in the soil, P is not as mobile and needs to be placed close to the roots to maximize availability in the year of application (Cynthia A. Grant and Flaten 2019). The side-band system offers a good compromise application strategy, where fertilizer is placed in a band about 1 inch to the side and 1 to 1.5 inch below the seed at the time of seeding (Mooleki et al. 2010). However, this close side banding option is not always available to producers and, depending on the seeding set-up they use, they may only be able to place the P fertilizer in the seed-row. However, fertilizer can be toxic if concentration near the seed is too high. A high rate of seed row P fertilizer placement such as that exceeding 30 kg P 2 O 5 ha À1 was observed to cause salt injury to young seedlings of some crops, including canola (P. Qian et al. 2012). Opener spread is the width of the furrow created by the opener on the seeding tool in which seed and fertilizer are placed together within the soil. Opener spread also can affect injury potential and fertilizer performance. A narrower opener spread results in seed and fertilizer in closer proximity, increasing the salt effect experienced by the seed. However, the narrower spread also reduces the distance for P diffusion from granule to rootlets and potential fixation by reaction with soil constituents.
While MAP is the dominant P fertilizer form placed in the seed-row by prairie small grain producers, there are other sources, such as Crystal Green TM (5-28-0, with 10% Mg), which is a struvite (magnesium ammonium phosphate) mineral extracted from wastewater streams. This product has recently become available and promoted for use as a more sustainable crop fertilizer in Western Canada. This less soluble P form may behave differently than MAP under different opener spreads. Limited research has been conducted on Canadian prairie soil to determine the optimum rate and opener spread of fertilizer P for modern canola cultivars, which have high yield potentials and high P requirement (40-50 kg kg P 2 O 5 ha À1 ) (Katanda et al. 2019).
The seed-row band application rate for MAP is not recommended to exceed 28 kg P 2 O 5 ha À1 (25 lb P2O5 ac-1) for canola in Saskatchewan. This rate, which is less than the P removal in grain of high yielding canola crops, may lead to soil P depletion and reduced yield of canola as well as the cereal and pulse crops that typically follow in rotation. Therefore, a growth chamber experiment was conducted to assess how P fertilizer form (MAP vs. struvite), along with opener spread (1" & 3") and P rate (i.e., 0, 20, 40, 60 kg P 2 O 5 ha À1 ) affected yield, P uptake and recovery by canola grown on a P deficient Brown Chernozem soil. Due to the immobility of P in soil, even under the controlled environment conditions, roots will not be able to take up all the P that is applied in a single season (Havlin 2014). Therefore, wheat and pea were grown in sequence after the canola to examine the influence of residual fertilizer P left behind in the soil in the seed row applied at different rates in the two different forms. The available P remaining in the seed-row soil at the end of the canola-wheat-pea growth sequence was also determined using chemical and ion exchange resin extraction techniques.

Soil description
The soil used for the growth chamber study was a P deficient Brown Chernozem soil of Ardill association collected from the field during the fall of 2018 from a pea stubble field. The soil used has loam texture, pH of 7.7, electrical conductivity (EC) of 0.25 dS/m (non-saline) in a 1:2 soil:water suspension and a modified Kelowna extractable soil test P concentration of 11 mg P kg À1 soil, which indicates deficiency in available P according to guidelines suggested for prairie soils (Cynthia A. Grant and Flaten 2019). Following collection from the field, the soil was air dried at room temperature and thoroughly mixed with a rotary soil mixer to ensure homogeneity.

