Phosphorus management for sensitive crops: Managing phosphorus through the rotation Abstract Introduction

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1 Phosphorus management for sensitive crops: Managing phosphorus through the rotation Cynthia Grant, Agriculture and Agri-Food Canada, Brandon Research Centre, Box 1000A, R.R.#3, Brandon, MB R7A 5Y3 Abstract Crop rotations in Manitoba are shifting towards greater production of crops such as soybean and canola that are sensitive to seed-placed fertilizers. A high-yielding canola or soybean crop will remove more phosphorus than can be safely applied in the seed-row, according to current recommendations. Side-banding can increase the amount of P that can safely be applied at the time of seeding. Both canola and soybean can effectively access phosphorus from the soil if there are sufficient reserves present. In many fields in Manitoba, soil phosphorus levels are relatively high due to long-term applications of manure or relatively high fertilizer phosphorus inputs. In contrast, many other soils are deficient in phosphorus and those deficiencies will be increased if more phosphorus is removed from the soil than is returned over time. The risk of soil depletion becomes greater with more frequent production of soybean and canola in the rotation if phosphorus applications are restricted to recommended seed-placed levels. Therefore, it is important to consider phosphorus input and off-take throughout the cropping sequence so that phosphorus can be managed in a way to optimize crop yield while avoiding either excess accumulation or depletion over time. This may be done by modifying the method of phosphorus fertilization in sensitive crops to allow applications that match crop removal. Manure can be a valuable P source, where available. Alternately, greater phosphorus inputs can be applied at other stages in the cropping sequence to compensate for the deficits in the sensitive crops and balance phosphorus input and off-take over time. Introduction Phosphorus fertilizer is a major input for crop production in Manitoba. Phosphorus is critical in the metabolism of plants, playing a role in cellular energy transfer, respiration, and photosynthesis. Phosphorus is also a structural component of the nucleic acids of genes and chromosomes and of many coenzymes, phosphoproteins and phospholipids. An adequate supply of P is essential from the earliest stages of plant growth, since phosphorus is required for all the growth processes required for seedling germination and establishment. Symptoms of P deficiency include decreased plant height, dark green or purpling coloration, delayed leaf emergence, reductions in tillering, secondary root development, and dry matter yield and seed production. Banding phosphorus near the seed improves phosphorus response If P supplied from the soil and seed reserves is inadequate to support optimum crop yield, fertilizer applications can supply P to the plant. Phosphorus supply during the first two to six weeks of growth tends to have a large impact on final crop yield in most crops; therefore, it is important that P fertilizer applications are managed in a way that ensures early season access of the fertilizer by the growing crop. Banding in or near the seed-row is the recommended placement method for phosphorus fertilizer on the prairies. Studies reported in the Westco training manual indicated that banding 20 kg phosphate ha -1 near the seed-row was as effective as broadcasting and incorporating 80 kg phosphate ha -1. Banding the P near the seed-row puts the fertilizer in a position where the roots can contact it early in growth, increasing the early season P supply. A large number of studies in many plant species have shown that early season P supply is critical for optimum crop yield. Withholding P during early plant growth will limit crop production and cause a

