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© 2008 Plant Management Network.
Accepted for publication 20 March 2008. Published 30 July 2008.

Enhanced-Efficiency Fertilizers for Use on the Canadian Prairies

Cynthia Grant and Ronggui Wu, Agriculture and Agri-Food Canada, Brandon Research Centre, Box 1000A, R.R.#3, Brandon, MB, Canada R7A 5Y3

Corresponding author: Cynthia Grant. cgrant@agr.gc.ca

Grant, C., and Wu, R. 2008. Enhanced-efficiency fertilizers for use on the Canadian prairies. Online. Crop Management doi:10.1094/CM-2008-0730-01-RV.

Abstract

In spite of the management practices adopted by Canadian producers to minimize fertilizer losses, N-use efficiency is normally estimated to be less than 50% in the year of application. Various types of enhanced-efficiency fertilizers such as nitrification inhibitors, urease inhibitors, and coated N fertilizers are available that attempt to address the constraints associated with traditional N management in order to improve N-use efficiency and/or the operational efficiency of Canadian agricultural systems. Enhanced-efficiency N fertilizers can chemically or physically influence the movement and transformations of N in order to improve synchrony between nutrient supply and crop uptake, reduce nutrient losses, and improve nutrient-use efficiency. Pathways and magnitude of N loss are influenced by soil characteristics, weather conditions, and crop management practices, as well as by fertilizer source and management practices. Therefore, the effectiveness of the various enhanced-efficiency fertilizers will be strongly dependent on the environmental conditions that influence potential losses. Under environmental conditions where the potential for N loss is high, enhanced-efficiency fertilizers may provide an effective method of improving N use, particularly where other agronomic factors are optimized so that the crop is able to convert the N supplied into usable yield with the greatest efficiency.

Introduction

The Canadian prairies are a major area of agricultural production in North America. In 2001, there was 67.5 Mha of farmland reported in Canada encompassing many ecological zones, with 54.8 ha or over 80% located in the three prairie provinces of Manitoba, Saskatchewan, and Alberta (41). The major crops grown are spring-seeded cereals [wheat (Triticum aestivum L.), durum wheat (Triticum durum Desf.), barley (Hordeum vulgare L.), and oats (Avena sativa L.)], and oilseeds [canola (primarily Brassica napus, L.), with limited areas of mustard (Brassica juncea L.) and flax (Linum usitatissimum L.)] (39). Pulse crops, potatoes (Solanum tuberosum L.), corn (Zea mays L.), canary seed (Phalaris canariensis L.), and a range of other minor specialty crops are also grown. There is also a major area dedicated to production of tame hay (i.e., cultivated forage legumes, grasses or legume-grass blends seeded for use primarily as stored feed for ruminant livestock) and improved and unimproved pastures. In the drier areas of the prairies, there is a significant area of summer fallow used to conserve water, with crops seeded every second year, or two years in three.

The prairie environment is characterized by a short growing season, ranging from 90 to 120 days, with cold winters and hot summers. The soils range from Aridic and Typic Borolls (Brown and Dark Brown Chernozems) in the more arid southern portions of the prairies, grading to Udic Borolls (Black Chernozems) and Boralfs and Mollic Cryoboralfs (Gray and Dark Gray Luvisols) as one moves to the more northerly regions where effective precipitation is higher (Fig. 1). Crop yield potential and fertilizer applications are strongly affected by available moisture, with the highest crop yields and the highest per unit fertilizer applications occurring in the Black soil zones.

Fig. 1. Soils of the Canadian prairies (AAFC, 2005).

The majority of soils in the Canadian prairies require applications of N and/or P in order to optimize crop yield and quality. Nearly 1.6 million metric tonnes of N are applied in Canada annually, with over 1.25 million or nearly 80% applied in the prairie provinces (40). Similarly, approximately 0.63 million tonnes of phosphate are applied in Canada, with 0.48 million tonnes or 75% being applied in the Prairie Provinces. Currently, it is estimated that cereal crops recover only about 30 to 50% of applied fertilizer N (5) and approximately 45% of applied P (46). Without major improvements in fertilizer-use efficiency, increasing fertilizer inputs may lead to negative environmental impacts if nutrients move from the soil-plant system to the air or water (5). Therefore, it is important to optimize fertilizer-use efficiency, both to improve the economics of crop production and to avoid negative environmental impacts from movement of excess nutrients into the air or water.