Experimental design
The growth chamber study was set up as a completely randomized design using elongated plastic trays of 0.73 m length, 0.16 m in width, and 0.16 m in depth. The trays each containing a treatment simplified movement for watering and enabled ease of random repositioning within the chamber each day. Canola (B. napus hybrid Invigor Liberty Link variety LL252), wheat (Triticum aestivum hard red spring wheat var AAFC Brandon), and pea (Pisum sativa dry green var CDC Stryker) were grown in sequence in the prepared soil trays in the University of Saskatchewan phytotron growth chamber facilities under controlled environment condition. The crop emergence, 30-days biomass yield, P uptake, apparent fertilizer P recovery, and soil residual P were determined. The 30 day above-ground crop sample P concentrations were analyzed by grinding of samples dried at 60 C, followed by a hot sulfuric acid-peroxide digestion (Thomas, Sheard, and Moyer 1967) and colorimetric analysis of P concentration in the digest using SEAL TM automated colorimetry.
To duplicate fertilizer application as it would occur in the field, N and S was side banded 2.5 cm from the seed row and at 2.5 cm depth at 200 kg N ha À1 and 20 kg S ha À1 , respectively as urea and ammonium sulfate to eliminate any potential N and S availability limitations to growth. Phosphorus fertilizer was added only to canola; the wheat and pea crops were grown in sequence after the canola in the same rows (1" and 3" spread) to examine the crop response to the residual fertilizer P. For wheat, while P fertilizer was not added, N and S were side banded at the rate of 200 kg N ha À1 and 20 kg S ha À1 . For pea, no additional fertilizer was added, but all seeds were inoculated with Rhizobium leguminosarum before seeding to promote biological nitrogen fixation.

Calculations and statistical analyses
The equations used to calculate crop P uptake, crop P recovery, and total P recovery by the three crops are provided below. Note that the control is the comparable placement and fertilizer type treatment without P fertilizer added. P uptake ¼ crop P concentration x crop above À ground biomass Individual crop e:g:, canola ð ÞP recovery ¼ treatment P uptake À control P uptake P application rate Total P recovery canola þ wheat þ pea ð Þ ¼ treatment P uptake À ðconala þ Wheat þ PeaÞ À control P uptake P application rate The statistical analyses were conducted using RStudio (ver. 1.2.1335) software. A multi-factor ANOVA was conducted with the means separated by Tukey-HSD test at a ¼ 0.05.

