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Wheat Fertility

Some basic points on using anhydrous ammonia for wheat production

As producers start thinking about anhydrous application for wheat this fall, there are a few basic points which they should keep in mind, especially regarding safety. Ammonia is a hazardous material and safety should be the highest priority of the operator.

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Safety practices

Always have your personal safety equipment available and use it. The word "anhydrous" means without water. Ammonia reacts rapidly with the water in tissue if it comes into contact with skin, eyes, and mucous membranes. It is extremely important that when working with ammonia, farmers and fertilizer plant employees use all the appropriate personal safety equipment. As a minimum, this includes wearing tight-fitting chemical goggles to protect your eyes, chemical-resistant gloves, and a long sleeve shirt or jacket. People working with ammonia should also carry a plastic eyewash bottle of water with them at all times, in addition to having access to safety water tanks on both the ammonia tank and the tractor/applicator.
Check over the equipment carefully before starting work. Make sure all hoses are in good shape, and valves and break-away disconnects are in good operating condition.

Application methods and ammonia retention

When using ammonia as an N source, there are a number of reactions which come into play that will affect ammonia retention in soils, N response and efficiency. These include chemical reactions, physical factors relating to soil conditions, and how deeply the ammonia is applied. One important question many years in Kansas in Kansas concerns dry soil. Will a dry soil be able to hold anhydrous ammonia or will some or most of the ammonia be lost shortly after application?

  • Chemical reactions of ammonia in soil. Ammonia (NH3) needs to react with water shortly after application in order to convert into ammonium (NH4+), which is the molecule that can adhere to clay and organic matter in the soil. Ammonia is very soluble in water. After it is placed in the soil, NH3 reacts with water in the soil to form ammonium-N (NH4+), which is retained on the soil cation exchange sites. This process takes a little time - it does not occur immediately upon contact with the soil. The main controlling factors in the conversion of NH3 to ammonium-N are soil temperature, soil moisture, and soil pH. The higher the soil temperature and the wetter the soil, the more rapid the conversion occurs. If the ammonia does not react with water, it will remain as a gas that could escape from the soil. Also, equilibrium between NH3 and NH4+ is affected by soil pH. More NH3 will remain unconverted in the soil longer at higher application rates and at higher soil pH levels.

  • Physical facts that influence sealing and ammonia loss. Dry soils may be cloddy, with large air spaces where the soil has cracked. Getting the soil sealed properly above the injection slot can also be a problem in dry soils. This can allow the gas to physically escape into the air before it has a chance to be converted into ammonium. On the other hand wet soils tend to smear, leaving application channels open to the surface and providing a pathway for ammonia loss also. It is very important to make sure at the time of application that the slot created by the shank is sealed shut and that there is adequate soil moisture present for the NH3 to be retained in the soil. If the soil is too dry to retain NH3, or is not sealed well, gaseous NH3 can escape into the atmosphere and be lost for crop use. At today's high N prices, this can quickly become very expensive.

  • Importance of application depth. The deeper the ammonia is applied, the more likely it is that the ammonia will have moisture to react with, and the easier the sealing. Anhydrous ammonia can be applied to dry soils, as long as the ammonia is applied deep enough to get it in some moisture and the soil is well sealed above the injection slot. If the soil is either dry and cloddy, or too wet, there may be considerable losses of ammonia within just a few days of application if the soil is not well sealed above the injection slot and/or the injection point is too shallow. A recent study near Topeka found little or not direct ammonia loss in the week after application when ammonia was applied at 5- or 9-inch depths under good soil conditions. However, under wet conditions, losses as high as 15% of the applied N were seen with shallow application.

Application rate and shank spacing will also have a strong influence on sealing and potential loss. Lower N rates and application with narrow spacings reduces the concentration of N at any one delivery point and reduces the risk of loss.

The human nose is a very good ammonia detector. Producers should be able to tell if anhydrous is escaping from the soil during application or if the ammonia isn't being applied deeply enough. If ammonia can be smelled, the producer should either change the equipment setup to get better sealing or deeper injection, or wait until the soil has better moisture conditions.

Shank spacing

What about shank spacing for wheat? A number of studies have been done looking at the spacing of anhydrous application on wheat yields. The results have been somewhat erratic, but in general, yields tend to be reduced at shank spacings wider than 20 inches. The differences seem to be greater at higher yield levels, on sandy soils, and at lower N rates.