Traditional Practices for Improving Fertilizer-Use Efficiency

Traditionally, Canadian producers have attempted to select combinations of nutrient source, timing, placement, and rate that would optimize fertilizer-use efficiency and economic return (14,18) by the crop. Most chemical N fertilizers supply N as ammonium (NH₄⁺), nitrate (NO₃^—), or as urea, which converts rapidly to NH₄⁺ through the action of the urease enzyme. Nitrogen fertilizers are subject to four major paths of loss – volatilization, immobilization, leaching, and denitrification. Ammonium or ammonium producing fertilizers that are applied on or near the soil surface may be lost from the plant-soil system through ammonia volatilization. Both ammonium and nitrate may be utilized by soil microorganisms and converted to organic forms through immobilization, particularly when in contact with crop residues. If fertilizers are placed under the soil surface below the crop residue, volatilization, and immobilization will be reduced and the major pathways of loss will be by denitrification (16) and leaching. The nitrate portions of ammonium nitrate and urea ammonium nitrate (UAN) are susceptible to leaching and denitrification losses as soon as they enter the soil. However, if an ammonium or ammonium–producing source of N is used (i.e. anhydrous ammonia or urea), the ammonium must convert to nitrate before significant losses occur. Rate of conversion and hence the risk of denitrification and leaching losses increases with increasing soil temperature and moisture (44,45).

In-soil banding refers to any application of fertilizer in a band below the soil surface, whether pre-plant banded in the fall or prior to seeding, seed-placed, or mid-row or side-banded during the seeding operation. In-soil banded applications of N are commonly used in the Canadian prairies to improve N-use efficiency. In-soil bands will reduce volatilization and immobilization losses, by placing the N below the soil surface and separated from crop residue. Banding an ammonium source of N will also reduce volatilization and leaching losses, since the band provides a highly concentrated region with limited contact with the soil microorganisms. Thus, banding slows the conversion from ammonium to nitrate, delaying the susceptibility of the fertilizer application to leaching and denitrification losses. In the Prairie Provinces, more than 75% of fertilizer is applied using some form of in-soil banding to reduce the risk of nutrient loss or conversion to unavailable forms prior to crop uptake (41).

The longer the fertilizer is present in the soil prior to crop uptake, the greater the risk of N losses. In principle, N losses should be reduced and N-use efficiency improved by matching the supply of N closely with the time crop uptake. The majority of N on the prairies is applied in the spring although fall applications are sometimes practiced (41). to spread the workload, avoid drying the seed-bed in spring with a separate spring banding operation and take advantages of lower fertilizer prices in the fall. Split applications of N are sometimes used to reduce initial investment in fertilizer and enhance wheat protein content, although this has not been a common practice in the Canadian prairies.

Under dry conditions, N fertilizer bands can remain in the soil for many months without significant losses, while under wet, warm conditions considerable losses may occur within days (44,45). In many areas of the prairies, particularly in the drier brown and dark brown soil zones and in well drained black soils, N losses from in-soil banded fertilizer may be insignificant (18). Nitrogen bands may be placed in the soil in the fall before the ground freezes, in the spring prior to seeding, or at the time of seeding in a one-pass seeding. During the seeding operation, the bands may be placed directly with the seed, or placed in a separate band at varying distances from the seed-row. Care must be taken to avoid placing too much N fertilizer too close to the seed-row as excess N fertilizer can lead to seeding toxicity from osmotic effects and/or ammonia toxicity (11). Since moisture is generally limiting to crop production in the prairies, particularly in the brown and dark brown soil zones, it is important to consider the impact of in-soil banding on soil moisture conservation as well as N losses.

In-soil banding can improve the efficiency of P fertilizer. Phosphorus is much less mobile in the soil than nitrate and consequently remains near the site of fertilizer placement. In the Canadian prairies, the majority of soils have a pH greater than 7.0, with the soil cation exchange saturated by calcium and magnesium. Phosphorus will react with the calcium and magnesium to form sparingly soluble calcium and magnesium phosphate compounds (34). In acid soils, similar reactions occur with iron and aluminum oxides. Band placement of P reduces contact with the soil resulting in less P fixation than broadcast application (47). In P-deficient soils with a high P fixation capacity, the optimal method of supplying P for early crop growth is by banding the fertilizer near or with the seed, during the seeding operation (i.e., use of "starter P"). Response to banding of "starter P" tends to be more frequent on colder soils, where P availability is often limited (12,36).

