Phosphorus Fertilizer Management

Table of contents

    Important tips for best management

    • Canola will respond well to starter phosphorus on low P soils.
    • Seed row placement is the best timing and placement for the first 15 to 20 lb./ac., which is the amount most likely to produce an economic return in the year of application. P is the most important nutrient to place in the seed row, due to limited mobility and the importance of early uptake.
    • Canola removes 0.9-1.0 pounds of phosphorus from the soil for each bushel of canola seed yield. This exceeds the amount that can be safely seed-placed, and exceeds the rate that many growers apply. This will deplete soils over time unless growers are consciously building soil P levels through higher application rates in other rotation crops.


    Canola response to fertilizer phosphorus

    Canola needs 1.3 to 1.6 pounds of phosphate fertilizer (P2O5) per bushel of yield. Harvest removes 0.9 to 1.1 pounds with the seed, leaving the balance on the field. A 40 bu./ac. canola crop takes up about 60 lb./ac. of P2O5 equivalent P, and removes 40 lb./ac. in the seed.

    Canola is very good, better than wheat, at extracting P both from the soil and from fertilizer granules. However, canola tends to take up more P than many growers apply, which will deplete soils over time unless growers are consciously building soil P levels through application rates that exceed removal in their other rotation crops.

    Yield

    Canola shows a yield response to P fertilizer no more than half the time.

    Without an adequate P supply, crop yield will be reduced. But while most agricultural soils in Canada have inadequate P for producing canola crops, canola is very good at recovering P from the soil and can make efficient use of soil P if soil P levels are moderate to high.

    With early seeding dates recommended for canola, some starter P fertilizer — 15 to 20 lb./ac. — placed in close proximity to the seed is warranted, even on soils with sufficient P for target yields. Canola response to P fertilizer depends mainly on the amount of plant available P in the soil but is also influenced by moisture and temperature. In cold soil, P availability and movement is reduced. Canola response to P fertilizer is greater under these conditions.

    Quality

    P fertilization has negligible effects on canola quality. Experiments in western Canada have found that P fertilizer increased, decreased or did not affect oil content. P fertilization has occasionally slightly increased canola protein content. A recent field experiment in Manitoba on two very deficient sites found that P fertilizer significantly increased both protein and oil content.

    Phosphorus fertilization can hasten maturity of canola crops by one or two days, primarily through more rapid emergence and early seedling growth. This may be important in short growing seasons.

    Economic

    In the short term, the greatest economic response to P fertilizer will come from the first 6-10 lb./ac. or so of P fertilizer placed in the seed row. (Note: This rate is too low to provide a prill or droplet in close proximity to each seed.) Higher rates placed in a band will restore soil P levels, but will have limited economic benefit in the short term unless soil P levels are very low.

    What is “very low?" Soil with 0-20 lb./ac. of soil test P is “very low” and  20 to 35 lb./ac. is “low.” Canola grown at these soil test levels is usually very responsive to phosphate fertilizer.

    Note, this is based on the modified Kelowna test for soil P. Manitoba government recommendations for P consider “very low” to be 0-10 lb./ac. and “low” to be 10-20 lb./ac., but this is based on a sodium bicarbonate test — the bicarb method. Generally, the modified Kelowna methods works much better over a wide range of soil pH levels. The bicarb method is fine in the neutral to alkaline pH range, which is typical in Manitoba, but does not correlate well to crop response on lower pH soils.

    Fate of applied fertilizer

    Soil interactions and availability

    P is not very mobile in the soil, moving only 1-2 cm (less than 1”) from point of placement. It ties up with calcium, magnesium, aluminum and iron. Roots must intercept P since P will only move short distances to roots.

    Prairie soils contain significant amounts of total P — 1,120 to 2,240 kg/ha (1,000 to 2,000 lb./ac.). However, most of this soil P is relatively insoluble with limited availability to plants. Canola roots obtain P by absorbing phosphate dissolved in the soil water. Since the amount of phosphate dissolved in the soil water is very small at any given moment, there must be constant replenishment into the soil water from the insoluble forms. This replenishment of soil solution P around roots arises from slightly soluble minerals, P desorption from surfaces, organic P mineralization and fertilizer.

    Both organic and inorganic P forms occur in soil, and both are important sources of plant available phosphate in soil water (soil solution P). Primary and secondary phosphate ions (H2PO4- and H2PO4-2) can be present in soil solution, with H2PO4- the major form at pH <7.2. This solution P has several possible fates: it may be absorbed by roots, adsorbed to mineral surfaces, precipitated with various cations such as Ca+2, or immobilized into microbial biomass and soil organic matter.

