Important tips for best management
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- Nitrogen is the most common limiting nutrient (other than water) for canola production.
- Follow the 4 Rs: Right rate, right timing, right source, right placement.
- Ensure adequate supply of N and S to support yield potential.
- High-yielding hybrid cultivars will require more N to support yield, but may be more efficient at extracting N from soil.
- Banding is the most efficient way to place nitrogen under Prairie conditions, but ensure adequate seed and fertilizer separation. Seed placed fertilizer guidelines were developed when researchers and growers were seeding at 8 lb./ac. Since that time, growers have adopted lower seeding rates. New hybrid varieties with larger and more vigorous seed may be somewhat more tolerant to stress, including effects of fertilizer placed in the seed row, but this may not be a worthwhile risk with such low initial seed population to begin with.
Canola needs 2.9 to 3.5 pounds per acre of available nitrogen per bushel of seed yield. A 35 bu./ac. canola crop takes up 100-123 pounds of N, and of that, 61-74 pounds are removed with the seed.1
Some of this will come from inorganic soil reserves and from organic matter (OM). OM is a large organic N storehouse but needs to be decomposed by soil microbes before becoming available for root uptake. The decomposition process is called mineralization. Mineralization of soil OM is fairly slow and variable, generating 6 to 20 pounds of available nitrogen per acre for each percentage point of OM. For example, soil with 4% OM will supply 24 to 80 pounds of nitrogen through the growing season. Warm conditions, ample moisture and high microbial activity increase the rate of mineralization. Soil building factors such as reduced tillage, application of manures, high yield production and return of crop residues can all increase organic matter content and potential mineralization.
The rest of the available N has to come from fertilizer, whether chemical fertilizers or organic fertilizers such as manure or compost. Canola responds very well to applied fertilizer N on deficient soils.
The most obvious response to fertilizer N is an overall increase in plant growth. Research generally confirms that N fertilizer mainly increases canola leaf area index, leaf duration, plant weight, growth rates, number of flowering branches, plant height, number of flowers, number and weight of pods and seed yield. Deficient plants are spindly and pale green when compared to those with adequate nitrogen. Therefore, good N fertility is necessary to produce a large, photosynthetically efficient leaf area that will support high numbers of flowers, pods and seed yield.
Moisture.Moisture is a major limiting factor for canola yield potential and must be considered when setting fertilizer rates and budgets. Canola needs 25 mm (1”) of available moisture through the growing season for every 4 bushels per acre of yield potential. A 35-bushel crop will need roughly 225 mm (9”) of moisture.
However, heavy N fertilization can reduce canola yields when excellent spring moisture conditions are followed by drought. Under this condition, the N stimulates larger leaves, and increases transpiration and moisture use. As a result, soil moisture can be depleted, leaving little for flowering, pod production and seed fill. Excessive N fertilization can also reduce yields by promoting lodging, delaying maturity that may increase fall frost damage, and increasing foliar disease due to the dense canopy and lodging.
Under dry soil conditions, root growth and activity are reduced, resulting in less N uptake. In addition, soil microbial growth is slower. This reduces N release from soil organic matter, but also reduces temporary tie-up by soil microbes through immobilization. Normally, more plant available N is left in the soil after a dry growing season than after wet seasons.
Canola yield potential strikes a balance between available nutrient and growing season moisture and temperature. Growers in a climate zone where canola can yield 60 bu./ac. will apply higher rates of fertilizer than growers in a climate where canola yields typically reach 40 bu./ac., for example.
The amount of nitrogen a crop will use also depends on the yield potential of the genetics selected for planting. Hybrid canola cultivars generally have a higher yield potential than OP cultivars grown under similar environmental conditions. 234 As such, they utilize more N to attain their optimum yield. A study published in 2009 concluded that the N requirement for hybrid canola was 50 to 60 kg N/ha (45-54 lb./ac. N) greater than for OP canola.5 This is under good growing conditions when canola has the opportunity to reach its full yield potential.
N fertilization, especially at higher rates, may slightly decrease the canola seed oil content. Plant breeders report that oil and protein contents are often inversely related — any attempt to change one causes an opposite change in the other component. However, while percentage oil content in each seed may decrease at high N rates, the total oil yield per acre still increases because yield increases more than oil level decreases.