Annual crops require P early in the season for the greatest benefit (12). Banding can improve the ability of plants to utilize fertilizer P by placing the fertilizer in moist soil in a position where the plant roots can contact the band early in the growing season. Also many plants are able to proliferate their roots when they contact a concentrated source of P in a fertilizer band (42,43). Root proliferation allows the plant to effectively extract the P from the band and utilize the P efficiently. Placement of P near the seed is particularly important for crops such as flax, which have poorly developed root systems early in the growing season (33). However, in many crops, seedling damage may occur if P is placed in the seed-row at rates required for optimum yield (27). Damage is more apt to occur when P sources such as monoammonium or diammonium phosphate are used since the ammonium portion contributes to seedling toxicity (30). Placement of the fertilizer band below or below and to the side of the seed-row can provide the improved P availability of band placement near the seed, but will reduce the likelihood of seedling toxicity where the risk of damage from seed-placed P is high.

Enhanced-Efficiency Nitrogen Fertilizers

In spite of the beneficial management practices adopted by Canadian producers, N-use efficiency is generally less than 50% in the year of application (18). Broadcast applications are simple but inefficient, particularly under no-till management. Banding improves nutrient-use efficiency, but a banding operation separate from seeding requires an extra operation to inject the fertilizer, utilizing time, labor, fuel, and equipment, and increasing soil moisture loss. Placement of fertilizer with the seed improves N-use efficiency and eliminates a separate banding operation, but only limited amounts of N can be placed with the seed without causing seedling damage (11). Separating the seed and fertilizer by side-banding or mid-row banding reduces risk of seedling damage in a one-pass operation, but increases equipment costs and draft requirements. Split application of N to synchronize N supply with crop uptake requires extra field operations and may be inefficient if volatilization and immobilization losses are high, or if moisture is not available to carry the fertilizer into the soil.

Various types of enhanced-efficiency fertilizers are available that attempt to address the constraints associated with traditional N management to improve N-use efficiency and/or the operational efficiency of Canadian agricultural systems. Enhanced-efficiency N fertilizers include nitrification inhibitors, urease inhibitors, and coated N fertilizers.

Nitrification inhibitors interfere with the action of Nitrosomonas bacteria that convert NH₄⁺ to NO₃^— and so directly reduce the amount of N₂O released (6). By slowing nitrification, ammonium-producing fertilizers remain longer in the ammonium form and reduce the amount of NO₃^— in the soil solution (25), thus reducing the risk of leaching or denitrification. The two major nitrification inhibitors that have been evaluated and marketed in Canada are nitropyrin (2-chloro-6-trichloromethyl-pyridine) (N-Serve) and dicyandiamide (DCD). Other inhibitors evaluated include ATC (4-amino-1,2,4-triazole hydrochloride) and thioourea (19,20). Ammonium thiosulphate (ATS), a fluid fertilizer source of N and S has been shown to limit nitrification to some extent (7).

Field studies conducted in Alberta showed that ATC, nitropyrin, and thiourea inhibited nitrification of fall-banded urea fertilizer under conditions where the fall applications were subject to nitrification losses (19,20). The inhibitors increased the amount of recoverable NH₄-N and NO₃-N in May, with approximately 30% of the N remaining as NH₄-N with the use of inhibitors while nearly none was recovered as NH₄-N when inhibitors were not used. However, crop yields were not increased consistently with the inhibitors. If fall fertilizer applications were banded late in the season when soil temperature was close to freezing, nitrification was restricted and use of the inhibitor provided no significant additional yield increase. In North Dakota, where growing conditions are very similar to the Canadian prairies, both nitropyrin and ammonium thiosulphate were effective at reducing nitrification from fall-applied aqua ammonia and increased spring wheat yield after a winter with high snowfall (8). Grain yield and N-use efficiency of winter wheat in Ontario, where conditions are wetter and slightly warmer than on the prairies, were increased by addition of the nitrification inhibitor DCD to large urea granules, with the fertilizers then fall-applied either at the time of seeding or one month after seeding (51). Use of the large granules with DCD in the fall produced yields higher than that with spring top-dressing and higher than that when the granules were applied without the DCD. Delay of fall application until the ground was cold improved N recovery and grain yield, which allowed the late fall applications of the large granules to be as effective as spring top-dressing. The spring-topdressed applications likely experienced significant volatilization loss, as the soil was alkaline.