Recent studies in Kansas showed a 5% yield difference between 15- and 30-inch spacings over 5 experiments. One general observation is that a wavy appearance will be common in fields fertilized with ammonia, with plants near or directly over an ammonia band being taller, and those between bands shorter. At low N rates, this will likely lead to a small yield reduction. But at rates more than 100 pounds of N, yields will likely not be impacted, especially on silt loam or heavier soils.


In short, ammonia is an excellent N source for wheat, but producers need to consider some basic issues to be able to apply it safely and to gain good efficiency.

  • Make sure the application equipment is in good condition, that water tanks on the nurse tanks and the applicator/tractor are full of clean water, and that they use their personal safety equipment and have a personal eye wash bottle with them at all times.
  • Apply anhydrous ammonia at the proper depth to ensure good sealing.
  • Where possible, use a narrow shank spacing, less than 20 inches.
  • Use covering disks behind the knives or sealing wings ("beaver tails") on the knives of conventional applicators.
  • Apply anhydrous ammonia at least 1 to 2 weeks before planting. This waiting period should be even longer if soils are dry.

-Dave Mengel, Soil Fertility Specialist

Correlation of soil test nitrate level, N rates, and wheat yields

Soil testing for nitrate-N in the fall for making nitrogen (N) recommendations on winter wheat is a valuable practice, particularly when using 24-inch profile sampling. Unfortunately, few farmers utilize this tool, and its value has been questioned in some areas due to the potential for overwinter N loss. However, with the exception of sands, N losses over winter in Kansas are normally quite low due to our low rainfall in December, January, and February.

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To evaluate the relationship between wheat yield and fall soil nitrate-N -- and to determine if it is still a viable practice to utilize in N management of wheat -- we summarized data from 26 different N management experiments conducted across Kansas from 2007 through 2014. Most were from 2010 through 2013.

The driving force behind this study is the growing interest in improving N management in winter wheat production. Recent efforts have been focused on improving nitrogen use efficiency (NUE), or the portion of the fertilizer N we apply which is used by the plant. This has resulted in the creation of N fertilizer products designed to reduce N loss, optical sensors that can evaluate wheat's N status, and changes in methods and timing of N applications. With so many new practices incorporated into N management systems, older practices are starting to be considered dated and discarded.

Taking fall soil profile-N samples has been a recommended practice for making an N recommendation for winter wheat for many years. However, due to the mobility of nitrate-N in the soil, soil test values observed in the fall may be completely different than values observed in the spring, particularly on soils prone to leaching. Because many producers wait until spring greenup to make their N application, does soil sampling in the fall for nitrate-N really provide useful information for N management in wheat? That's a legitimate question.

The objective of our study was to evaluate the relationship between N fertilizer response by wheat and fall soil nitrate-N and determine if it is still a viable practice to utilize in N management of wheat.


Data were drawn from 26 dryland wheat experiments conducted in 2007 through 2014 throughout Kansas in cooperation with producers and Kansas State University experiment stations. Locations included Manhattan, Tribune, Partridge, Johnson, Randolph, Rossville, Ottawa, Sterling, Pittsburg, Silver Lake, Solomon, and Gypsum.

Soil samples to a depth of 24 inches were taken prior to planting and fertilization. Samples from 0 to 6 inches were analyzed for soil organic matter, phosphorus, potassium, pH, and zinc. Soil profile 0- to 24-inch samples were analyzed for nitrate-N, chloride, and sulfate. Fertilizer needs other than N were applied in the fall at or near seeding.


1) Analysis of yields taken from plots that received no N fertilizer shows a strong positive relationship with fall soil profile nitrate-N (Figure 1). Wheat yields increased rapidly as soil N levels increased to about 80 pounds soil N per acre, and then leveled off.


Figure 1. Relationship between fall soil profile nitrate-N level and wheat yield with no N fertilizer applied.

2) We then converted check plot yields to a relative yield, or percentage of the maximum fertilized yield obtained at each location (Figure 2). The results reveal not only the yield of the check plot, but also the N responsiveness of the site. This shows that at low soil nitrate levels, sites respond well to applied fertilizer. When fall soil profile nitrate-N levels are greater than 80 to 100 lb/acre, relative yield is approaching 100%, and it is unlikely the site will respond to additional fertilizer N applied in the spring.


Figure 2. Relationship between fall soil profile nitrate-N and Relative Yield, or percent check plot yield of the maximum obtained with fertilizer at each site.

3) A third way to show this relationship between fall soil nitrate and N response is to calculate the Delta Yield, or the increase in yield obtained from the addition of fertilizer at each site. This is a good measure of N responsiveness of an individual research site. The relationship between fall profile N level and Delta Yield is shown in Figure 3. It is clear from this graph that at low soil nitrate levels in the profile, sites respond well to applied nitrogen fertilizer. However, as the profile N level increases beyond 75 to 80 pounds N per acre, little or no N fertilizer response was found.