Higher amounts of available N later in the growing season can increase protein in the seed. However, when N fertilizer is added under conditions of S deficiency, there may not be a protein increase but a rise in free amino acids due to hampered protein synthesis.
Ample to excessive nitrogen availability can also extend crop maturity. This increases the risk that a fall frost will prevent the plant from reaching full maturity. Seed will be smaller and seed chlorophyll content will increase, reducing quality. Excess N placed too close to the seed-row may delay emergence or decrease stand density, which will also delay maturity.
When it comes to crop nutrition, the biggest return on investment comes from nitrogen.6 Full economic value of high-yielding canola cultivars is realized when fertilizer rates are at or above the current recommended rates.7
Fate of applied fertilizer N
Research in western Canada has found that N fertilizer use efficiency (fertilizer N recovered in seed) rarely exceeds 50%. Of the rest, some remains in plant parts other than seed and eventually returns to the soil, and a significant portion remains in the soil as mineral or organic N and will be recovered by subsequent crops. In the Prairies, a relatively small amount of the N seems to actually be “lost” in terms of irrecoverably gone via leaching, denitrification or volatilization.
Soil interactions and availability
Several different mechanisms contribute to N loss from soil. Understanding the conditions that promote such losses can help growers avoid such conditions in the field and improve the N fertilizer efficiency.
Denitrification. One major N loss from soil occurs through a microbial process called denitrification. Denitrification occurs regardless whether the nitrate source is from fertilizer, manure or decomposition. As shown in Figure 10, nitrate can be changed by certain bacteria to gases such as N2O and N2, which escape back to the atmosphere. These soil bacteria have the ability to switch their respiration from using oxygen to nitrate or sulphate. Since respiration is more efficient using oxygen, these denitrifying bacteria will only switch to nitrate or sulphate if oxygen is absent, such as in prolonged waterlogged soil.
Figure 1. Nitrogen Cycle and Transformations
Significant denitrification losses normally occur only under warm, saturated conditions. (Crop residues and neutral to alkaline pH can make it worse.) On the Prairies, combinations of warm and saturated soil conditions are not that common. In spring, although we get saturated conditions, it is usually cold so microbial activity is low. In the summer, soils are warm but not usually saturated. Research on the Prairies has shown that denitrification losses that do occur tend to be during spring thaw. For this reason, effective N fertilizer management strives to avoid having large amounts of nitrate present during spring melts.
Summerfallow is especially prone to denitrification losses since large amounts of nitrate and moisture are stored during the fallow year, which increases the denitrification potential in the next spring thaw. Summerfallow also is a major contributor to leaching losses of nitrate.
Immobilization. Immobilization is the second major N transformation that reduces the plant available N supply. Soil bacteria may use either nitrate or ammonium for their own growth, temporarily tying up the N in the soil organic N storehouse. Immobilization essentially is the reverse of mineralization, and occurs when residues with low N content (high ratio of carbon to nitrogen, like cereal straw) are being decomposed. Since these residues don’t contain enough N for the microbes to make their own protein, they need to use the nitrate and ammonium from the soil solution. Soil microbes thus compete with plant roots for the available N and plant growth suffers when N supplies are inadequate for both microbial and plant growth needs. The poor crop growth in heavy chaff rows is due in part to immobilization of N and other nutrients by the decomposing microbes. One effective fertilization strategy is to place the fertilizer away from residues and thus avoid immobilization losses. Immobilization is only a temporary loss. N tied up in this way becomes a part of the soil organic matter and will contribute to the mineralizable N in the system.
Leaching. Nitrate is not adsorbed to the soil and moves readily with soil water. Nitrate lost to leaching can be significant in sandy soils in high rainfall areas or under summerfallow, but overall leaching probably contributes to less than 10% of the available N losses on the Prairies. To reduce leaching losses, time fertilizer applications to avoid prolonged exposure to wet conditions, and consider band placement of ammonium or ammonium-producing sources to delay the conversion to the vulnerable nitrate form.