Work with oilseed rape in Manitoba also showed that use of the nitrification inhibitor nitropyrin with fall-banded urea and UAN increased crop yield and N accumulation (Table 1) (3). In these studies, the fall fertilizer was applied in mid-September, approximately six weeks before the soils froze. Use of nitropyrin did not influence crop yield or N recovery with spring-banded applications of urea or UAN. Yield and N-recovery of oilseed rape was higher with spring-banded urea or UAN than when the fall treatments were applied without nitropyrin. However, nitropyrin improved the yield and N-recovery of the fall-applied urea and UAN so that the fall-applied N with nitropyrin produced yields and N-recovery equivalent to the spring-banded fertilizers. Nitropyrin did not influence the performance of ammonium nitrate. The author also pointed out that use of nitropyrin appeared to reduce oil concentration of oilseed rate, beyond the effect that would be expected due to the conservation of N (3). Protein content was not affected and the mechanism for the oil reduction could not be determined, but the author suggested that nitropyrin should not be used with oilseed rape because of the risk of reducing oil concentration below acceptable levels (3).

Table 1. Effect of N fertilizers with and without nitropyrin on the seed yield of Tower rape (average of 4 years on two soils) [(adapted from (3)].

 * a-e Values in a column followed by the same letter are not significantly different (Tukey’s Test, P < 0.05).
r-s Values in a row followed by the same letter are not significantly different (Tukey’s Test, P < 0.05).

Delaying nitrification may increase the proportion of the N supplied to the growing crop in the form of ammonium and decrease the amount of N available as nitrate. The ratio of ammonium to nitrate may influence plant physiology and acidification of the soil rhizosphere. In water culture and potted soils, yield of rapeseed with ammonium nutrition was less than yield with nitrate nutrition, possibly associated with an accumulation of unassimilated ammonium within the plant (21). Use of an inhibitor with the ammonium-based fertilizers accentuated the problem. While the ammonium:nitrate ratio of the N supply appeared important in controlled environment studies, the effect was not observed under field conditions. In 33 field experiments using spring banded or broadcast N, rapeseed seed yield was not affected by N source. The use of inhibitors slowed nitrification but yield in the field were not negatively affected by maintaining the N in the ammonium form.

Use of nitrification inhibitors should provide environmental benefits in areas where significant denitrification or leaching losses could occur. Nitrification produces both nitrous oxide and nitric oxide as intermediaries. Nitrous oxide is a potent greenhouse gas while nitric oxide can destroy stratospheric ozone (6). Nitrate in the soil solution can move into ground and surface water, where it can create negative environmental and health effects. Slowing the conversion of ammonium to nitrate will reduce the concentration of nitrate present in the soil solution and consequently reduce the potential production of gaseous nitrous oxide and the risk of nitrate leaching. Environmental benefits from nitrification inhibition may occur even under conditions where yield benefits are not observed.

The agronomic and environmental benefits of nitrification inhibition will be greatest under conditions that promote losses by leaching and denitrification, such as wet soil conditions. Benefits are unlikely in dry or well-drained soils, where leaching and denitrification losses are limited. The longer the fertilizer remains in the soil before crop uptake, the greater the potential benefit of an inhibitor, thus inhibitors have more consistently increased crop yield with fall than spring applications (19,20,37). Yield increases have also been minimal when banding operations have been delayed until late in the fall, when soils have cooled (51). Adoption of nitrification inhibitors in the Canadian prairies has been low because they have not been shown to provide a significant economic benefit above traditional N fertilization practices.

Urease Inhibitors

Urease inhibitors can also be used to improve the efficiency of urea-based fertilizer (49). Urea fertilizer is widely used in Canada, because it is the most concentrated granular N source available. It is also used as a component in UAN. However, efficiency of urea fertilizer can be reduced through ammonia volatilization, if urea remains near or at the soil surface (50). If urea is place too near the germinating seedling, it can lead to seedling damage, through osmotic damage and ammonia toxicity (11). Volatilization can be reduced by incorporating or banding the fertilizer beneath the soil surface. Seeding damage can be reduced by separating the fertilizer from the seed-row. Incorporation, banding, and separation of seed and fertilizer tend to increase soil disturbance and fertilizer application costs. Urease inhibitors can be used in place of traditional management practices to decrease volatilization from surface applications and seedling damage from seed-placed fertilizer, reducing soil disturbance and application costs.

