Mineral Nitrogen Fertilizers and Nuances of Their Use in Utah Soils
Mineral N fertilizers (as opposed to N from manure, compost, or other organic matter sources) in most cases, begins with the manufacture of Ammonia (NH3) gas. Ammonia is produced through the Haber-Bosch process where atmospheric nitrogen (nearly 80% of Earth’s atmosphere) is combusted with methane (natural gas) at high temperature and pressure. Gaseous ammonia then is liquefied under pressure to be used as a fertilizer itself, or reacted with various acids or gases to form the common gaseous, solid and liquid mineral N fertilizers.
The mineral N fertilizers provide one or both of the primary forms of N that are taken up by plants, namely, Ammonium (NH4+) and Nitrate (NO3-). I will discuss the main N fertilizer sources commonly used in Utah, with nuances regarding their management in alkaline, calcareous Western U.S. soils. Discussion of N-containing phosphates will be treated in a future post on phosphorus fertilizers.
Gaserous Mineral N Fertilizers
Anhydrous Ammonia (82-0-0)
Liquefied ammonia gas is used extensively as a fertilizer directly. Because liquid ammonia boils above -33 C (-27 F), it is injected with specialized equipment below the soil surface so it does not escape as a gas directly back to the atmosphere. Ammonia gas almost instantly reacts with soil water and converts to the relatively soil-stable NH4+ form that adsorbs to soil surfaces and is readily available for plant uptake. Over time (1 to 2 weeks), NH4+ is microbially converted to NO3-, which is also readily available for plant uptake. Nitrate, however, is not adsorbed to soil surfaces and therefore is highly mobile in soils and potentially lost due to leaching and/or denitrification (microbial conversion of nitrate to various gaseous N forms).
In Utah and many Western U.S. locations, high soil pH can cause volatilization of ammonia gas after application, as the high concentration of hydroxide (OH-) in alkaline soils, drives a reverse reaction converting NH4+ back to NH3 gas. Care should be taken to ensure that ammonia is injected deep enough to prevent gaseous N losses (four inches minimum, deeper in drier soils). Injection when the soil is moist is desirable so ammonia (either directly applied or created as a result of secondary reversion reactions) has ample opportunity to form NH4+. It is possible, and even advisable to keep N in the NH4+ form using nitrification inhibitors that suppress microbial conversion to NO3-, thereby restricting the mobility and potential for additional loss of N from the root zone from leaching or denitrification.
Anhydrous ammonia can be dissolved directly into water to form aqua ammonia (20 to 24-0-0), which is also an ammonia-based fertilizer but requires less specialized application equipment and fewer handling safety concerns compared to anhydrous, but is not in common use in Utah. Because the soil reactions are similar to anhydrous ammonia, the same measures used to minimize gaseous N losses in high pH soil, should be observed.
Solid Mineral N Fertilizers
Urea is formed by reacting ammonia and carbon dioxide at high temperature and pressure. The reaction forms a molten solid that gradually cools and is formed into prills of solid fertilizer having high N content. Urea is a form of organic N (e.g., it is naturally excreted in animal urine), but urea fertilizer is industrially manufactured in the quantities and purities required for agricultural use.
Plants can adsorb small amounts of urea directly from soils, but the bulk of N made available to plants from urea is the result of rapid, enzyme-mediated hydrolysis to NH3 gas, then reaction with soil water to NH4+. The enzyme, urease, which is found in all soils, is responsible for catalyzing this reaction, which happens within a few days of urea application to the soil.
It is important to plan urea applications just before tillage, an irrigation event, or rain storm so that the highly soluble urea can be incorporated at least two inches into the soil, thus preventing unnecessary gaseous N loss back to the atmosphere. This requires at least 0.5 inches of water application. Leaving urea on the soil surface for more than 48 hours can result in large or nearly complete N loss as NH3. The process of hydrolysis also forms large quantities of OH-, which can drive additional reversion of newly formed NH4+ back into NH3 gas in our alkaline soils that are already high in native levels of OH-. Urease inhibitors are often used in conjunction with urea to slow the enzyme-mediated transformation and help reduce gaseous losses of N.