Volatilization. Volatilization occurs when ammonia escapes from the soil to the atmosphere. Such losses happen in a variety of ways. One obvious loss occurs when anhydrous ammonia fertilizer is applied too shallow, into a soil that is too dry, or into soil too wet for furrows to seal properly behind the openers. Broadcasting urea fertilizer on the surface without incorporation can also lead to significant volatilization losses if more than 6 mm (1/4”) of rainfall does not occur soon after application. All ammonium based fertilizers are subject to volatilization if broadcast on the surface of soils with high pH, surface lime salts, low soil organic matter, warm temperatures and dry conditions. To reduce volatilization loss, ensure proper fertilizer placement into the soil, broadcast just before a rain, or use a urease inhibitor such as Agrotain. Product choice is also a factor. Ammonium sulphate would have very little volatilization, and would be preferred in a situation where incorporation won’t take place.
Weeds. Weeds can contribute to poor N fertilizer efficiency by competing with crops for uptake. The competitive ability of the crop for fertilizer uptake can be improved by banding rather than broadcasting fertilizer.
Erosion. Erosion of topsoil carries significant N, organic matter and other nutrients away from the field. Use soil conservation techniques to minimize such losses.
Clay particles. A final minor loss mechanism occurs when ammonium is fixed into the crystal structure of certain clays. Some soils contain expanding type clays that allow ammonium to enter within the plates of the crystal structure and become “fixed.” Such ammonium trapped within the crystal lattice is held tightly and is unavailable for root uptake.
Identifying deficiency symptoms
Nitrogen deficiencies look like:
- Leaves at the bottom of the plant are yellowing prematurely.
- Plants have reduced biomass. They are thin and spindly.
Healthy canola plants with adequate N have dark green leaves. Nitrogen is mobile within the plant and can be moved from older to younger leaves and pods. Therefore, N deficiency symptoms first show up in older leaves as pale green to yellow colouring, and sometimes purpling. These older leaves tend to die early, turn brown and drop off prematurely. Overall plant growth is slow, with short thin stems, small leaves, and few branches. The amount and time of flowering is restricted, and pod numbers are low.
In healthy canola, plant tissue tests of above ground material at flowering will show more than 2.5% N.
How to confirm
Proper diagnosis of nutrient deficiency should use most of the following tools:
- 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. Consider soil quality variation within’ the field. N is soil mobile, and sandy areas, for example, may show symptoms earlier or be more obvious.
- 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.
- Tissue test. Do not use this alone. A tissue test may show that the plant is deficient but this could be temporary due to environmental stresses, for example. Nitrogen may be in the soil at adequate rates, but the plant simply can’t access it. So even if the tissue test shows deficiency, top dressing may not be the answer. Plants may recover without top dressing once the environmental stress subsides.
- 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.
- Environment. Cold, wet, hot and dry can all stress canola plants, creating symptoms that may look like nutrient deficiencies. If most of your canola fields in the area are affected and neighbours’ fields have similar symptoms, the cause is more likely to be environmental — frost, excess moisture, etc.
- History of the land. Recently broken forage land is likely to be depleted in a lot of nutrients. Conversely, land that has had maximum rates of manure application or heavy rates of inorganic fertilizer in recent years may be less likely to exhibit deficiencies, making other causes more likely, especially if environment had kept yields below target levels.
- 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. If your farm is depleted of copper for example, this deficiency is likely to show up in wheat before any other crop.
Placement of fertilizer
Canola is sensitive to seed row N. Canola seedlings are injured by excessive seed row N by the “salt effect” that reduces water uptake by the seed, and by ammonia toxicity.
Numerous trials have examined the safe seed row N amounts with various openers and configurations. Research conducted in Alberta from 1992-96 illustrates the effect of seed row N on canola emergence and yield. The research involved 32 site years with canola at various Alberta locations. The highest N rate and least SBU reduced canola emergence 90% of the time and reduced yield 45% of the time. Sites with limited moisture (due to sandy texture, low seedbed moisture or dry conditions) two weeks after seeding experienced the greatest reduction in emergence and yield with 101 kg N/ha (90 lb N/ac) as urea and low SBU.
Figure 2. Seed Row N Fertilizer Effect on Canola Emergence
Figure 3. Effect of Moisture and SBU on Canola Emergence [101 kg N/ha (90 lb N/ac)] in Alberta
The seed-placed nitrogen table shows the approximate safe rates of seed row granular N fertilizer based on Prairie research to date. In canola, there is no significant difference in seed row safety between urea (46-0-0), ammonium sulphate (21-0-0-24) or ammonium nitrate (34-0-0). Anhydrous ammonia (82-0-0) must be placed separately from the seed. If moisture conditions are dry, reduce the safe amount of seed row N by half. The safe N rates in the table are in addition to N contained in safe rates of seed-row phosphate fertilizer.