Urea is not subject to volatilization loss until after it has hydrolysed to ammonium in a reaction catalysed by the urease enzyme. During hydrolysis, OH^— is released, raising the soil pH. Ammonium and ammonia interact in an equilibrium reaction in the soil solution, with the formation of ammonia being favoured by increasing soil pH. The more ammonia is present, the greater the risk of volatilization. Also, as ammonia is directly toxic to germinating seedlings, a higher proportion of ammonia increases the risk of seedling damage. The major factors influencing ammonia concentration, and hence the risk of volatilization or seedling damage, include the rate of urea hydrolysis and the soil pH surrounding the application site (16). Slowing the rate of hydrolysis reduces the concentration of ammonium and ammonia present in the soil solution, thus reducing the volatilization gradient. Delaying urea hydrolysis provides time for rainfall to move the urea into the soil where the released ammonia will be protected from movement to the atmosphere (50) or for the urea to move away from the germinating seedling, which reduces the potential toxicity (12,32,48). While many different compounds have been assessed for use as urease inhibitors, the most widely tested inhibitor currently available is N–(n-butyl) thiophosphoric triamide (NBPT), which is marketed as Agrotain.

Many of the soils on the Canadian prairies are highly calcareous, increasing the potential benefit from urease inhibitors. The urease inhibitor NBPT was effective at reducing ammonia emissions from surface applications of both urea and UAN, when applied to the surface of high pH soils in Manitoba (Table 2) (13). The magnitude of ammonia loss and the potential benefit derived from use of NBPT were higher when urea rather than UAN was the fertilizer source. Losses tend to be higher from urea than UAN because only a portion of the N in the UAN is in the form of urea and hence less is susceptible to losses. In addition UAN does not increase the pH surrounding the fertilizer to the same extent as urea, thus the ratio of ammonia to ammonium is lower with UAN than urea. Volatilization will be reduced if rainfall occurs soon after fertilizer application, carrying the urea into the soil where it is protected from loss. Ammonia losses from surface-applied urea were reduced when simulated rainfall was applied to move the fertilizer into the soil, but the lowest loss occurred when urea plus NBPT was combined with simulated rainfall (Fig. 2) (31).

Fig. 2. Effect of N-(n-butyl)thiophosphoric triamide (NBPT) and simulated rainfall (2.0 cm on day 4 and day 7) on volatilization losses from surface applied urea fertilizer (29).

Use of NBPT has been more consistent in reducing measured loss of ammonia from surface applications of urea and urea ammonium nitrate than in increasing crop yield. For a yield increase to occur from the use of NBPT, the N supply must be limiting to crop production and the potential volatile losses of N from the urea must be high enough to restrict crop yield. Maximum benefits of urease inhibitors will occur when crop yield potential is high, soil N levels are low, and soil and environmental conditions promote extensive volatilisation losses. Potential volatilization, and hence potential benefits from the use of urease inhibitors, will be higher where incorporation is difficult, where there is little opportunity for urea to move into the soil with infiltrating water, or where the soil has a high urease activity because of lack of cultivation or the accumulation of organic material (4). In the Canadian prairies, there are large areas of pastures and forages, where urea may be broadcast on heavy residues and left unincorporated. Surface applications of urea or UAN may also be used for production of winter cereals and as split applications to synchronize N supply with crop uptake or to increase protein concentration of wheat grain. In management systems where surface application is used with no incorporation, urease inhibitors may be effective in increasing fertilizer-use efficiency and economics of production.

No-till is a popular management practice in the prairies. Under no-till, surface applications cannot be incorporated and the efficiency of surface applied urea may be very low due to the lack of incorporation and the presence of large amounts of crop residue (18). In Manitoba, barley yield was increased when NBPT was added to broadcast urea under no-till, but under conventional till yields with broadcast urea were similar with and without NBPT (10).

The potential benefits from use of urease inhibitors are likely lower with in-soil applied urea, as in-soil banding will reduce the potential for volatilisation losses. However, if fertilizers are banded in the fall, use of urease inhibitors might be beneficial in slowing the release of ammonia and its subsequent conversion to nitrate, thus reducing the risk of nitrate leaching or denitrification. In unpublished studies, Grant et al. (10) observed higher yield under no-till conditions with the use of the urease inhibitor NBPT when fertilizer was banded in the fall, but no benefit when NBPT was used with spring-banded N. Combination of a urease inhibitor with a nitrification inhibitor could increase the time that the fertilizer could be in he soil before significant losses occurred. However, combined use of a urease inhibitor and a nitrification inhibitor with fall banded urea did not increase crop yield or N recovery in studies in Manitoba (44,45).