Results
Fertilizer type (MAP, struvite) did not have a significant (a ¼ 0.05) effect on the first crop canola biomass yield and P uptake but had a significant effect on the following wheat crop. The last crop, pea, was not significantly affected by treatments. Application rate of P fertilizer significantly affected biomass yield and P uptake in canola and wheat but not pea. Opener spread significantly affected biomass yield in all three crops and P uptake in pea and canola. Opener spread and its interaction with application rate were significant for canola P uptake and wheat 30-day biomass yield. The calculated % recovery of added P fertilizer in the above-ground biomass was significantly affected only by opener spread for canola, which received the P fertilizer as the first crop grown. The proportion of P fertilizer added to the canola crop that was recovered in all crops in the sequence (canola plus the following wheat and pea crop ¼ total P recovery) was significantly affected by fertilizer type at a ¼ 0.05.
The MAP and struvite performed similarly in their effect on canola biomass and P uptake ( Figure 1). The effect of fertilizer type is more pronounced in wheat as the second crop in rotation, where struvite resulted in slightly greater biomass yields and P uptake. Pea did not significantly respond to fertilizer type when biomass means were compared with Tukeys HSD test at alpha ¼ 0.05. However, the MAP fertilizer resulted in a slightly greater P uptake in pea. Differences in opener spread significantly affected all crops in their 30-day aboveground biomass yield and P uptake response. The 1" opener resulted in a greater biomass yield and P uptake for canola and the following wheat crop ( Figure 2). However, for pea, the last crop in the rotation sequence, the 3" spread produced slightly greater P uptake, which might be due to a shallower and more spreading nature of the pea root system. Phosphorus fertilizer application rate significantly affected yield and P uptake by the P fertilized canola and the following wheat crop (Table 1).
In canola, addition of 20 kg P 2 O 5 ha À1 significantly increased canola biomass yield above the unfertilized control. Canola biomass yield at rates of 40 and 60 kg P 2 O 5 ha À1 were not significantly different from 20 kg P 2 O 5 ha À1 . The highest canola yield was produced at 40 kg P 2 O 5 ha À1 . The crop P uptake was more responsive to P fertilization than biomass yield, with 40 kg P 2 O 5 ha À1 resulting in significantly higher P uptake by canola than lower rates. Increasing P uptake beyond the point of maximum yield may be considered luxury uptake. Highest mean P uptake of canola was achieved at the highest rate of 60 kg P 2 O 5 ha À1 . Wheat was responsive to the residual P fertilizer left after the canola crop, with significantly higher 30 days biomass yield and P uptake at 60 kg P 2 O 5 ha À1 rate compared to lower rates. Pea, as the crop grown following wheat and canola, did not respond in biomass yield or P uptake to the P fertilizer application rates made to canola. This may reflect depletion of soil P by the previous crops as well as peas being good scavengers of soil P.
Canola and wheat had significant responses to opener spread width, and its interaction with P fertilization rate (Figure 3). The narrower 1" opener spread with 60 kg P 2 O 5 ha À1 rate of fertilizer application resulted in a significantly higher biomass yield and greater P uptake response in both the canola and wheat crop. In canola, the fertilization rate of 40 and 60 kg P 2 O 5 ha À1 resulted a similar 30-day biomass yield and P uptake, while 60 kg P 2 O 5 ha À1 fertilization rate resulted significantly greater 30 days wheat biomass yield over the rate of 40 kg P 2 O 5 ha À1 . Pea, the third crop in the crop rotation, did not show any significant response to the treatments. Increasing P Figure 1. Aboveground biomass yield (g) and P uptake (mg) of canola followed by wheat and pea in response to P fertilizer type applied to the canola in the chamber study. Means were separated using Tukeys HSD test (a ¼ 0.05). For a crop for a single variable (e.g., Biomass), bars with different letter indicate significant difference. Figure 2. Aboveground biomass yield (g) and P uptake (mg) of canola followed by wheat and pea in response to opener spread in the chamber study. Means were separated using Tukeys HSD test (a ¼ 0.05). For a crop for a single variable (e.g., Biomass), bars with different letter indicate significant difference.  fertilizer rate resulted in a trend of lower mean emergence with increasing rate (Figure 3). This was observed for all crops including the canola to which the P fertilizer was added to the seed row, as well as for the following wheat and pea crops to which no P fertilizer was added and the crops were seeded into the seed row of the previous crop. However, the effects on emergence were not significant at p ¼ 0.05 for any of the treatments (rate, spread, P fertilizer type). The higher P uptake with the 1" spread versus the 3" spread, which was significant at the highest P rate, may be explained by reduced soil-fertilizer contact and interaction, reducing fixation by adsorption and precipitation reactions.
In the canola crop to which the fertilizer P was applied, opener spread is the major variable that affects the apparent P recovery, where 1" spread resulted in significantly higher P recovery compared to 3" opener spread (Figure 4). For P fertilizer added to canola crop that was recovered in all three crops grown (canola plus the following wheat and pea crops), the only significant response was to fertilizer type, where MAP resulted in slightly, but significantly greater apparent total P recovery compared to struvite ( Figure 5). This may reflect slightly greater solubility of MAP fertilizer reaction products in soil compared to struvite.
Both soil available P assessment methods (Modified Kelowna and Ion Exchange Resin) showed similar treatment effects, where residual plant available P in the soil is significantly affected by fertilizer type, application rate and their interaction (Table 2). Both assessment methods indicate residual soil available P increased with increasing P fertilization application rate (Table 2). This suggests that fertilizer P applications made to the canola (first crop in the rotation) and subsequently utilized as residual P by wheat and pea would continue the residual benefit in enhanced soil P availability to crops in a second phase of rotation, especially at higher rates. As well, both assessments show that at higher application rates above 20 kg P 2 O 5 ha À1 , there is significantly less residual available P in the soil with struvite P form than with MAP. At high application rates such as 40 and 60 kg P 2 O 5 ha À1 , greater amounts of P fertilizer applied at the beginning of the rotation remained unutilized, thus increasing the opportunity for P to convert to less soluble forms. The lower residual available P observed with struvite at the higher rates cannot be explained by greater plant uptake and removal but may reflect presence and/or formation of less soluble reaction products from struvite in the soil compared to MAP.