Putting down seed and all fertilizer in one pass is common practice on the Prairies, but for canola, most of that fertilizer has to go in a side band or mid-row band. The table shows safe rates of seed-placed urea, given in pounds of actual nitrogen per acre. (Multiply by 0.46 to get pounds of urea.)
As shown in the table, Manitoba has lower safe rates than Saskatchewan and Alberta. This distinction is based on soil characteristics. Manitoba soils tend to have high soil pH, and high levels of free lime or carbonates. Under these conditions, urea tends to release more of its nitrogen in the free ammonia form (NH3), which is more damaging than the ammonium form (NH4+).
When selecting seed placed nitrogen rates, note that the rates in the table assume that the grower is already applying a safe rate of seed-placed monoammonium phosphate (MAP). MAP is usually safe at rates below 45 kg/ha (40 lb./ac.) of product. Some growers also use diammonium phosphate (DAP) for starter phosphorus, but this form has a higher nitrogen component. Safe rates of seed-placed nitrogen will be lower if the grower is using DAP instead of MAP.
Factors that affect safety of seed-placed nitrogen are:
Soil texture: The lighter the soil texture, the higher the risk to emergence damage and yield loss. Sandier soils are more risky than clays. Clay soils hold more nitrogen in the ammonium (NH4+) form. In sandy soils, more ammonium converts to ammonia, which is the form that causes seed and seedling damage because it is more toxic to plant tissues.
Seedbed moisture conditions at seeding: With moist soil conditions, water dilutes the concentration of nitrogen molecules around the seed and seedling. Water also disperses nitrogen molecules throughout the soil, reducing concentrations around the seed. In dry conditions, seed-placed nitrogen fertilizer tends to produce higher concentrations of ammonia and ammonium that can damage young seedlings. Seed-placed nitrogen rates should be lowered when conditions are drier than normal.
Fertilizer source: Ammonia can damage crops through direct toxicity while nitrate will damage seedlings by desiccation through the salt effect. Canola is sensitive to both the salt effect and direct toxicity, which is why there is no distinction made between products containing straight ammonium and those containing a mixture of ammonium and nitrate N when it comes to safe seed-placed N rates for canola. Products such as polymer coating (e.g.ESN) and urease inhibitors (e.g. Agrotain) can allow for higher safe-rates of seed-placed urea by lowering the concentration of ammonia that is in contact with the seedling. Anhydrous ammonia should never be placed in the seed row. Link to Product Choices section.
Row space: Wider rows increase the risk of seedling damage and yield loss, assuming that fertilizer rates per hectare (or acre) stay the same. The same amount of fertilizer directed into fewer rows (due to wider spacing) increases the concentration of fertilizer in each seed row. (Link to paragraph on RWU.)
Application rate: The higher the N rate, the higher the risk of emergence damage and yield loss.
Crop type: Generally, small seeded crops such as canola are more sensitive to seed-placed nitrogen. Do not use the same rates for canola as for wheat, which is less sensitive to ammonia toxicity and salt effect.
Soil pH: Safe rates are lower in high pH soils. At higher pH, more of the N from urea is in the free ammonia form (NH3) vs. ammonium (NH4+). Ammonia is more damaging.
Band placement away from the seed row can be used to avoid toxicity and to improve fertilizer use efficiency. Banded N fertilizer is usually more efficient than broadcast incorporation because concentrating fertilizer in the band reduces the contact with soil and microbes. This slows the conversion of ammonium to nitrate, reducing the risk of denitrification and leaching. Placing fertilizer below the crop residue also reduces the risk of immobilization. The banding benefit varies between soils and years mainly due to differences in moisture and susceptibility to loss.
Pre-plant banding can be difficult to work in the spring, and may do excessive damage to the canola seedbed. Fall banding on relatively well-drained soils after the soil has cooled can spread the workload without significantly lowering N efficiency, and often allows growers to buy fertilizer at lower cost than in the spring.8 The banding depth often is 8-10 cm (3-4"). Under dry or sandy soil conditions, ensure anhydrous ammonia is placed deep enough to prevent visible gaseous loss, and ensure the soil flows well around the openers to permit a good seal behind the shanks. Spring banding can be shallower due to better moisture and tilth. However, in dry springs the banding operation can reduce seedbed moisture and quality.