Another use for urease inhibitors is to increase the safety of seed-placed N fertilizer (Table 3). Seed-placed fertilizers are an efficient method of applying N fertilizer. Seed-placing fertilizer allows for an in-soil band application to be applied in a one-pass operation. Placing the fertilizer directly in the seed-row eliminates the need for use of an extra opener or a wider opener that physically separates the seed and fertilizer, so seed-placement reduces implement cost, draft and soil disturbance relative to systems that band the seed and fertilizer separately. However, excess application of N fertilizers can lead to seedling damage, through direct ammonia toxicity or osmotic effects. Unless yield potential and fertilizer needs are low, there is a risk of seedling damage with rates of N required to optimize crop yield. Use of urease inhibitors or slow release fertilizers can reduce seedling damage by reducing the concentration of ammonia/ammonium close to the seed (48). Field studies with barley (11) and durum wheat (Table 3) (22) showed a significant increase in stand density and final crop yield when seed-placed urea was treated with the urease inhibitor. Based on a series of field studies across the Prairie Provinces, it was suggested that the average guidelines for seed-placed urea for spring wheat crops grown on medium textured soils with 10% seedbed utilization could be increased by 20 kg of N per ha by treatment with NBPT (17).

Table 3. Effect of N fertilizer applied as uncoated urea, urea treated with N-(n-butyl) thiophosphoric triamide (NBPT), or polymer coated urea (CRU) on stand density and grain yield of durum wheat [adapted from (22)].

* P < 0.05, **P < 0.01, and ***P < 0.001.

Fig. 3. Effect of fertilizer type and application rate on stand density of winter wheat at the two- to three-leaf stage in the spring when fertilizer was seed-placed at 9 different experimental site-years. The 20-day CRU and 40-day CRU refers to polymer coated urea formulated to release over 20 or 40 days, respectively. Values are expressed relative to the average stand density of side-banded treatments, which were unaffected by fertilizer type or application rate. Error bars are standard deviations. Significance of treatment effects: * P < 0.05, **P < 0.01 and ***P < 0.001, NS = not significant [adapted from (23)].

Controlled Release Nitrogen

Controlled release N fertilizer products also hold potential for improving fertilizer-use efficiency, enhancing economics of production and reducing the risk of negative environmental effects for movement of N into the air or water (15). A number of different types of controlled release fertilizer products are available but in Canada, they are most commonly used in turf-grass or horticultural crops as they have generally proven too costly for use in grains and oilseed production (35). Lower-priced controlled release formulations have been developed that may be cost-effective for agronomic crop production systems. Currently, the only product registered in Canada in ESN (Environmentally Smart Nitrogen, Agrium, Calgary, Alberta). This controlled release fertilizer has a polymer coating that releases urea fertilizer into the soil solution at a rate limited by moisture, but controlled by soil temperature (15). The rate of release is designed to match the N uptake pattern of the crop being produced. By closely matching the time of N release into the solution to the uptake pattern of the growing crop, the concentration of nitrate and the length of time the nitrate is present in the soil solution before crop uptake are both minimized. For crops such as wheat, where high protein content may be an important quality consideration, controlled release fertilizers could be used to ensure that N is available late in the season to enhance protein, while avoiding inefficiencies due to excess N in the soil solution during the early stages of growth.

Potential benefits from use of controlled release urea will increase with the potential for N loss. Since losses of N from in-soil applications increase with increasing moisture, benefits from use of controlled release fertilizers are also likely to be greatest under wet, warm conditions. The potential for reduction in N loss will also increase with the length of time that the fertilizer is in the soil before crop uptake. However, under dry conditions where losses of urea may be low, controlled release urea may not provide a significant yield increase over in-soil banded uncoated urea (26). Controlled release urea has also not generally produced a significant advantage with spring broadcast fertilizer applications in winter wheat (23,26) or in timothy (24).

