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.
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.
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.
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