Crop response to fertilizer rate
In the growth chamber study, canola showed a significant 30-day biomass yield response to P fertilization at the 20 kg P 2 O 5 ha À1 rate, while further rate increases produced no further significant yield increases. The rate response observed in this study is corroborated by results reported by others in the literature. A four-year field study in Manitoba found canola yield to be optimized at . Canola P recovery (% of P fertilizer applied to canola present in above -ground biomass after 30 days) in response to opener spread. Means were separated using Tukeys HSD test (a ¼ 0.05). Bars with different letter are significantly different. 24.7 kg P 2 O 5 ha À1 (22 lb P 2 O 5 ac À1 ) (C. A. Grant et al. 2009). McKenzie et al. (2003) noted the most profitable P fertilization rate was between 10-20 kg P 2 O 5 ha À1 for canola. As canola is sensitive to seed-row placed P fertilizer, a high rate of MAP, for example above 30 kg P 2 O 5 ha À1 rate, can lead to seedling damage and yield reduction (Bailey and Grant 1990;C. A. Grant 2013;Grenkow 2013). Previous growth chamber studies have found that canola was quite sensitive to seed-row MAP fertilization, with a significant negative impact occurring at rates of 30-40 kg P 2 O 5 ha À1 and above (Qian and Schoenau 2010). In the current chamber study, there was a trend of reduced emergence after 5 days with increasing rate of seed-placed MAP and struvite, going from about 95 percent emergence at the 0 kg P 2 O 5 ha À1 rate to 85 percent emergence at the 60 kg P 2 O 5 ha À1 rate, but the effects were not significant at a ¼ 0.05 level of significance. Furthermore, emergence rates were similar between MAP and struvite. Reflecting the ability of canola to compensate for reduced emergence, no negative impact was observed on canola 30 day above ground biomass yield and P uptake with high P fertilization rate. The canola P uptake was maximized at 40 and 60 kg P 2 O 5 ha À1 , which might be considered as luxury P uptake.
Modern hybrid canola cultivars have not only greater yield potential than old non-hybrid cultivars but also greater vigor that may enable them to tolerate high rates of fertilizer P with the seed better, and to be able to better compensate for any stand reduction. However, one must be careful in extrapolating the results from trays in a growth chamber with controlled environmental conditions to a field situation where conditions are less than optimal.
Interestingly, the following wheat crop was very responsive to the P fertilizer that was added to the previous canola crop, with a more pronounced rate effect observed than canola. The presence of residual fertilizer P in soil after canola growth significantly increased the wheat above ground biomass yield in comparison to the control, and the soil with 60 kg P 2 O 5 ha À1 added to the previous canola produced the greatest wheat 30 day biomass yield. Overall, the following wheat crop appeared to benefit significantly from the fertilizer P applied to canola that was in excess of the canola P requirement and uptake potential. Figure 5. Total P recovery (% of P fertilizer applied to canola recovered in above-ground biomass of canola þ wheat þ pea crops after 30 days) in response to fertilizer type. Means were separated using Tukeys HSD test (a ¼ 0.05). Bars with different letter are significantly different. Table 2. Effect of fertilizer type and its interaction with application rate on residual soil available P assessments made at the end of study after pea harvest. Means in a column followed by the same letter are not significantly different based on Tukeys HSD test at a ¼ 0.05 level of significance.

Fertilizer
Rate kg P2O5 ha À1 The lack of response of the pea crop as the last crop in the rotation, to P fertilizer treatment added to canola, might be due to the depletion or fixation of soil residual P. However, significantly higher extractable available P levels remaining in the soil after the pea crop in the higher P rate fertilizer treatments suggest that the previous crops did not use all the fertilizer P applied, and that differences in P availability were present at the start of pea growth among the treatments that might have been expected to produce differences in pea yield and P uptake. Yield response of pulse crop to P fertilization is normally not large even under P deficient soil conditions (Gervais 2009). Furthermore, as a pulse crop, pea is a good scavenger of P from the soil, and the differences in residual soil P level may not influence the pea crop significantly. Peas can develop strong mycorrhizal relationships when soil P levels are low and acidify the rhizosphere to solubilize P minerals (Xie, Schoenau, and Warkentin 2017).