Another potential use of controlled release urea, as with urease inhibitors, is in the reduction of seeding toxicity (15). In research conducted near Melfort, Saskatchewan, controlled release urea was more effective than Agrotain in reducing seedling damage from seed-placed urea fertilizer (Table 3) (22). In southern Alberta, controlled release urea was seed-placed with winter wheat at rates as high as 120 kg of N per ha without producing significant seedling injury whereas stand density declined significantly when uncoated urea was applied at rates between 30 and 60 kg of N per ha (Fig. 3) (23,26).

Controlled release urea may have particular benefits for application in winter wheat. Application of N fertilizer at seeding or in winter wheat are generally considered undesirable due to potential for N loss and/enhanced winter damage. Current N management for winter wheat generally involves application of the bulk of the N requirements early in the spring. However, spring applications may be delayed due to poor weather conditions, leading to early-season N deficiency. Traditionally, ammonium nitrate was used to minimize volatilization losses from broadcast applications. Ammonium nitrate is no longer widely available, so producers are using more urea and UAN, both of which are subject to losses by volatilization and immobilization. Seed-placement or side-banding of controlled release urea could be effective methods of placement for N in winter wheat (26). However, under dry conditions where fall losses of urea may be low, controlled release urea may not provide a significant benefit over uncoated urea as long as the urea is side-banded far enough away from the seed to avoid seedling damage (26).

Environmental benefits are likely to occur with the use of polymer coated urea; however published data from Canada are limited. Under wet conditions, controlled release urea should reduce the risk of leaching and/or denitrification, by more closely matching uptake with supply, hence reducing losses.

Enhanced-Efficiency Phosphorus Fertilizers

Availability of P is reduced by reaction with Ca and Mg in high pH soils and Fe and Al in low pH soils (34). Fertilizer-use efficiency may be enhanced by products that reduce these P reactions. In greenhouse studies, the action of a controlled release monoammonium phosphate (MAP), diammonium phosphate (DAP) and ammonium polyphosphate were simulated by making small, periodic additions of fertilizer P, so that the plants would rapidly use the supplied P from the soil solution, minimizing precipitation (28). Supplying the P to the plant gradually over several weeks reduced P fixation and increased P uptake as compared to a single application of P at the start of the growing period, with the effect being greater with DAP than MAP. Growth chamber studies showed that polymer and shrink wrap coatings could be used to slow the release of P from MAP or DAP. Coating MAP improved P uptake, fertilizer efficiency, and barley dry matter yield (Table 4), but did not affect performance of DAP (29). While studies are underway evaluating controlled release phosphate fertilizers, there currently is little published information from Canada evaluating the performance of enhanced-efficiency P fertilizers under field conditions.

Table 4. Barley dry matter yield, P uptake, and net fertilizer P efficiency (NFPE) after 52 days of growth in pot trials as affected by application of non-coated or coated monoammonium phosphate fertilizers [adapted from (29)].

Avail (Malefic Itaconic Copolymer) was reported to work by sequestering antagonistic ions that react with P in the soil solution, reducing precipitation and keeping the P in available forms for longer (1). Unpublished data from trials that are reported on the website of Specialty Fertilizer Products indicate yield increases in canola when Avail was used with monoammonium phosphate on a calcareous soil in Manitoba (2). However, there is a lack of published information on the performance of Avail under Canadian conditions.

Role of Enhanced-Efficiency Fertilizers in Canada

Enhanced-efficiency N fertilizers can chemically or physically influence the movement and transformations of N or P in order to improve synchrony between nutrient supply and crop uptake, reduce nutrient losses and improve nutrient-use efficiency. However, these products have not yet been widely adopted for commercial production on the Canadian prairies. The major constraint currently associated with the wide-spread use of enhanced-efficacy fertilizers in grains and oilseed production on the Canadian prairies is the product cost. Currently, in Canada most controlled release fertilizers are used in non-farm uses such as golf courses, home gardening, landscaping and nurseries, where the perceived benefits are greater than the cost of the technology (35).