Figure 3. Increase in yield due to N fertilization, Delta Yield, as a function of soil N level.

4) A commonly used way to measure the efficiency of N use is to determine the amount of N fertilizer required to produce one additional bushel of yield. This relationship is shown in Figure 4.


Figure 4. Pounds of N fertilizer required per bushel of yield increase at different levels of N responsiveness, or Delta Yield.

On highly N-responsive sites, those with a large Delta Yield, the amount of N required to increase yield by one bushel is relatively low, near the 2.4 pounds N per bushel used in the K-State fertilizer recommendations. However, as the yield response decreases, the amount of N required to obtain that response increases dramatically. This relationship provides a good explanation of why fertilizer recommendations are generally made not to obtain the maximum yield, but rather the economic optimum yield. The efficiency of squeezing out those last one or two bushels is just too low. The cost of the added fertilizer will exceed the value of the extra grain produced. A number of additional conditions such as drought, disease, and poor root growth can influence this relationship. Many of the new technologies being developed to enhance N management and NUE, should help reduce the pounds of N fertilizer required to obtain a bushel of N response.


Wheat yield with no N fertilizer applied was compared with fall nitrate-N levels and a strong relationship was established. Although new practices have been developed to improve N management in winter wheat, soil sampling in the fall for nitrate-N remains an important practice to manage N efficiently and can result in considerable savings for producers.

When soil sampling for N is not done, the K-State fertilizer recommendation formula defaults to a standard value of 30 lb/acre available N. In this particular dataset, the average profile N level was 39 lb N/acre. However the N level at individual sites ranged from 11 to 197 lbs N/acre. Most recommendation systems default to a standardized set of N recommendations based on yield goal and/or the cost of N. Without sampling for N or using some alternative method of measuring the soil's ability to supply N to a crop, such as crop sensing, the recommendations made for N will be inaccurate, resulting in a reduction in yield or profit per acre and increased environmental impact.

Due to the drought of the past three years, there have been many situations where large amounts of N have been present in the soil at planting of wheat or summer crops such as corn or grain sorghum. Early samples requesting soil N tests from western Kansas coming to the lab are already showing high soil N levels from some areas. Failure to account for that valuable resource can result in excess foliage, increased plant disease, inefficient use of soil water, and reduced yield.

Soil sampling in fall for nitrate-N can have a significant impact on N recommendations for winter wheat, thus improving N management, and is still strongly recommended.

-Dave Mengel, Soil Fertility Specialist
-Ray Asebedo, Agronomy Graduate Student

Foliar nitrogen fertilizer products for wheat

Various foliar nitrogen (N) fertilizer products are sometimes promoted as an option for spring fertilization of wheat. These products range in analysis and can include straight nitrogen products or mixtures of N plus other macro and micro nutrients. The straight nitrogen products will typically have an analysis similar to traditional liquid N fertilizers, such as 25 to 30 percent N.

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One of the main differences between traditional UAN and the foliar products is that a certain percentage of the N in the foliar products is that a certain percentage of the N in the foliar fertilizers is commonly in some type of slow-release form. As a result, these specialty products are generally safer for application directly to the foliage in later stages of growth and result in less leaf burn than traditional UAN products.

K-State has tested many different types of foliar N fertilizer products over the years. Foliar N fertilizer products are just as effective as traditional N fertilizers on a pound-for-pound basis, but they are not more effective than traditional N fertilizers. They can be applied in a broadcast spray application at later growth stages of wheat growth than traditional N fertilizer products without damaging the wheat.

One of the reasons the foliar products have not been found to be more effective than traditional soil application is that only a small portion of the N applied as a foliar application to wheat actually moves into the plant through the leaf tissue. An excellent study done in Canada a few years ago found that when care was taken to prevent foliar applied N from reaching the soil, only 8-12% of the applied N was recovered by the plant, compared to 35 to 70% of soil applied N being taken up by the plant. Thus it is very likely that many foliar applied fertilizers are actually taken up through the roots once they wash off the plant.

At the normal topdress time (prior to jointing), producers should simply compare a foliar product to a traditional N fertilizer product based on the cost of a pound of N per acre to determine which product gives the best value. Invariably, the foliar products will be higher in terms of cost-per-pound-of-N than the traditional N fertilizers. In unusual situations (well after jointing or when trying to increase protein levels), the foliar N products would have some premium value since traditional N products would have the potential to burn the foliage if applied in a broadcast spray application.