Assessment Method
These findings support the concept of making larger P fertilizer applications to a crop than required to maximize yield, with the intention of having the unutilized fertilizer P carry over and provide benefit to subsequent crops. It has long been known that on low P soil, the buildup of background soil P levels combined with low rates of starter P fertilizer application can provide crops with very good growth benefits (Alessi and Power 1980). However, the effect and degree of observed benefit from residual fertilizer P will depend on the following crop type. In the controlled environment study of this thesis, wheat greatly benefited from increased residual soil P level, while pea did not show much response.

Effect of opener spread
In comparison to the 3" opener spread, the 1" opener spread resulted in a better biomass yield, P uptake, and P recovery in both canola and wheat. The soils from the Northern Great Plains are generally high in pH with high levels of calcium. The soil used in the pot study had a pH of 7.7, and in similar soils, phosphorus was shown to react strongly with calcium present in the soil and form sparingly soluble calcium compounds like brushite (Peak et al. 2012). Over time, these compounds would become increasingly less available by conversion to insoluble forms like apatite (Kar et al. 2017). The narrow 1" opener spread, which distributes the fertilizer across a lower proportion of the seed bed soil than the wide 3" spread opener, would be expected to result in a higher concentration of P per unit of soil and therefore greater potential for saturation of soil adsorption sites in the application zone as the fertilizer dissolves, keeping more P in solution. A wider spread may also increase the time for root access, enhancing the formation of insoluble P minerals (greater fixation) and the removal of available P from solution by sorption, all of which reduce the solubility and availability of added P fertilizer, and the carry over benefits for the following crop.
While seed-row P fertilizer application is an effective method that provides crop with earlyseason access to the fertilizer, plants may experience the fertilizer toxicity and root growth inhibition if the application rate is too high, especially when applied with seed in a narrow band (Bailey and Grant 1990). The damage from high rate P fertilizer is related to salt damage (dissolution of the fertilizer salt) and ammonia (N) toxicity. The MAP (5211-0) used in the pot study contains 11% N by weight (14 kg N ha À1 when MAP @60 kg P 2 O 5 ha À1 ). Canola is sensitive to N which can damage the seedling and negatively influence the crop (Grant et al. 2011;Malhi and Gill 2004). The negative effect of a highly concentrated fertilizer band on early root growth is likely to be less in the ideal environment of a growth chamber compared to the field. The pots in the growth chamber were watered every two days, which would dilute or maintain the fertilizer concentration in the root zone at a safer level in comparison to where moisture is limited. Under controlled environment conditions, 1" opener spread resulted in a higher above-ground biomass yield and P uptake compared to wider opener spread. No fertilizer toxicity symptoms were observed in the controlled environment study of this thesis, suggesting that modern canola varieties may have a greater tolerance level under high P fertilization rates. As well as the sufficient moisture under the controlled environment condition dilutes the P concentration and reduces the P fertilizer toxicity.
Before the seeding of wheat, the crop root residues from the previous canola crop were removed to enable seeding, and the seed row was disturbed by the seeding of the wheat into the seed-row of the previous canola crop. However, wheat as the second crop in the rotation still showed response to the different opener spread. This also provides support for the concept that seed-row placed P fertilizer in the 1" band had less fixation and sorption, which provided the following crop with greater level of plant available P.