    Soil phosphate supply is usually highest in the pH range of 6.5 to 7.0. At high pH levels (>7.5), calcium and magnesium cations can precipitate with phosphate to form salts with low solubility. At lower pH levels (pH<6), iron and aluminum cations react with the phosphate to form insoluble compounds. Phosphate is not a mobile nutrient in soil due to these soil constituent reactions.

    The natural soil weathering process encourages the eventual conversion of primary P to secondary minerals and unavailable forms. This transformation to unavailable forms takes centuries.

    The organic P pool also contributes to the maintenance of phosphate in the soil solution. Organic P in Prairie surface soil constitutes about 25-55% of total P and is a large pool of potential plant available P. Microbial processes drive the organic section of the P cycle. Phosphate from organic matter can be released through decomposition and then incorporated into new microbial biomass or enter into the soil solution. Most organic P compounds released during decomposition are quickly degraded and exist briefly in soil. Some organic P compounds can be stabilized in soil through adsorption to soil constituents or by physical isolation within aggregates. Tillage decreases the soil organic P content by exposing stabilized forms to new or more vigorous microbial attack. Organic P can be degraded to phosphate by enzymes (phosphatases) released by soil microbes and by canola roots.

    Significant seasonal fluctuations occur in both organic and inorganic P pools. However, reports conflict on the direction and magnitude of the fluctuations. For example, several experiments reported decreases in organic P during the summer growing season and gains over the winter, while another experiment measured major declines over the winter. Inorganic P (soil test extractable P) can also vary widely and inconsistently between fall and spring.

    Identifying deficiency symptoms

    Phosphorus Deficiency 1
    Phosphorus Deficiency 2
    Phosphorus Deficiency 3
    Phosphorus Deficiency 4
    Phosphorus Deficiency 5

    Phosphorus deficiencies look like:

    • Delayed maturity
    • Spindly plants
    • P is mobile in the plant, so older leaves will show symptoms first

    The visual symptoms of plant P deficiency are generally neither definitive nor pronounced enough in the field to be very diagnostic. These symptoms can be mistaken for symptoms of other stresses unless direct comparison of growth in adjacent P-fertilized and non-treated areas within the field is possible.

    Canola plants suffering from strong P deficiency can experience slow leaf expansion, smaller and fewer leaves. Deficiency symptoms do not appear until at least the second week of growth since canola seedlings are able to obtain sufficient P from seed reserves for the first week of growth.

    Phosphorus deficient leaves may have a dark green or bluish green colour since chlorophyll and protein formation are less affected than cell and leaf expansion. Under severe P deficiency, purple colouration can arise from accumulation of anthocyanin pigments (although this stress response can have many causes). Mildly deficient plants may look normal but are small. Above-ground plant P content at flowering should be above 0.24%.

    Root growth is less affected by P deficiency than shoot growth, leading to a typical decrease in the shoot-root ratio. With a more severe deficiency, root development is restricted, but not as dramatically as stem and leaf growth. Although overall root branching is restricted in P deficient soils, root hair length and density usually increase.

    P deficiency affects the maturity and development of reproductive tissue. Even a mild P deficiency can result in maturity delays of several days compared to plants with adequate P. In addition to a flowering delay, P deficiency can reduce the number of flowers and seeds per pod. Also, a P deficiency can cause leaves to die and drop early, which contributes to the overall yield loss.

    How to confirm

    Proper diagnosis of nutrient deficiency should use most of the following tools:

    1. Soil test. Are any obvious shortages evident? Other soil test parameters such as texture, pH and electrical conductivity may also provide clues in the diagnosis.
       
    2. Fertilizer history. In past years, what rates and source products have been applied on that field? Include crop yields and consider whether rates have been adequate to match removal.
       
    3. Tissue test. Do not use this alone. Follow sample guidelines provided by the lab. Also keep in mind that nutrients may be in the soil at adequate rates, but due to cold stress or some other stress, the plant simply can’t access them. In the case of cold or excess moisture, plants may recover on their own when soils warm up or dry out again. Plants stressed by other factors may show elevated content of some nutrients due to concentration effects.
       
    4. Herbicide history. Is there any chance of carryover from products applied one or two years ago? In dry conditions or very wet conditions, herbicides can take longer than expected to break down to safe recropping levels for canola.
       