Spring banded anhydrous ammonia can be immediately followed by seeding, providing there are several inches of vertical separation between the injection point and seed depth. Canola emergence directly over the bands may be slightly reduced, but yields are generally not affected.
Figure 15 illustrates research conducted at the Agriculture and Agri-Food Canada (AAFC) Scott, SK Research Centre(Citation needed) on the safety of seeding directly after banding anhydrous ammonia 10 to 15 cm (4 to 6") deep. Yields varied between seeding dates following anhydrous banding over the years with no relationship between yields and plant stands. Proper soil packing over the seed row to firm the soil disrupted by the banding operation was deemed more important to avoid stand reduction than was potential injury from the banded ammonia. No difference was found between banding ammonia parallel to and perpendicular to the seed row.
Side bandinginvolves placing the fertilizer band to the side and often below the seed during the seeding operation. This method has good N use efficiency and avoids seed row toxicity if the separation is maintained under field conditions. However, side banding at seeding does involve more complicated, costly openers, increased draught and wear.
Mid-row bandinginvolves placing fertilizer between every second seed row or between a paired row during the seeding operation. This method also has good N use efficiency and avoids seed row toxicity if the separation can be maintained under field conditions.
In recent years, openers have been designed that allow anhydrous ammonia to be side-row or mid-row banded during seeding. Ensure the anhydrous ammonia is horizontally separated from the seed by at least 5 cm (2").
Dry or liquid N fertilizer can be applied to the soil surface then incorporated into the soil with a tillage implement. Although this method can be time and labour saving, fertilizer efficiency is usually sacrificed. Under dry conditions, broadcast-incorporated fertilizer can be stranded in dry surface soil, and not accessible by plant roots growing down into moisture. Broadcast applications can have relatively good efficiency if a good rain follows immediately afterward. However, in wet conditions, broadcast-incorporated fertilizer is more vulnerable to losses due to denitrification, immobilization and leaching than banded fertilizer. Use of a urease inhibitor such as Agrotain can reduce these losses. Broadcasting after crop emergence or topdressing generally has lower efficiency, but it can serve as a rescue treatment when poor fertility leads to plant deficiencies following emergence, or when improved environmental conditions require a reassessment of initial yield targets.
A nitrogen top up can make financial sense for canola if:
- Growing conditions improve after seeding. If conditions were too wet or too dry at the time of seeding, growers may have cut back fertilizer rates in response to lower yield projections. If conditions improve in June and a good stand emerges, growers may see a yield benefit from a nitrogen top up. In dry conditions, applying at least 66% of the recommended nitrogen rate at seeding then topping up if conditions improve is a risk management option.9
- Saturated soils impede good seed placement. This expands on the previous point. When the only choices to get canola seeded are mudding in or broadcast, cutting back nitrogen rates at seeding may be good practice to reduce nutrient losses and to assess stand establishment. If the crop becomes well established, an investment in more nitrogen fertilizer would be warranted.
- High nitrogen losses are likely to have occurred. Wet soil conditions can accelerate nitrogen losses through leaching and denitrification. Fields may need a nitrogen top up to reach their yield potential, but make sure canola survived the wet conditions before investing in a fertilizer top up.
- The crop is showing signs of nitrogen deficiency. Nitrogen deficiency symptoms first show up in older leaves as pale green to yellow colouring, and sometimes purpling. Tissue analysis may confirm these observations, but be sure to follow lab rules for sampling. Turnaround time is another hurdle. Results may not come back in time to take action. As an alternative, growers or agronomists could do a small experiment by spreading fertilizer on the surface and watering it in to see how plants respond. A small patch could be done by hand. If the plants in the patch green up, this suggests a nitrogen deficiency.
- A grower cannot efficiently place all the fertilizer needed through the seeding tool. Some growers will address this with a top dress application of nitrogen after crop emergence.