Influence of fertilizer MAP vs struvite
There was no significant difference between MAP and struvite applied in the seed-row of canola on the 30-day biomass yield, P uptake and recovery of fertilizer P by the canola crop. Canola was responsive to P fertilizer application in both MAP and struvite form as it has high P demand and apparent ability to use P fertilizer effectively. It has been well documented that canola will positively respond to P fertilization with MAP, especially when soil P test values are less than 10 ppm. Canola has a combination of tap and fibrous root system that can explore significant soil volume and uptake P from the soil solution. Canola can proliferate its roots in areas with high P concentration, which enhances the ability of utilizing P fertilizer (Strong and Soper 1974). When the concentration of fertilizer salts in the band is low, like many other crops, canola can acidify its rhizosphere by the exudation of organic acid which increases P availability (Hoffland 1992;Hoffland, Findenegg, and Nelemans 1989). One study found that canola roots could lower the pH by 0.8 units (McKenzie et al. 1995). Unlike MAP, which is highly soluble in water, struvite is less soluble, but its solubility can be increased under acidic conditions (Ackerman et al. 2013). The organic acid released by root system of canola may give canola the ability to utilize applied P effectively in the form of struvite. (Cynthia A. Grant and Flaten (2019) suggested that a low solubility P fertilizer like struvite may show improved performance when soil -fertilizer contact is increased, such as by broadcasting or using a wider opener spread. However, there was no significant fertilizer form by opener spread interaction observed in the current study for any of the crops. The results of this thesis work demonstrate that struvite can be as effective as MAP for canola fertilization, at least under controlled environment conditions with suitable moisture and temperature, in agreement with results of (Ackerman et al. 2013). (Ackerman et al. 2013) Wheat in this study, as a following crop relying on residual fertilizer P left behind after the canola crop, had a slightly higher biomass and uptake response to struvite than MAP. Like canola, wheat can develop roots that proliferate in soil areas with higher concentrations of available P, which is beneficial in accessing P from fertilizer (Strong and Soper 1974). Addition of Mg in struvite might be expected to slow the conversion to less soluble apatite forms over time as noted by (Kar et al. 2017). However, the time elapsed in this growth chamber experiment from beginning to end (3 months) was much less than that which would occur in the field (3 years) using the same rotation. A detailed evaluation of the P reaction product species formed when MAP and struvite granules undergo dissolution, and their changes over time in the soil as the products age would be beneficial in helping to explain differences in plant yield response and P uptake.
Pea showed very little response to P fertilizer application form, rate and opener spread. As a pulse crop, pea has a capability of forming strong mycorrhizal associations to assist in accessing soil P (Bailey and Grant 1990), which makes pea a good scavenger for soil P. Therefore, as the third crop in the rotation, pea did not show much response to P fertilizer treatment and soil residual P from either P source.
When considering total P recovery, calculated by summing the P uptake attributable to fertilization for all three crops in relation to the P fertilizer applied at the beginning in the seed row of canola, the struvite had slightly lower apparent % recovery of the fertilizer P compared to MAP. This may be due to reduced availability to the plant of the fertilizer reaction products. Overall, the findings of the growth chamber study indicate that struvite is a good alternative P source for canola that can also benefit following crops in rotation.

Residual soil P
Modified Kelowna and Ion Exchange Resin Membrane techniques showed a similar pattern in residual soil available P at the end of the three crops rotation (canola -wheat -pea). As expected, fertilization rate had strong effect on residual soil P, where higher application rate left more fertilizer P in the soil behind as unused P, some of which is present in the labile, plant available P fraction extracted by chemical solution and ion exchange techniques. A noteworthy finding is that for the same high rate of P application of 60 kg P 2 O 5 ha À1 , the MAP resulted in significantly greater residual available P according to soil assessment than struvite, which might be due to the lower solubility of struvite and its reaction products upon dissolution. However, the ability of successive crops to access residual struvite P was similar to MAP P in terms of crop P uptake and recovery, which may also reflect the inability of the soil P residual analysis methods to account for plant rhizosphere and P solubilization effects.

Conclusion
MAP and struvite were effective P sources for the crops in this controlled environment study. A narrower opener spread that creates less distance between fertilizer P and the seed and reduces contact between fertilizer and soil appears to have benefit in increasing crop yield, P uptake and recovery. Canola, wheat, and pea were grown in sequence to provide a contrast in rooting system and crop P demand and to represent a typical oilseed-cereal-pulse crop rotation sequence. Significant residual benefit to following wheat crop was observed from carryover of fertilizer P applied to the preceding canola crop. Depending on the crop type and growth conditions, plants may increase root development, exude organic acids, or establish associations with mycorrhizal fungi to improve access to P. Therefore, the crop yield response to P fertilizer application will depend on the amount of P required by the plant related to yield potential, its ability to access P from soil that is affected by root characteristics and microbial relationships, and the ability of the soil to supply P to roots in the amounts and time that the P is needed. Rate, placement, form, and time of fertilizer P application (Grant and Flaten 2019) need to be considered as this affects the success of the P fertilizer application in providing the supplemental P that the crop needs.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The financial support of the Canola Agronomic Research Program (CARP) is gratefully acknowledged.