    5. Environment. Cold, wet, hot and dry can all stress canola plants, creating symptoms that may look like nutrient deficiencies. If neighbors have similar symptoms, the cause is probably environmental — frost, excess moisture, etc.
       
    6. History of the land. Recently broken forage land is likely to be depleted in a lot of nutrients. Canola seeded into a field that was in long-term alfalfa, for example, is one case where you may see severe crop stunting and delayed maturity as a result of phosphorus deficiency. Otherwise it is rare to see severe phosphorus deficiency in canola.
       
    7. Look at other fields for similar symptoms. When diagnosing for a specific nutrient, target the crop that tends to be most sensitive to that nutrient.


    Placement of fertilizer

    The majority of P fertilizer is not absorbed by canola in the application year. Instead, most of the fertilizer P reacts with soil constituents to form relatively insoluble salts or stabilized P compounds. Proper timing and placement will maximize P fertilizer uptake and yield response.

    Research comparisons of P fertilizer placement methods at typical rates show that highest yields are frequently obtained with seed-placed and split applications, followed closely by pre-plant band methods. Broadcast-incorporated methods produce significantly lower yield responses in the year of application.

    Seed row

    Phosphorus supply during the first two to six weeks of canola growth is critical to achieve optimal yield. Given that soil supply is reduced under wet, cold conditions, canola benefits from a seed-placed supply of fertilizer P.  Cold soil decreases phosphate solubility and diffusion in the soil solution, slowing P movement to roots and root uptake rates. This condition increases the likelihood of response to readily accessible seed row P.

    A seed-placed application of 15 to 20 lb./ac. will ensure that each canola seed has a phosphate prill or droplet close in close enough proximity to access some P readily, since P is not very mobile in the soil. This is assuming that the seed and fertilizer is being placed in a relatively narrow band. Reducing the seed-placed rate below that level may restrict availability, especially if combined with a wider spread width of the seed and fertilizer band. 

    The maximum safe rate of seed row P fertilizer for canola depends on seedbed utilization and soil moisture conditions. Under dry soil conditions with low seedbed utilization (such as disc opener), the maximum safe P2O5 seed-placed rate is approximately 22 kg/ha (20 lb./ac.). The rate can be safely increased to 28 kg/ha (25 lb./ac.) under good moisture conditions with low seedbed utilization.

    As seedbed utilization increases, growers can proportionally increase seed-placed P fertilizer rates. This may be necessary in fact because the greater the seedbed utilization, the wider apart the granules are spaced. Higher rates will ensure that P prills or droplets maintain close proximity to each seed. 

    Safe rates are dictated by the N component of P fertilizers. Toxicity is mainly related to salt effect from N portion of monoammonium phosphate (MAP) fertilizer. Canola has low tolerance to seed placed N. The ammonium component of MAP is all the nitrogen a canola plants needs to generate a root system to reach nitrogen in a side band or other placement choice.

    Band

    Pre-plant bandreduces fertilizer contact with sensitive canola seed and with soil constituents that will fix P making it less available over time. Also, the deeper fertilizer placement tends to be more accessible to roots as they normally grow down to moist soil. Banding fertilizer prior to seeding reduces the fertilizer handled during seeding and can provide time and labour benefits. Phosphorus fertilizer can be banded in late fall or spring prior to seeding. Given that canola benefits from early-season access to P fertilizer, some P should be placed in the seed row as well, especially if the soil P availability is low enough that the full fertilizer P requirement cannot be safely seed-placed.

    The application of both N and P fertilizer in a single band is called dual banding. At low to moderate rates of N, the uptake efficiency of P is sometimes increased from a dual band. At N rates above 90 kg/ha (80 lb./ac.) the concentrated N in the band can reduce early season P uptake due to ammonia and nitrite toxicity that hinders root entry into the band. This P uptake interference appears to be strongest in recent band applications, and could be a problem with dual spring banded N and P fertilizer applied immediately before or during seeding.

    Side-bandingplaces fertilizer near the seed row during seeding. The fertilizer normally is banded 2.5-5 cm (1-2") below and beside the seed row. Several direct seeding machines use a mid-row or paired-row method of banding fertilizer. Air seeders with shovels or knives are used with shank spacings ranging from 20-36 cm (8-14"). The fertilizer is usually banded to a depth of 5-13 cm (2-5"). No consistent agronomic benefits accrue to banding deeper than 8 cm (3") and fuel costs increase significantly with deeper depths. In high P-fixing soils, place fall P bands deeper than subsequent tillage depths to avoid mixing the band with soil. Because P is fairly immobile in the soil, it will take more time for young canola roots to reach the band than it would take to reach fertilizer placed in the seed row.