- In cases of poor root growth. Plants may be able to reach leached nitrogen later in the season as their root systems fully develop, but canola in fields with excess moisture may not develop the root system to reach that far. If roots have been growing to the side and there is no dominant tap root, then the tap root is unlikely to develop fully. Lateral roots may start to turn downward as the top layer dries, but they may not have the reach of a good tap root. In this case, a top dress of nitrogen may help the crop — as long as good growing conditions have returned, improving the potential return on that nitrogen investment.
Top up timing: Ideally, a nitrogen top dress should occur before the 4- to 6-leaf stage of the crop. For canola, maximum nitrogen uptake is from the start of flowering to the end of pod formation.10 Application by the 4- to 6-leaf stage gives time for rainfall to move fertilizer into the root zone making it available before the peak uptake period begins. Top ups can be effective after the 4- to 6-leaf stage — if weather prevents earlier application — assuming that soil nitrogen levels are enough to carry a crop through the first few weeks of peak growth. In that case, a later top up may provide the nitrogen necessary to carry the crop through to full yield potential after original reserves are drawn down.
Top up,nitrogen source: Nitrogen options for in crop application are urea (dry), UAN (liquid) or ammonium sulphate if sulphur shortages are also expected.
UAN dribbled on the surface is less prone to losses than dry urea broadcast on the surface, but both surface applications require rain soon after application to move the fertilizer into the soil and limit volatilization losses. The key for UAN is that it reaches the soil surface. UAN applied in no-till situations with high trash cover can be absorbed by the residue and be unavailable to the crop. Surface dribble bands are required. Spray applications are less effective.
Ammonium sulphate is less volatile than urea and will not lose nitrogen as quickly when surface applied, except under high pH carbonated conditions.
Agrotain will reduce losses for surface applied urea and UAN.
ESN is not recommended for post seeding applications because its slow-release feature may not release the nitrogen in time.
If using granular, apply when leaves are dry to make sure prills roll off onto the ground and don’t cause leaf burn.
If using liquid, apply when leaves are moist from early dew or a light rain so liquid nitrogen fertilizer runs off quickly, if possible. Applying when hot and dry can increase absorption of liquid into the plant, increasing the amount of burn. Consider adding some extra water to the tank in these conditions if waiting is not an option. Use straight stream applicators, not fan nozzles, and use some pressure to drive the stream to the ground. Keep in mind that windy conditions may disrupt the streams and lead to increased plant coverage and potential for damaging greater leaf area.
Top up rates: Even a small amount — 20 pounds of actual nitrogen per acre, for example — can provide an economic return if crops are deficient and conditions are such that the plant can access the N. Some growers will apply higher rates, but remember that a nitrogen top up will extend canola’s vegetative period and delay maturity. Consider the calendar date and the fall frost risk when making nitrogen top up decisions.
Growers may need to top dress up to 30 pounds per acre of actual nitrogen to see a noticeable improvement with the naked eye, but yield differences for rates lower than that may show up at harvest.
Top up application techniques: Broadcast spreading of urea or surface dribble banding of UAN are the most common and fastest methods, but these methods can also result in the highest losses if rain doesn’t come quickly. (Losses with UAN tend to be lower than with urea.) Urease inhibitors like Agrotain can help to minimize these losses.
Fertilizer placement in the soil, as opposed to on the surface, reduces losses from volatilization and immobilization and enhances overall nitrogen fertilizer recovery.11 This may be difficult to achieve in practice because spoke wheel applicators are hard to find these days.
Tank mixing liquid nitrogen with herbicide and applying through the sprayer tank provides only a few pounds of actual nitrogen per acre. This practice also applies fertilizer to the leaves instead of the soil surface. Fertilizer applied to leaves can damage the crop, plus the crop can’t take up much nutrient through the leaves. Most of the uptake will occur through the roots once rain washes the fertilizer off the plants and into the soil.
A variable rate applicator using optical sensors (e.g. GreenSeeker) is a high tech option. Applicators with optical sensors estimate crop biomass, yield potential and crop nutrient requirements based on reflectance of certain wavelengths of light emitted from the sensor and then reflected back to it. The sensors can be attached to a nitrogen applicator and provide continuous and instantaneous readings for variable rate applications.
