Nitrogen management in crops

Overview

Nitrogen (N) is a significant component of proteins, which are essential for crop growth. It is crucial to determine the best timing, placement, and rates of N fertilizer to apply.

Timing

Fertilizer timing depends on environmental conditions and the seasonal growth patterns of the plant. There are advantages to both spring and fall fertilizing, but N supply is critical for early plant growth.

Adding additional N should be done within 4 to 5 weeks of seeding, but it can be more cost effective to apply it at seeding. N fertilizer requires moisture with canola and cereal crops, so N will be more effective after spring rains.

Winter fertilization should only take place if snow is less than 4 inches deep and there is no ice layer, to allow granules to be ‘driven’ into the soil. Compare costs, fertilizer can be more expensive in the spring making winter applications of urea viable, if there is limited loss from run-off, leaching and volatilization.

A 2005 study in Lacombe by Agriculture and Agri-food Canada found ammonium nitrate applied at 100 kg N/hectare doubled productivity over unfertilized forage fields. Early winter applications of fertilizer appeared to be the worst, productivity dropped by as much as 15% over the same application in early spring. Broadcast fertilization in the spring yielded the best results.

A number of factors can cause N immobilization and losses, see ‘Nitrogen immobilization and losses’ below. See the following fact sheet for fall fertilizer information:

• Fall-applied Nitrogen: Risks and Benefits

Growth stage

Wheat starts to require N at the 5-leaf stage, while the overall plant growth lags behind. The N uptake pattern and dry matter accumulation of spring wheat by growth stage is depicted in Figure 1 below. Canola’s N uptake pattern is similar to that of wheat.

Canola N requirements are highest prior to bolting. N fertilizer must be in the root zone and available for uptake as crops enter their rapid growth period in the first month.

Figure 1. shows growth stage where the flag leaf is fully expanded at Stage 8, the spike has cleared the collar at Stage 11, and kernel hard is Stage 15.

Figure 1. Wheat dry matter accumulation and nitrogen uptake according to growth stage 1

¹Adapted from Aerial parts of hard red spring wheat. I. Dry matter distribution by plan development stage, and Aerial parts of hard red spring wheat. II Nitrogen and phosphorus and content by plant development stage by Armand Brauer, A.B. Frank, and A.L. Black. Agronomy Journal. 79: 845-852 and 852-858.

See wheat nutrition and fertilizer requirements – nitrogen for more information.

Nitrogen immobilization and losses

Immobilization

Soil micro-organisms can convert nitrate and ammonium-N (inorganic N or plant available N) to organic N, which is not plant available. Organic N nust be mineralized by soil micro-organisms back into inorganic N forms for plants to utilize the N. Soil microbes compete with growing crops for N fertilizer, which may result in reduced crop growth. Immobilization of ammonium nitrogen is slightly greater than immobilization of nitrate-nitrogen. Immobilization can remove between 20 to 40% of inorganic N from the soil. Under minimum till management, higher N immobilization in surface soil may reduce available N to crops.

Banding N fertilizer in the soil can reduce immobilization losses.

Volatilization

Nitrogen from fertilizers containing ammonia or urea can be lost through volatilization as ammonia gas to the atmosphere in dry conditions. Ammonia volatilization increases with increasing soil pH (alkaline soils), soil carbonate content and pH of the added fertilizer. These types of losses are far greater with urea (46-0-0) than with ammonium nitrate fertilizers (34-0-0) and ammonium sulphate (21-0-0-24).

The losses from sandy soils are usually higher than from heavier textured soils and are greater at high temperatures than at low temperatures. Greatest volatile losses can occur where there is just enough moisture to put fertilizer into solution – but not enough to move it into the soil – followed by hot, dry windy conditions.

Banding or incorporating N fertilizer can reduce loss due to volatilization.

Leaching

Nitrogen fertilizer leaching can happen in the late fall or early spring in sandy or irrigated soils. The N loss occurs when nitrate N moves out of the root zone and cannot be used by plants. Losses of this nature are minimal during winter and the spring growing season on most soils, particularly in clay soils.

Denitrification

Soil micro-organisms convert nitrate-nitrogen to gases such as nitrogen (N₂) and nitrous oxide (N₂O), which are lost to the atmosphere, this is denitrification. Considerable nitrate-nitrogen may be lost by denitrification when soils are temporarily wet (early spring or after heavy rainfall), this is the best condition for micro-organism conversion.

West central Alberta is often saturated in the spring and prone to denitrification losses. Fall N fertilizer should be avoided in areas where flooding can occur. Denitrification losses do not occur as long as the fertilizer nitrogen is present in the ammonium form.

To reduce denitrification losses, band fall-applied N in the ammonium form and apply as late as possible. This way the nitrogen will remain in the ammonium form.

Note: Ammonium fixation may be a partial factor. This is when the ammonium (NH₄⁺) form of N is retained by some clay minerals, making it no longer available to plants.

Some Saskatchewan studies have shown that minimum and zero-till moist cropping conditions have a greater N loss through denitrification than conventional tillage.

Leaching and denitrification losses can be significant on summer fallow fields in high moisture conditions because the available nitrogen is present in the nitrate form.

Erosion

N fertilizer can be lost in runoff waters (rain, snow or frozen soils) and through soil erosion caused by either wind or water. Banding N fertilizer in the soil can reduce runoff losses. See Soil erosion for methods to control water or wind erosion.

For more details on nitrogen losses, see Assessing effects of agronomic nitrogen management on crop nitrogen use and nitrogen losses in the western Canadian prairies.