2 restriction in crop growth from which the plant may not recover. Phosphorus limitation later in the season has a much smaller impact on crop production than do limitations early in growth. Concentration of P in the soil solution is usually low since when phosphate is added to the soil, it reacts relatively quickly with calcium (Ca), magnesiumm (Mg), and iron (Fe) and aluminum (Al) to form less soluble phosphates. Most phosphate movess to the plant by diffusion rather than mass flow, and as P movement through the soil to the root is restricted, diffusion is generally considered to be the rate-limiting factor in P absorption by plants. It is estimated that, on average, phosphate could only diffuse approximately 0.22 to 0.5 mm,, so that only phosphate within 0.5 mm of a plant root is in a position where it cann be accessed by the crop. Placing the fertilizer near the seed-row allows the first plant roots to contact the fertilizer and begin uptake early in growth. As the crop root system grows, it is ablee to access P from a larger volume of soil and begins to rely less on the P from the fertilizer near the seed and more from P in the bulk soil (Figure 1). Having a combinationn of P banded near the seed-row and an adequate concentration of P in the bulk soil provides the plant withh the P required throughout the growing season. Figure 1: Absorption of P from seed-placed P fertilizer and from the bulk soil in three crops through the growing season (Kalra and Soper 1968). Uptake of P by the plant is proportional to the root density, so enlargement of the root surface area increases the ability of the plant to access and absorb P from the soil. Therefore, many plants respond to low soil P concentrations by enlarging the root system and developing highly branched roots with abundant root hairs to enhance their ability to explore new soil reservess of P and efficiently extract P from the soil when areas of high P are encountered. Having the fertilizer placed in a band near the seed-row provides thee opportunity for the plant to contact the fertilizer granule and to begin root proliferation in the high-concentration reaction zone. This increasess the plant s ability to utilize the fertilizer when itt is needed to plant establishment. Cold soil temperatures slow diffusion of P in soil, reduce P solubility and decrease root growth. Therefore, under typical Prairie conditions at planting, cold soil temperatures at seeding may increase the benefit of banding P near the seed-row.

3 Phosphorus deficit in sensitive crops While seed-placed phosphorus is an efficient method of fertilizer placement, excess seedplaced P may lead to seedling damage in sensitive crops (Figure 2). Canola, soybean and flax are all sensitive to seedling damage from monoammonium phosphate fertilizer, with stand density decreasing mainly due to the salt effect from the N portion of the fertilizer. Blending ammonium sulphate and monoammonium phosphate together can increase the damage additively. Because of the sensitivity of canola, soybean and flax to seedling damage, the amount of phosphate fertilizer that can be safely applied with the seed is low. If the provincial guidelines for safe placement of phosphate fertilizer with the seed are followed for canola and soybean, a good crop will remove more P from the soil than is added (Table 1). The simplest way of avoiding this problem is to shift the placement of the phosphate away from the seed-row. Applying the fertilizer as a side-band near the seed-row provides greater seedling safety yet is still an efficient method of P placement. However, if the fertilizer is more than an inch or two away from the seed-row, it may not be in a position where the roots can access it early in the season and so may not provide the crop a starter benefit. Stand Density (plants m -2 ) kg S ha-1 9 kg S ha-1 18 kg S ha Phosphate (kg ha -1 ) Figure 2: Impact of seed placed monoammonium phosphate and ammonium sulphate on stand density of canola (Grant and Grenkow unpublished). Table 1: Balance between phosphate removal and recommended safe limits for seed-placed monoammonium phosphate for common Manitoba crops Yield Removal Seed Limit Balance Crop bu/acre lb/acre lb/acre lb/acre Wheat Canola Soybeans Barley Flax Peas Oats

4 Historically in Manitoba, inputs of phosphate fertilizer and off-take of P in the plant was fairly well-balanced, because shortfalls in P input during production of canola was compensated by higher additions in the cereal years. However, cropping patterns in Manitoba are changing, with higher production of canola and soybean and lower production of cereal crops (Table 2). If removal of P is greater than input of P over time, the soil may become P depleted. Conversely if input of P exceeds P removal over time, as may occur with manure applications, soil P levels may increase. Long-term studies conducted by Fernando Selles at the AAFC research centre in Swift Current showed a good relationship between Olsen-P soil phosphorus levels and the balance between P applied and P removed in the crop (Figure 3). Table 2: Production of various field crops (000 acres) in Manitoba between 2001 and 2012 (Statistics Canada). Crop Wheat Canola Soybeans Barley Peas Flaxseed Oats Corn (grain) P depletion P build-up 60 k and 95% confidence Interval Olsen-P (kg ha -1 ) intercept and 95% confidence interval y * Pbal P-balance (kg ha -1 ) when Pbal <= k y * Pbal 1.1*( Pbal k) otherwise Figure 3: Soil test P values reflected the balance between P input and P removal in the crop in long-term studies conducted at Swift Current, Saskatchewan (Selles et al. 2011).