Top up economics. Whether top dressing makes financial sense may depend on the conditions of the crop and whether the crop is deficient in N. If rates were more than adequate for a high-yielding crop, some nitrogen could be lost to leaching or denitrification and not hurt yield potential too much. The healthy crop may also access leached nitrogen lower in the soil profile later in the season. But if N application rates were reduced in response to rising costs or lower yield targets at seeding time and the crop is better than expected, this crop might benefit more from a top up.
Timing of application
Nitrogen fertilizer efficiency is greatly affected by the placement method and the application date. However, differences in crop response between placement methods or timings vary widely between years or fields due to variability in weather, soil type and drainage.
Nitrogen fertilizer banded too early in the fall increases potential for losses on wet, poorly drained soils. Delay banding until soil temperatures have cooled to less than 10 °C on well drained sites, and less than 5 °C on poorly drained sites.
This will significantly reduce losses by reducing the rate of change from ammonium to the more vulnerable nitrate form. Unfortunately, the opportunity to fall band fertilizer into cold soil is quite short since snow or frozen soil can occur without much warning. On average, fall soil temperatures on the prairies drop 1 °C every 5 days. In the Black and Gray Wooded soil zones, fall soil temperatures usually decline to 10 °C by the last week of September. In these zones, since the banding benefit is significant due to typically wet spring conditions and moderately high N rates, begin fall banding in late September on well-drained soils.12 This is earlier than the normal practice.
Generally, N fertilizer applied at or near seeding is the most efficient. The major disadvantages of applying all the fertilizer N at seeding are:
- increased time required to seed and fertilize during the short seeding season
- general trend of higher fertilizer prices during the spring season compared to fall
- risk of reduced seedbed quality due to extra tillage or soil disturbance at or near seeding
- seed row N limits
- fertilizer applicators being unavailable during the busy spring season
Considerable fertilizer placement and timing research has been conducted on the prairies.
Table 5. N Fertilizer Effect on Pod, Seed Number and Yield
|Time and Placement
||Relative Efficiency (%)
Anhydrous Ammonia (NH3) 82-0-0. Anhydrous ammonia is delivered as a liquid in pressurized tanks, and converts to gas at atmospheric pressure. When NH3 is injected into the soil, an individual NH3 molecule reacts with a water molecule in the soil to form an ammonium and a hydroxyl ion (NH3 + H2O → NH4+ and OH-). This results in the formation of a zone of a high pH solution of ammonium hydroxide localized around the point of injection. The NH4+ ions will at first be adsorbed onto the negatively-charged soil colloids but will begin to spread out slowly due to chemical diffusion. The oxidizing bacteria present in or near the zone of application will cause nitrification of the NH4+ to NO3-. The mobile NO3- ions will then flow with water in the soil and spread out into the soil and be intercepted by crop roots. It does take time for nitrification to occur because it is a microbial process. The nitrifying bacteria need adequate moisture and temperature to function. If NH3 is applied late in the spring when soil temperatures are cool and soil moisture is limited, the spreading out of NO3- from the zones of NH3 injection may be delayed. This is sometimes observed as striping in fields, with dark green crop where the fertilizer was applied and yellowing crop in the deficient soils between each injection zone. This usually clears up with rain. Denitrification and leaching losses from NH3 applications are usually low unless the NH3 is applied very early during a warm long fall. Nitrification of most of the NH4+ may occur and if waterlogged conditions are experienced the following spring denitrification of NO3- may be a problem. If NH3 is applied later in the fall when soil temperatures are reduced below 10 °C or in the spring, denitrification losses are normally small.
Ammonium Sulphate, 21-0-0-24S. This fertilizer is not subject to denitrification losses in the short term, and is well suited to surface applications due to low or no NH3 volatilization losses, except on high pH or carbonated soils. Its disadvantage is a lower N-analysis than other N sources, resulting in higher transportation costs. It is an excellent sulphur fertilizer source.
Urea, 46-0-0. This fertilizer has the highest analysis of N of all granular fertilizers. It is hydrolyzed in the presence of the naturally occurring urease enzymes in the soil or on crop residues to release plant available nitrogen. A rainfall of 15-20 mm (1/2-3/4”) is usually sufficient to dissolve and move the soluble urea into the soil before losses occur. If incorporated or banded in the soil, volatile losses are minimal. Urea is also commonly used as a granular source in a fertilizer blend that is broadcast and then incorporated into the soil using tillage. It can be a good choice for surface applications in late fall or early spring.