Fertilizer type

• Ammonium-based fertilizers include urea (which converts into ammonia) and anhydrous ammonia.
• Urea is an inexpensive form of nitrogen fertilizer with an NPK (nitrogen-phosphorus-potassium) content. Manure contains urea. See also: Ammonia Volatilization from Manure Application
• As long as the soil flows around the opener and seals, clay and organic matter can retain anhydrous ammonia even in dry conditions. In sandy soils with low organic matter the fertilizer should be applied deeper than usual to reduce losses to the atmosphere. See also: Anhydrous Ammonia Use at Seeding
• Ammonium sulphate can overload the soil with Sulphur (S), reducing the amount of S required for the next 3 cropping years.
• Elemental Sulphur (S₀) can work well in the fall as the granules need precipitation and freeze/thaw cycles to be broken down. Spring field operations will further degrade and disperse the particles so that microbial oxidation to sulfate can occur.

Lowering fertilizer cost

Green manure ploughdown

Growing legumes and working the whole crop into the soil as a ploughdown is a way to lower fertilizer costs. Since legumes fix their own nitrogen from the atmosphere, green manuring can maintain or improve soil fertility without direct fertilizer costs. However, there are still costs involved to fix nitrogen this way, including:

• seed and seeding costs
• stand termination and residue incorporation costs
• lost revenue as there is no cash crop produced during the year of ploughdown

Best green manure crops

The best crop for green manuring has low production costs but still fixes enough nitrogen to meet the requirements for the following crop. Several crops will work well for green manuring.

Deciding which crop to grow depends on:

• seed availability
• seed costs
• local environmental conditions

Peas

Peas have good dry matter production and nitrogen fixation capabilities in a range of environments. Most varieties have a large seed size so seeding costs can be significant. Small seeded ‘silage’ varieties do not offer savings as they are usually priced to be comparable to other peas on a $/ac basis.

You can use a wide range of field pea varieties for ploughdown. Common seed is available in most regions so shopping around can minimize seed costs. Lush plant material, incorporated at mid- to late-flowering, breaks down relatively quickly. It can meet the nitrogen requirements of following cereal crops.

Lentils (Indianhead)

These lentils have a small, black seed size. Total plant growth and nitrogen fixation with lentils is often lower than other legumes but is still acceptable in dryland situations. Another positive factor is that plant material breaks down quickly following incorporation.

Chickling vetch

Chickling vetch has reasonably good nitrogen fixation capabilities. Incorporated plant materials break down almost as quickly as lentils. In dryland experiments, chickling vetch had enough fixed nitrogen to meet the requirements for a following cereal crop.

Faba beans

Unlike many other legumes, faba beans will fix nitrogen throughout the growing season. If conditions are good, they will produce a lot of dry matter and fix high rates of nitrogen.

However, this crop does not have very good nitrogen fixation capabilities if it has access to less than 8 inches of water. Another disadvantage is that it is large-seeded, with high seeding costs.

Sweet clover

Sweet clover’s biennial growth allows you to seed it with the crop prior to the green manure ploughdown year. This eliminates costs for a separate seeding operation. Weed control options may be limited in this mixed crop year. But sweet clover can suppress weed growth while the crop is growing, throughout the fall, and the following growing season.

Seed is generally available at reasonable cost. A small seed size and low seeding rate also reduce seeding costs. Sweet clover has good dry matter production with high nitrogen-fixing capabilities.

Crop removal/ploughdown timing can be critical. Sweet clover uses more soil moisture than other green manure crops. This soil drying can suppress the yield of subsequent crops.

Fixed nitrogen

Under irrigation, legume crops can fix well over 100 lb N/ac (Table 1). You can only achieve these high rates of symbiotically fixed nitrogen under low soil N conditions. This is where inoculation is most effective and where a lack of moisture will not limit crop growth.

Table 1. Nitrogen fixation in legumes grown under irrigation*

* From R.J. Rennie as cited in Soil Improvement with Legumes

Resources

Government of Saskatchewan – Green Manuring with Legumes

Michigan State University – Field crops

Nitrogen placement (application method)

• Banding N fertilizer keeps fertilizer in a form that cannot be lost from the soil. When you band urea or anhydrous ammonia, the concentrated ammonia temporarily hinders bacteria that normally convert ammonia to nitrate reducing denitrification losses.
• Broadcasting N fertilizer after seeding is about 20% less effective than buried N. (Table 2)
  • Broadcast urea (46-0-0) or UAN (urea ammonium nitrate, usually 28-0-0) can gas off and lose N back to the atmosphere. Fertilizer losses from these products depend on a combination of soil and environmental factors (Table 3).
  • Broadcast urea or UAN applied when the soil is cold, in late April and early May, has minimal N losses during the first 1 to 3 days. However, 25 to 50% N losses can occur in the first 7 days with the same products broadcast on warm soils.
• Burying nitrogen may prevent N immobilization and N volatilization. To be effective, the soil temperature at fertilization depth should be less than 10°C during mid-afternoon. Fertilizer applied with this method during the growing season is side dressing.
  • Canola and cereal crop residues will immobilize N during the initial phases of decomposition, N fertilizer can be trapped in these residues for weeks or months. Every ton of cereal crop residue from the previous year will immobilize 20 to 30 lbs/ac.
• Topdressing (surface N fertilizer) is rarely used as it relies on rain to get the fertilizer to the roots. Also due to losses from gassing-off or immobilization, this method can lose up to 70% of the N fertilizer. Cereal and canola crops require 2 to 3 pounds of N for every bushel of grain produced. Therefore topdressing would require a minimum of 25 to 30 pounds of actual N per acre.

Table 2. Relative effectiveness of placement (method) and timing of nitrogen application for increasing crop yield in dry soil conditions

Table 3. Soil and environmental factors that affect nitrogen losses from urea or UAN (UAN is a solution of urea and ammonium nitrate in water)

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Related

• Alberta Farm Fertilizer Information and Recommendation Manager
• Soil health management resources