5 Similar results were found in studies across the prairies that evaluated the effect of annual inputs of approximately 0, 40, 80 and 160 lb of phosphate per acre from 2002 to 2010, in a durum wheat-flax cropping sequence (Figure 4). In these studies, withholding P fertilizer led to a large depletion in soil P while applications of 80 lb ac -1 or above led to a large increase. Application of 40 lb ac -1 produced minor changes in soil-test P, depending on the soil type. In this study, similar rates of P were applied to both crops in the rotation, even though the flax crop tends to remove lower amounts of P. In rotations with canola that removes greater amounts of P than are normally applied, the depletion would be greater than observed with this study. 80 Change in Olsen P (ppm) Carman Carstairs Brandon Ft. Sask Phillips Phosphate applied annually (lb/ac) Figure 4: Change in Olsen P values with annual P application after 8 years of cropping following a durum wheat-flax cropping sequence on five soils in Western Canada (Grant et al., unpublished). Either excess depletion or excess accumulation of P in soils can cause problems. Excess P accumulation can increase the risk of P movement into water bodies, leading to eutrophication, as is currently seen in in Lake Winnipeg. Conversely, depletion of soil P can reduce the supply of P from the soil to the crop, potentially limiting yield, especially in situations where the P application is reduced to meet safe limits for seed-placement. Where soils are depleted, the plant may not be able to access sufficient P from the soil to optimize yield. Studies conducted in Saskatchewan evaluated the effect of applications of seed-placed MAP on soils with and without application of initial large rates of broadcast MAP (Figure 5). Seed-placed P alone was not sufficient to optimize crop yields on soils that had very low background P levels, as measured by Olsen P. In order to maintain the long-term productivity of the soil, it is important to manage phosphorus through the rotation to maintain reasonable levels of available soil P, in the range of 15 ppm.

6 Figure 5: Effect of a single broadcast and annual seed-placed phosphate applications on wheat grain yield (Wager et al. 1986). While it is often stated that the efficiency of phosphate fertilization in the year of application is low, in the range of 30% or less, the P that is not used by the crop in the year of application will normally remain available to following crops, unless it is removed from the field by run-off or erosion. When phosphorus fertilizer is applied to a soil, it initially dissolves and enters the soil solution, but this shifts the equilibrium towards precipitation of P as Ca, Mg, Fe or Al-phosphates that have a lower solubility than the fertilizer that was applied. A large proportion of these phosphate compounds still remain predominantly in "labile" forms that can be accessed by the plant over time. As the plant feeds on the P in the soil solution, it shifts the equilibrium back towards the dissolution of these phosphates in order to replenish the soil solution (Figure 6) ). So, in any particular season, the plant will access newly applied P from the fertilizer, P that had recently precipitated and re-dissolved from the fertilizer applied in that season, as well as P derived from fertilizer that had been applied and precipitated in previous years. The <30% figure for nutrient use efficiency that is often used refers to the amount of P recovered in a single crop from the fertilizer thatt was applied in that growing season. When you consider the recovery of fertilizer P over multiple years, the fertilizer use efficiency is substantially higher. Studies in Rothamstad, England showed P recoveries over time of greater than 90% while in Swift Current, SK, nearly all of the P fertilizer applied was recovered over time if N was not deficient (Figure 7).

7 Figure 6: Phosphorus that is precipitated as sparing soluble phosphates can remobilize to replenish the soil solution when it is depletedd by plant uptake. Precipitation-dissolution is a two way reaction. Figure 7: In studies conducted by Selles et al. (2011), long-term multi-year recovery of applied phosphate fertilizer approached 100% when nitrogen was not limiting.