Urea Ammonium Nitrate (UAN), 28-0-0. This liquid fertilizer is a mixture of dissolved ammonium nitrate and urea in an approximate 50:50 blend along with enough water to keep it in solution. It has loss properties that are characteristic of a blend of urea and ammonium nitrate. Dribble banding is recommended for surface applications, because if sprayed evenly over the surface of the soil into a thatch cover, much of the fertilizer may adhere to the surface residue and not move readily into the soil beneath the residue. It will be immobilized in this residue (e.g. established forage crop or a no-till seeded crop). If banded into the soil it will perform similarly to banded urea or banded ammonia in the spring. In the fall, it will be more subject to loss than the ammonium or ammonium-producing fertilizers because a greater proportion is initially in the nitrate form. There is some mention that this form of N-Fertilizer is more available to plants because it comes pre-dissolved in water. However, this is a misconception because the amount of water in the fertilizer is minuscule compared to the amount of water present in a soil near normal field capacity.
Manure: Manure can be a very good source of N, but manure should be tested for nutrient content per tonne before setting application rates per hectare or acre.
ESN wraps urea prills in polymer that lets water in slowly, and then slowly releases ammonia and ammonium back out into the soil. Rate of release is controlled by the soil temperature, with release increasing as the soil temperature increases. This keeps the concentration of ammonia and ammonium lower early in the season when seedlings are most sensitive and releases the N in greater amounts as plant growth increases later in the season.
An Agriculture and Agri-Food Canada study found that using ESN for seed-row placed urea actually increased seed safety more than increasing the seed row width from 1.9 cm (3/4”) to 7.6 cm (3”).14
Inhibitors and coatings
While increasing seedbed utilization increases the safety of seed-placed fertilizer, the higher disturbance and draft is not always desirable. Openers with sidebanding capability add cost and draft. Even with sidebanding, seedling damage can sometimes occur, particularly with wide row spacing, high rates of N application or insufficient separation between seed and fertilizer. Therefore, there is interest in “safening” urea fertilizer so that seedling injury is reduced, allowing more freedom for seed-placed N.
Agrotain (N-n-butyl-thiophosphoric triamid) “safens” urea by inactivating urease enzymes in the soil adjacent to the granule. This slows the breakdown of urea to ammonia/ammonium, reducing the potential for seedling damage from seed-placed or sidebanded applications of urea or urea ammonium nitrate. As long as N remains in the urea form, the risk of damage is minimal. In addition, since Agrotain delays the release of ammonia, there is more time for the uncharged urea to move away from the seed-row in the soil water or with rainfall. Movement of the urea away from the seed, combined with a slower release of ammonia from the urea, will decrease the concentration of ammonia in contact with the germinating seedlings, thereby reducing seedling damage.
In field studies, Agrotain was effective in reducing seedling damage from side-banded urea and urea ammonium nitrate, where soil and environmental conditions led to seedling damage from the untreated fertilizer. The improved stand did not always lead to a higher crop yield because canola has the ability to compensate for reduced stands. The studies also showed that canola oil and chlorophyll content were often improved by using Agrotain.
Urease inhibitors slow the conversion of urea into ammonia and ammonium, reducing concentrations early in the season. Agrotain allows for safe seed placement of nitrogen at rates 50% higher than provincial recommendations for seed-placed nitrogen.15,16
1. Canadian Fertilizer Institute, “Nutrient uptake and removal by field crops, Western Canada 2001” The table says a 35 bu./ac. canola crop takes up 100-123 pounds of N and removes 61-74 pounds of N.
2. Harker et al, 2012, “High-yield no-till canola production on the Canadian prairies.” Canadian Journal of Plant Science. Findings: In 2008, higher than recommended nitrogen rates (150%) increased canola yields by 0.12 tonnes per hectare (t/ha), on average, compared to the recommended (100%) rate. The actual yields were 2.98 t/ha for 100% and 3.10 t/ha for 150%. In 2010, high rates increased canola yields by 0.25 t/ha compared to the recommended rate. The actual yields were 3.10 t/ha for 100% and 3.35 t/ha for 150%. This supports previous studies indicating that hybrid canola yields are often optimized at higher than ‘‘normal’’ rates of nitrogen. These results were statistically significant both years.