To reduce the potential for leaf burn, there are alternative ways to apply traditional liquid N sources other than the standard spray nozzle. Streamer bars, a 10- to 15-inch long plastic bar which can be used with traditional spray booms in place of the nozzle, provide a solid stream of liquid fertilizer spaced every 5-6 inches. These streams of liquid greatly reduce foliar burn as compared to complete foliage coverage with a standard flat fan spray nozzle. Broadcast granular urea also produces limited leaf burn as compared to sprayed UAN. The bottom line is, foliar N products can be used for later applications, but the limited amounts of N which can be applied based on the labels of many of these foliar products limits their use in situations where large amounts of N are needed.

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Nitrogen topdressing on wheat

Now is a good time to start planning for topdressing nitrogen (N) of the winter wheat crop. With the temperature swings and seemingly warm winter during the end of January and the first of February, there are some key elements that need to be considered when deciding on the exact program you plan to use. These include: timing, nitrogen source, application method and nitrogen rate.

Ideally, the N in topdress applications will be moved into the root zone with precipitation well before jointing begins in order to be most efficiently utilized by wheat. With a great deal of the wheat not having a chance to tiller last fall, having adequate N available to support spring tillering when it breaks dormancy will be important. Some combination of fall preplant or at-seeding N, and/or early topdressed N, is also normally needed to supply adequate N to support head differentiation. This is the stage when head size is being determined and can begin about two weeks before jointing. The following will discuss some of the issues to consider when making topdressing decisions.

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Timing. The most important factor in getting a good return on topdress N is usually timing. It is critical to get the N on early enough to have the maximum potential impact on yield. While some producers often wait until spring just prior to jointing, this can be too late in some years, especially when little or no N was applied in the fall. For the well-drained medium- to fine-textured soils that dominate our wheat acres, the odds of losing much of the N that is topdress-applied in the winter is low since we typically don't get enough precipitation over the winter to cause significant denitrification or leaching. For these soils, topdressing can begin anytime now, and usually the earlier the better.

For wheat grown on sandier soils, earlier is not necessarily better for N applications. On these soils, there is a greater chance that N applied in the fall or early winter could leach completely out of the root zone if precipitation is unusually heavy during the winter. Waiting until closer to spring green-up to make topdress N applications on sandier soils will help manage this risk.

Also, keep in mind that N should not be applied to the soil surface when the ground is deeply frozen and especially when snow covered. This will help prevent runoff losses with snow melt or heavy precipitation.

Application method. Most topdressing is broadcast applied. In high-residue situations, this can result in some immobilization of N, especially where liquid UAN is used. If no herbicides are applied with the N, producers can get some benefit from applying the N in a dribble band on 15- to 18-inch centers. This can help avoid immobilization and may provide for a little more consistent crop response.

Source. The typical sources of N used for topdressing wheat are UAN solution and dry urea. Numerous trials by K-State over the years have shown that both are equally effective. In no-till situations, there may be some slight advantage to applying dry urea since it falls to the soil surface and may be less affected by immobilization than broadcast liquid UAN, which tends to get hung up on surface residues. Dribble (surface band) UAN applications would avoid much of this tie-up on surface crop residues as well. But if producers plan to tank-mix with a herbicide, they'll have to use liquid UAN and broadcast it.

Some of the new controlled-release products such as polyurethane coated urea (ESN) might be considered on very sandy soils prone to leaching or poorly drained soils prone to denitrification. Generally, a 50:50 blend of standard urea and the coated urea--which will provide some N immediately to support tillering and head development and also continue to release some N in later stages of development--works best in settings with high loss potential.

Rate. Producers should have started the season with a certain N recommendation in hand, ideally based on a profile N soil test done before the crop is planted and before any N has been applied. If some N has already been applied to the wheat crop, it is too late to use the profile N soil test since it is not reliable in measuring recently applied N. Topdressing should complement or supplement the N applied in the fall, with the total application amount equaling that targeted rate.

Nitrogen management considerations for wheat

Tillage systems affect nitrogen (N) management for wheat in many ways. Nitrogen rates, application methods, and timing can all be affected by tillage considerations.

Some of the most important factors include the following.

* Even distribution of previous crop residues is extremely important for no-till wheat. Nitrogen applied on the surface can get tied up, or immobilized, on crop residues and be unavailable to the currently growing crop for several months or longer. Where there are windrows of residue, the N immobilization potential is especially high for surface-applied materials. Also, where wheat is planted into fields with an uneven distribution of crop residue, the wheat may have poor stand establishment and root development in the areas of especially heavy residue.