    Split applicationscombine band and seed row placement. This takes advantage of the consistent benefit of seed-placed P fertilizer up to 22 kg P2O5/ha (20 lb./ac.), and avoids seedling injury by placing the remainder of the P fertilizer in a band (usually with N and S).

    Broadcast

    Broadcast-incorporatedplacement involves spreading P fertilizer on the surface followed by cultivation to work it into the soil. This method is significantly less effective than seed-placed or banded P fertilizer due to increased contact between the P and reactive soil constituents. Application rates with broadcast-incorporated P fertilizer usually have to be 2 to 4 times seed-placed or banded rates to get an equal response. Therefore, broadcast-incorporated methods are less economical at low rates.

    Broadcast without incorporation leaves P, which is immobile in the soil, on the soil surface. Canola uptake will be very low. Broadcast without incorporation is only recommended for established forage crops because alfalfa and grasses have feeder roots very near the soil surface.

    Foliar

    P needs to be in place early, and because P is relatively immobile in the soil, it works best when placed in the root zone. Otherwise roots may take a while to find P for timely uptake. For these reasons, in-season top up applications have limited success in addressing a P deficiency in the season of application. A top up isn’t a complete loss if there isn’t a response in the year of application, as any unused phosphate will remain in the soil and be available to future crops. This should not however change your fertilizer plans for the following year as top up rates tend to be quite low and seed-placed P will remain a worthwhile practice.

    Timing of application

    Fall or spring

    Since phosphate reacts with soil constituents to form insoluble compounds over time, P fertilizer efficiency may be increased with a spring application, which limits the time between application date and crop uptake. However, the P fertilizer application date affects canola yield much less than placement method. P fertilizer applied in the seed row at seeding minimizes P availability losses by reducing reaction time. For P not placed in the seed row, research in Western Canada has shown the effectiveness of fall and spring banding of P fertilizer is similar.

    Product choices

    Plants need a given amount of P to yield accordingly, regardless of the form applied.

    Available forms

    Monoammonium phosphate (MAP) 11-52-0 or 12-51-0 is the standard dry fertilizer source for the Prairies. Diammonium phosphate (DAP) 18-46-0 is also available. P is in orthophosphate form. Ammonium in the formulation enhances efficiency.

    Ammonium polyphosphate (APP) 10-34-0 is the standard liquid P fertilizer source. With this formulation, P is in both orthophosphate and polyphosphate forms. Polyphosphate is a chain of orthophosphate molecules, which converts readily into orthophosphate through reaction with soil moisture.

    Liquid or granular? Whether P fertilizer is liquid or granular does not affect utilization by plants. P in liquid sources is not more available than P in granular sources — even in a dry year. The selection of a liquid or granular P source should be based on adaptation to the farmer's operation, convenience and economics. Liquid may allow for better distribution and metering of the product.

    Manure. Manure can be a very good source of P (and N) but manure should be tested for nutrient content per tonne before setting application rates per hectare or acre. Manure contains a combination of inorganic and organic P. Most of the inorganic P is orthophosphate form. Organic P has to be mineralized before plants can take it up. Approximately 60-80% of the total P in manure is available to crops in the first year.

    Special formulations

    Fluid orthophosphates (such as Alpine) have P in orthophosphate form entirely, which is the form that plants take up. Liquid APP has a mix of polyphosphate and orthophosphate, usually in about equal parts. Field performance differences between orthophosphate and polyphosphate sources are minimal, since polyphosphate converts to orthophosphate in soils rapidly, with half usually converted within a week. Conversion may be slower if soils are cool and dry.

    Avail, an additive for granular or liquid phosphorus fertilizer, claims to keep phosphate fertilizer in a plant available form for longer. Canola experiments in Manitoba got higher yields in one year and one soil type (fine sandy loam) with high rates of Avail-treated MAP compared to high rates of controlled release phosphate or uncoated MAP. Otherwise that study and other studies across the Prairies have been inconclusive, often because getting any yield response to P fertilizer is so variable.