Ammonium Nitrate (34-0-0)
Ammonium nitrate is formed by the reaction of NH3 gas and nitric acid (HNO3) to form a liquid ammonium nitrate solution that is then dried and granulated. The result is a highly stable, very low volatility, moderately high N content fertilizer that has approximately 50% each of the soil-adsorbable ammonium-N form, and the mobile nitrate-N form. This has long been the fertilizer of choice in agriculture since the end of World War II when the manufacture of reactive N compounds turned from making explosives to peacetime applications.
Ammonium nitrate is very soluble, provides an attractive mix of labile and mobile N, presents the lowest risk for N volatilization of all mineral N fertilizers, and provides tremendous flexibility for wide, broadcast-type application on the soil surface. It is typically the N contributor in multi-nutrient NPK formulations (like 10-10-10, 16-16-16, etc.) for a wide variety of agricultural and horticultural uses. Care still needs to be taken to minimize leaching and denitrification gaseous N losses, both of which are promoted under excess irrigation conditions, but there are many fewer specific concerns with the management of this fertilizer in Utah and Western U.S. soils.
Over recent years, ammonium nitrate has come under increased regulation because of its explosive properties when mixed with oxidizable fuels. Significant permitting is now required on the part of transporters, sellers, custom applicators, and bulk purchasers under ATF and DHS rules. The flexibility of use and relatively high N content of ammonium nitrate, however, continue to create high demand for this N fertilizer, so it is still widely available. That said, fewer manufacturing plants are making it in the U.S. resulting in urea becoming more and more attractive economically, despite its special soil management requirements.
Ammonium Sulfate (21-0-0-24S)
Ammonium sulfate is produced by reacting sulfuric acid with heated ammonia gas to produce crystals of the product that are then screened and sized for various applications. Because all N in ammonium sulfate is in the NH4+ form, many of the concerns already covered regarding ammonia volatilization in high pH soils are in play with this fertilizer.
One often hears this fertilizer referred to as a soil “acidifier” due to fact that the reaction by-product of nitrification (the microbial conversion of NH4+ to NO3-) is H+, or acid. However, in high pH soils of Utah and the Western U.S., correspondingly high lime, or calcium carbonate, content also is present (in some cases as high as 50% by weight). As acid is formed during nitrification, it is almost immediately consumed by reaction with lime, producing additional OH- in the end. This reaction actually increases soil pH, which we have already discussed as a promoter of the reversion of NH4+ to ammonia gas, presenting potential N loss from soils. If not incorporated by tillage, irrigation, or rainfall, gaseous N losses from ammonium sulfate can be quite high from alkaline, calcareous Utah soils.
Ammonium sulfate does offer additional value in terms of sulfur content for cases where sulfur may be deficient. Sulfur deficiency is more and more frequent in Utah soils that are sandier and under intense irrigated crop production (such as alfalfa production). This fertilizer may present an attractive alternative N source in such conditions, though it is less attractive economically than urea or ammonium nitrate as a sole N source.
Liquid Mineral N Fertilizers
The two most common liquid N fertilizer solutions available in Utah start with a base of dissolved Ammonium Nitrate. These are Urea-Ammonium Nitrate (or UAN32, 32-0-0) and Calcium Ammonium Nitrate (or CAN17, 17-0-0-8Ca). The advantages of these liquid formulations is the desirable mix of both ammonium and nitrate N forms, but without the oxidative reactivity and associated security and handling issues associated with the solid formulations. In addition, CAN17 contains eight to nine percent soluble calcium (Ca) useful for sodic soil reclamation.
The same issues with the incorporation of solid urea previously discussed, are also applicable to the UAN32 liquid formulation. Care must be taken to ensure that adequate irrigation or rainfall occurs soon after (within 48 hours) surface UAN32 application in order to incorporate the fertilizer below the surface and minimize potential NH3 gas losses.
Both UAN32 and CAN17 are saturated fertilizer salt solutions. Because the solubility of the mineral fertilizer salts varies with temperature, then at sufficiently low temperatures, there is potential for “salting out” or the precipitation of solid fertilizer crystals out of solution. In the case of UAN32, the salting out point is about 28 F (-2 C) and for CAN17 it is about 25 F (-4 C).
Grant Cardon, USU Extension Soils Specialist