* No-till wheat may require an extra 20 to 30 lbs of N per acre compared to conventional-till wheat. One reason for this is that no-till soils are generally cooler, and have lower N mineralization rates. Another reason is that organic matter levels tend to build up slowly in no-till soils, and this process uses and stores soil N. For example, every percent organic matter in the top 6-7 inches of soil contains about 1,000 pounds of N per acre. If the soil organic matter level were to increase by a full percentage point over 20 years, an extra 50 lbs N per year per acre would need to be invested just to build up the organic matter level. That's over and above the needs of the crop.

* Topdress N should be in the root zone by jointing. Producers should not wait too long to apply topdress N. Topdress applications should be applied early enough to have a good chance of moving down into the root zone by jointing. By waiting until the last minute, producers run the risk of being prevented by wet weather from applying the N in time. On the other extreme, late-applied topdress N may not receive any precipitation for a time after it is applied, and thus may not get into the root zone when the plant needs it most to maximize yield potential.

* Urea-containing N fertilizer products (such as dry urea or N solutions) should not be placed in direct seed contact as a starter fertilizer.

* On medium- and fine-textures soils with adequate internal drainage, applying N in a subsurface band is generally more efficient and consistent than surface-applied N in no-till. A subsurface band minimizes or eliminates the potential for immobilization, places N in the active root zone where it is needed, and would eliminate any potential for volatilization losses if it exists.

* For surface applications of N, applying it in a dribble band is generally more efficient and consistent that broadcast N, but not as consistent as subsurface applications.

* Although broadcast surface applications of N are often somewhat less efficient and consistent overall than subsurface band applications, there are can be many reasons that broadcast applications better fit many producers operations;

-- Can cover more acres per day

-- Does not require specialized equipment

-- Does not require extra horsepower or fuel use

-- Allowing producers to tank mix with herbicides in a single application

While topdress N applications to wheat may sometimes result in some leaf burn (especially late applications in early spring), it is generally cosmetic only and has not resulted in noticeable yield loss in Kansas trials.

* On well drained, medium- and fine-textured soils, there is generally no agronomic advantage to making multiple split applications of N. For no-till wheat, however, there may be an advantage to applying at least 20 to 30 lbs of N preplant or at planting time in order to supply adequate N for fall growth.

* On poorly drained or claypan soils, topdress applications of N are preferred. Fall applications of N are subject to denitrification losses on these soils.

Late topdressing of wheat

The heavy dose of precipitation in Kansas during late December and January may mean that soils will stay wet through at least late winter or early spring. Producers who were planning to topdress nitrogen (N) on their wheat during this time period may have to wait longer than they'd like for soils to dry out enough to make the application. This could mean that the N will go on later than usual. How late can producers topdress their wheat and still get a yield benefit?

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Unless wheat is not severely deficient in N at the boot stage, it's unlikely that an N application at that time will increase yields. If the wheat is severely deficient in N at that time, then an application of N can increase yields.

In almost all situations, N has to be in the root zone of wheat by the jointing stage to have a significant effect on yields. After that stage of growth, additional N has much less, if any, yield benefit. Nitrogen that is surface-applied just prior to or at jointing has less chance of moving into the root zone than N applied in fall or winter. That's why we recommend applying topdress N as early as possible on medium- to fine-textured soils where the chances of losses from either leaching or denitrification are low. Nitrogen applied on the surface at or after jointing has little chance of increasing yields much, unless the wheat is severely deficient in N.

A three-year study conducted by Barney Gordon at the North Central Experiment Field has tested different N rates and application timing on wheat, and demonstrates a typical response on medium- to fine-textured soils.

Spring applications were made in mid-February to mid-March each year. The soil N level in the top 24 inches in 2003 was 26 ppm; 6.2 ppm in 2004; and 10.2 ppm in 2006. The variety was 2145 and the previous crop each year was soybeans.

When part or all of the N was surface-applied at F6 (jointing), it had only half the effect on yield as where it had been applied earlier. When part or all of the N was surface-applied at F8 (flag leaf emergence), it had little or no effect on yield.

In contrast, a treatment of 80 lbs/A N resulted in wheat yields of about 90 bu/A whether it was applied all in the fall, split between fall and early spring, or all in early spring. In each of these years, there was enough moisture after the early spring applications to move the N into the root zone of wheat.

The wheat in these tests was not severely deficient in N at the time of the applications made at jointing or flag leaf. If that had been the case, there could have been more of a yield benefit from the flag leaf application of N.