    Rock Phosphate.  Rock phosphate is the relatively insoluble, gray-black powdery material that is refined into soluble phosphate fertilizer. High rates of rock phosphate do slightly improve canola yields on some soils, but this is not cost effective compared to fertilizer phosphate. Typically, rock phosphate application rates need to be six to eight times that of fertilizer phosphate for equivalent yield response. Research by Alberta Agriculture Food and Rural Development at Ellerslie, Alberta illustrates the poor performance of rock phosphate compared to fertilizer P. 

    Inhibitors and coatings

    Polymer-coated phosphate fertilizer.Polymer coating will control the release of fertilizer into the soil. This can increase the safety of very high rates of seed-placed MAP, but high rates of seed-placed MAP are not needed for crops to reach their yield potential. A Western Canadian study found that, in general, a rate of 30 kg P2O5 ha (roughly 30 lb./ac.)was sufficient to produce maximum early season biomass yield and P uptake for both conventional MAP and controlled release phosphate (CRP) fertilizers. Large differences in early P availability were not evident between the conventional P and controlled released P fertilizer products.

    Inoculants

    Penicillium bilaii (which can be applied to seed using the product Jumpstart) is a fungus that mobilizes P in rhizosphere (root zone), making it more available to the crop. Research in wheat has shown inconsistent results, with some positive results and some negative results. Canola is better than wheat at extracting P from the soil all on its own, and is less likely to benefit from soil treatments such as P.bilaii.

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    Canadian Fertilizer Institute, “Nutrient uptake and removal by field crops,” 2001

    Rigas E. Karamanosand John Heard, 2011, Updated Phosphorus Recommendations for Wheat, Barley and Canola in Manitoba

    Central Alberta research in the 1980s found that 23 of 48 sites responded to P fertilizer. An Alberta study from 1991-93 found a statistically significant response to P fertilizer at 42 site-years while 81 site-years had no response. Economic analysis of the results suggested that 70% of the canola sites responded to 7 kg (15 lb.) P2O5/ac and 53% responded economically to 14 kg (30 lb.) given canola at $352 per tonne ($8/bu.) and $0.75/kg ($0.34/lb.) P2O5.

    Nyborg, M., Malhi, S. S., Mumey, G., Penney, D. C. and Laverty, D. H. 1999. “Economics of phosphorus fertilization of barley as influenced by concentration of extractable phosphorus in the soil.” Commun. Soil Sci. Plant Anal. 30: 1789–1795.

    Grant, C.A., D.N. Flaten, D.J. Tomasiewicz and S.C. Sheppard, “The importance of early season phosphorus nutrition,” Canadian Journal of Plant Science, 81, pp. 211-224

    R. H. McKenzie, E. Bremer, L. Kryzanowski, A. B. Middleton, E. D. Solberg, D. Heaney, G. Coy, J. Harapiak, 2003,Yield benefit of phosphorus fertilizer for wheat, barley and canola in Alberta, Canadian Journal of Soil Science 83(4): 431-441

    R. H. McKenzie and A.B. Middleton, 1997, “Phosphorus fertilizer application in crop production,” Alberta Agriculture and Rural Development Agdex 542-3

    Ross McKenzie, Research Scientist - Agronomy, Alberta Agriculture and Rural Development, Lethbridge, personal communication.

    Grant, C.A., D.N. Flaten, D.J. Tomasiewicz and S.C. Sheppard, “The importance of early season phosphorus nutrition,” Canadian Journal of Plant Science, 81, pp. 211-224

    George Rehm, Michael Schmitt, John Lamb, Gyes Randall, and Lowell Busman, 2002, reviewed 2010, “Understanding phosphorus fertilizers,” University of Minnesota Extension, Link: http://www.extension.umn.edu/distribution/cropsystems/DC6288.html

    IPNI Insights Newsletter, July 2009, includes summaries of various studies looking into phosphate fertilizer enhancers. The one positive response to Avail came from “Impact of Traditional and Enhanced Efficiency Phosphorus Fertilizers on Canola Emergence, Yield, Maturity, and Quality in Manitoba,” led by Cynthia Grant, Agriculture & Agri-Food Canada, Brandon

    Qian, Peiyuan, and Jeff Schoenau, 2010, “Effects of conventional and controlled release phosphorus fertilizer on crop emergence and growth response under controlled environment conditions,”  Journal of Plant Nutrition, Volume 33, Issue 9, pages 1253-1263.

    Karamanos, R. E., Flore, N. A. and Harapiak, J. T. 2010. Re-visiting Use of Penicillium bilaii with phosphorus fertilization of hard red spring wheat. Can. J. Plant Sci. 90:265-277.