8 The long-term availability of phosphorus in the labile pool provides a number of options for enhancing the P supply through the rotation for sensitivee crops. One option is to apply very large single applications of P fertilizer, particularly if P fertilizer prices were low for some reason. In studiess conductedd by Bailey et al. (1977) in two locations in Manitoba, a single broadcast application of 200, 400 or 800 lb P 2 O 5 ac -1 increased crop yields and maintained soil P at levels above the deficiency level, even after eight years of cropping. Similar results were seen in the work by Wager (1986) shown in Figure 5. The high cost may make application of such large amounts of P fertilizer unattractive. A more economicc approach may be to take advantage of the livestock manure that is available in the province. Livestock manure is rich is phosphate, with ratios of available N:P 2 O 5 usually below 1:1. However, plants require and remove a N:P 2 O 5 ratioo in the range of 2 to 3:1. Therefore, applying manure at a rate to satisfy the N demands of a crop will apply much more P that the crop requires and can provide sufficient P for several years of crop production (Table 3). Table 3: Liquid hog manure or solid cattle manure applied at a rate to supply crop N requirements will supply several years of P needs (Courtesy of Don Flaten). 1 Manure analyses are from the Tri-Provinciais spring applied, by subsurface injection, with no significant volatilization loss of Manure Application and Use Guidelines 2 Assume s all manure NH 4 -N 3 Assume s that P is removed as grain only. Phosphorus may also be built up in the soil more gradually, by using maximum recommended seed-row application rates of P in crops such as wheat or barley that are tolerant of seed-placed MAP. In studies conducted at two locations in Manitoba, P concentration in the tissue of flax at six weeks was increased by application of P fertilizer to preceding wheat or canola crops, indicating that P from the preceding year remained available for the flax to use (Figure 8). Althoughh the flax seed yield in this study did not increasee with the P applied either to the preceding crop or directly as a side-band application to the flax itself, the increase in tissue P indicates that P applied to preceding crops will remain available and so P can be managed through the rotation to improve P status of sensitive crops (Figure 8).

9 Figure 8: Effect of P applied as MAP to Canola or wheat at two Manitoba locations (RC- tissue at six Research Centre and NTF No-till et al. 2009). Farm) on P concentration in the following flax weeks of growth (Grant Summary Crop rotations in Manitoba are ncluding large proportions of crops such as soybean and canola that are sensitive to seed-placed d fertilizers. A high-yielding canola or soybean crop will remove more phosphorus than can be safely applied in the seed-row, according to current recommendations. If the P removed in the harvested crop is not replaced throughout the rotation, soils may become depleted over time. The risk of soil depletion becomes greater with more frequent production of soybean and canola in the rotation if phosphorus applications are restricted to recommended seed-placed levels. Therefore, it is important to consider phosphorus input and off-take throughout the cropping sequence so that phosphorus can be managed in a way to optimize crop yield while avoiding either excess accumulation or depletion over time. This may be done by modifying the method off phosphorus fertilization, for example using side-banded or mid-row banded P, in sensitive crops to allow applications that match crop removal. Where available, manure application can be a good source of P for several crops. Alternately, greater phosphorus inputs can be applied att other stages in the cropping sequence to balance phosphorus input and off-take over time. Maintaining good soil fertility while using starter P near the seed-row to provide adequate early season P, particularly on cold soils, can provide sensitive crops with the P required to optimize yield.

10 Reference Bailey, L. D., Spratt, E. D., Read, D. W. L., Warder, F. G. and Ferguson, W. S Residual effects of phosphorus fertilizer. II. For wheat and flax grown on Chernozemic soils in Manitoba. Can. J. Soil Sci. 57, Grant, C. A., Monreal, M. A., Irvine, R. B., Mohr, R. M., McLaren, D. L. and Khakbazan, M Crop response to current and previous season applications of phosphorus as affected by crop sequence and tillage. Can. J. Plant Sci. 89, Kalra, Y. P. and Soper, R. J Efficiency of rape, oat soybean and flax in absorbing soil and fertilizer phosphorus at seven stages of growth. Agron. J. 60, Selles, F., Campbell, C.A., Zentner, R.P., Curtin, D., James, D.C., Basnyat, P Phosphorus use efficiency and long-term trends in soil available phosphorus in wheat production systems with and without nitrogen fertilizer. Can. J. Soil; Sci. 91: Wager, B.I., J.W.B. Stewart, and J.L. Henry Comparison of single large broadcast and small annual seed-placed phosphorus treatments on yield and phosphorus and zinc contents of wheat on Chernozemic soils. Can J. Soil Sci. 66: