Increasing nitrogen rates produces diminishing returns

In this second part of our look at nitrogen, we will examine more sources, rates, timing and location of application.

Liebig’s Law of the Minimum applies, like all of the other nutrients we have looked at. Most crops, with the exclusion of legumes, exhibit the classic response curve we have seen with other nutrients.

Nitrogen is, in the majority of situations, the most limiting nutrient, and along with water, one of the two most limiting plant growth factors encountered in Western Canada and the Great Plains.

A hundred years ago, nitrogen was amply supplied by the decomposition of organic matter that mineralizes or liberates from the organic matter. Due to the dominant vegetation being native grasses, organic levels were comparatively high and for the next 50 to 75 years, farmers relied on this mineralization process to provide the nitrogen for growing crops.

For example, 55 barley studies reviewed by Cowell and Doyle from 1950-69 showed a 26 percent yield response, while studies from 1970-91 showed a 44 percent yield response.

The Law of Diminishing Returns — Nitrogen, as seen in the accompanying graphic, shows a classic yield response curve. This data, which was taken from the Revised Nitrogen Fertilizer Guidelines for Wheat, Barley and Canola in Manitoba, illustrates the wheat response curve from 147 sites in Western Canada across three moisture regimes.

Available on the internet at, it also uses crop and nitrogen values to calculate the anticipated break-even point for applying nitrogen. For example, using wheat in the parkland with an anticipated value of $7 per bushel and urea costing $650 per tonne, a return of $115 could be anticipated when 100 pounds of nitrogen per acre is applied.

When increased to 110 lb., a return of $119.50 could be achieved, or $4.50 extra. At 120 lb., $122.10 is returned, or an additional $2.60. By 130 lb., while the yield is still increasing, the return is still in the $122 range because of the increased cost of the fertilizer.

Now, let’s look at the 4Rs for managing nitrogen.

With the exception of potash (0-0-60 or 0-0-62) and elemental sulfur (0-0-0-90 or 0-0-0-95), most of the commonly used fertilizer products contain some nitrogen. However, because many of these have been discussed previously, I will focus on the products used as a nitrogen source.

Anhydrous ammonia, 82-0-0, is the most concentrated form of nitrogen. It is the base product that is manufactured by fertilizer plants and is the product from which others are manufactured. It is manufactured as a compressed gas and needs special storage, transportation and application equipment. There are also specialized safety requirements and handling techniques because it is listed as a dangerous product.

Anhydrous ammonia is usually the least costly nitrogen product available. Because all of the nitrogen is in the ammonium form, it is ideally suited for fall applications as well as spring pre-planting. Modern seeding tools have been developed that allow it to be applied at planting and as a post-planting side dress application.

Urea, 46-0-0, is the next most concentrated form of nitrogen. It is a granular product that is manufactured by reacting anhydrous ammonia with carbon dioxide. It is the most common nitrogen fertilizer used. It is easy to handle and quite flexible with regards to application.

Like NH3, all of the nitrogen is in the ammonium form and is ideally suited for fall applications on the Canadian Prairies and U.S. northern Plains, as well as spring pre-planting. Modern seeding tools have been developed that allow it to be applied at planting and as a post-planting side dress application.

Urea is not suited for broadcast applications unless it is incorporated shortly after application or is treated with a unease inhibitor or coated with a polymer. This is because the enzyme urease hydrolyzes urea into ammonium carbonate. Ammonium carbonate, also known as smelling salts, volatilizes into the air and is lost.

Ammonium nitrate, 34-0-0, is a very effective and flexible nitrogen source. It is a granular product that can be effectively applied in the spring as a band, broadcast or seed row product. Unlike urea, it does not volatilize when broadcasted and is also safer to the seed when applied in the seed row. It does have some storage issues because it absorbs moisture readily and can cake in the bin.

Sadly, most sources of ammonium nitrate have disappeared because it can also be used as an explosive and was used in the 1995 Oklahoma City bombing. Today’s regulations make it difficult to buy, store or use.

Urea and ammonium nitrate as a liquid, 28-0-0 or 32-0-0, is a product made up of approximately equal parts urea, ammonium nitrate and water. This product is the highest concentrated nitrogen liquid product.

UAN is popular in many areas because it’s safe to handle, convenient to mix with other nutrients and chemicals and is easily applied. Liquid urea-ammonium nitrate fertilizer is relatively simple to produce. A heated solution containing dissolved urea is mixed with a heated solution of ammonium nitrate to make a clear liquid fertilizer. Half of the total nitrogen comes from the urea solution and half from the ammonium nitrate solution.

It is made in batches in some facilities or in a continual process in others. No emissions or waste products occur during mixing.

Like ammonium nitrate, it can be effectively applied in the spring as a band or seed row product. When treated with a urease inhibitor, it can also be effectively broadcasted post-emergence to many crops, either as a foliar spray or dribble band applications.

Aqua ammonia, 20-0-0, is anhydrous ammonia dissolved in water. It is a low-pressure solution and contains free ammonia. The amount of free ammonia increases as air temperatures increase. It is stored in closed low-pressure tanks and is injected into the soil much like anhydrous ammonia.

Because aqua ammonia is a low-pressure solution and contains only a small amount of free ammonia, direct soil application does not need to be as deep as with anhydrous ammonia. It is not commonly used because of the cost of handling the water in the product.

There are many other products that include nitrogen and another nutrient.

Ammonium sulfate or AmSul, 20 or 21-0-0-0 + 24S, is a dry crystalline material produced by reacting anhydrous ammonia with sulfuric acid. It is a common byproduct of many industrial processes and contains 20 or 21 percent nitrogen and 24 percent sulfur, which makes AmSul an excellent source when both nutrients are needed.

The price of AmSul can vary depending on quality, and availability can swing wildly. Supplies can also vary, depending on the processing of oil and gas or other refining processes.

While all nitrogen fertilizers have an acidifying effect, ammonium sulfate is more acidifying than other nitrogen sources, which limits its use in areas where soil requires liming. Its most common use would be before canola planting where sulfur is also needed.

Most other products that contain nitrogen contain it as a combination of one of the above products and other nutrients.

The amount of nitrogen that is in a soil at application time can vary widely based on the previous crop, its nitrogen removal and the nitrogen in its residue, soil texture, the weather and a number of other factors. This, in addition to the supply rate of the soil by a process called mineralization, all will play a role in how much nitrogen should be applied.

New models have been developed that do a good job of predicting application rates under a number of scenarios.

Compared to other nutrients, nitrogen is very flexible in its timing options. However, we have to categorize it based on the products that it contains.

Ammonium containing products such as urea, anhydrous ammonia and aqua ammonia are excellent options for fall applications. That is based on the fact that the ammonia ion NH4+ carries a positive charge. Soil particles have a negative charge. Like a magnet, opposite charges attract, so the NH4+ is tightly held onto the soil and is not affected by moving in the water in springtime.

The nitrate ion NO3-, on the other hand, has a negative charge so remains in the soil solution and can move or leach through the soil with water movement. Nitrate is also susceptible to a microbial process called denitrification. This process occurs in warm soils when oxygen is limited. It results in oxygen being removed by microbes and nitrogen being lost to the atmosphere as nitrous oxide and other nitrogen containing gasses.

Nitrate containing products are usually limited to springtime applications close to, at or following seeding.

For fall applications of ammonium products such as urea or anhydrous ammonia, banding into the soil is by far the best option for placement, and the only option for anhydrous.

For a broadcast application of urea in the fall, the use of a nitrate inhibitor product such as Koch’s SuperU is usually effective in eliminating volatilization and denitrification losses. Fall applications are recommended to begin only after soil temperatures are below 10 C to reduce the conversion of NH4+ to NO3- in a process called nitrification.

Spring applications are a little more flexible with broadcasting and incorporation, seed row mid-row or side-banding all working effectively. If broadcasting is being used without incorporation, the use of a urease inhibitor is recommended to reduce volatilization losses. It is important to note however, that banding of nitrogen products two to three inches deep is the gold standard for nitrogen applications.

Referring back to the first column on nitrogen, remember that nitrogen is the number one nutrient related to yield and quality. Properly selecting nitrogen products, using them in the proper manner and applying them at the proper time will result in the highest returns for your input dollars.

And as a bonus, they are also the same practices that are used to reduce losses into the environment, either through water pollution or into the atmosphere as greenhouse gasses.

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