For enhanced-efficiency fertilizers to be widely used, they must have advantages both over traditional fertilizer sources and over other beneficial management practices available, such as in-soil banding or split application. Pathways and magnitude of N loss are both influenced by soil characteristics, weather conditions, and crop management practices, as well as by the fertilizer source and management practices used. The effectiveness of the various enhanced-efficiency fertilizers will be strongly dependent on the environmental conditions that influence the potential losses that the fertilizer is attempting to address. For example, as discussed previously, the potential benefits from use of nitrification inhibitors or controlled release N to reduce denitrification and leaching losses will be greater under wetter conditions where potential losses are greater. Under dry soil conditions, as are prevalent in many parts of the prairies, the reduction in N loss associated with the use of inhibitors or controlled release N products may not be sufficient to recover the increased cost of the product. However, large variations in loss can occur even on a small scale within a field, due to variation in drainage and micro-environment (9). Therefore integration of use of enhanced-efficacy fertilizers with site specific management techniques may increase the relative benefit achieved from use of both of these new technologies.

Substitution of enhanced-efficiency fertilizers for other traditional management practices may provide economic or operational benefits, particularly in no-till systems. Seed equipment that can apply fertilizer separately from the seed-row can be costly and may also increase draft requirements, soil disturbance and moisture loss. enhanced-efficiency fertilizers that allow the use of simplified, less expensive equipment or practices (for example seed-placed as compared to mid-row or side-band systems; surface applications rather than in-soil band) may be economically and operationally attractive. Similarly, enhanced-efficiency fertilizers can be beneficial if they allow for reduction in the number of field operations. For example, enhanced-efficiency fertilizers may facilitate one-pass seeding and fertilizing in direct seeding systems, eliminating the need for repeated passes for separate in-soil banding or for split applications. enhanced-efficiency fertilizers increase the flexibility in the timing of application, so fertilizer can be applied efficiently in a single application during a longer time period. enhanced-efficiency fertilizers may allow for efficient fall application for crops such as winter wheat and can avoid the potential for missing correct timing for split applications due to poor weather, physical condition of the field or time constraints.

Enhanced-efficiency fertilizers can be used in combination with other technologies to optimize efficiency. For example, in-soil banding of inhibited or controlled release products can capture benefits both of the banding action and of the slow release, allowing fertilizers to remain in the soil for a longer period with reduced losses. This allows the producer and the industry to distribute operations when resources are in lower demand, improving the efficiency of use of capital investments and available labor.

Compared to split applications, use of slow release production or inhibitors with all N applied prior to seeding still requires assessment of yield potential at the time of nutrient application. Therefore, it leaves the system sensitive to changes in yield potential which can result in either excess or inadequate application. In regions of the prairie where moisture supply is highly variable from year to year, use of urease inhibitors with split applications of N and methods of in-crop analysis of N deficiency can allow for split applications to reduce the N application prior to seeding or to more closely match N application to crop demand, while reducing the risk of volatilization losses from surface applications.

Analysis of the benefits of the use of enhanced-efficiency fertilizers compared to alternative methods to increase fertilizer-use efficiency must consider not just the cost of the material, but also factors such as differences in machinery costs, reductions in time, labor, and fuel costs, improved efficiency of operations, effects on seed-bed, soil moisture or other agronomic factors, and management skill required to apply the technologies. It is important that a production system optimizes the use of all available resources. In the Canadian prairies, moisture is frequently limiting so technologies that encourage moisture conservation can be highly beneficial. Similarly, the prairie growing season is generally very short, and time and labor are frequently limited. Reducing time and labor associated with fertilizer management can have advantages in the overall efficiency of the farm. The current trends towards increasing energy costs, increased cost of fertilizer N, and scarcity of agricultural labor may make enhanced-efficiency fertilizers more attractive.

Currently, the value of enhanced-efficacy fertilizers is primarily based on the increased yields or reduced production costs to the farmer. In many jurisdictions the environmental benefits to society are not ascribed an economic value. Life-cycle analysis of environmental impacts could clarify the environmental value of use of enhanced-efficiency fertilizers to society and provide guidance for methods of transferring some of the costs of development and use of the products to those in society that benefit from the technology, possibly through subsidies or incentives for adoption of the technology.

For crops to take advantage of improved fertilizer efficiency through enhanced-efficiency fertilizers, the overall cropping system must be optimized. Tillage management, crop genetics, pest control, water management and soil tilth must all be managed effectively so that the crop is able to convert the N supplied into usable yield with the greatest efficiency. Increasing the yield potential of the crop through better use of water, balanced fertility, higher yielding cultivars, disease control, timeliness of operations or improvements in soil structure improves the efficiency of use of all resources. It is estimated that good agronomic practices could raise the average N-use efficiency by at least 25 to 30% during the next two generations (38). Use of enhanced-efficiency fertilizers can play an important role in meeting that goal.

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