Urea-Ammonium Nitrate Fertilizer

Liquid fertilizer solutions or fluid fertilizers are popular in many areas because they are safe to handle, convenient to mix with other nutrients and chemicals, and are easily applied. A solution of urea [CO(NH2)2] and ammonium nitrate [NH4NO3] containing between 28 and 32% N is the most popular N fluid fertilizer.

Production

Liquid urea-ammonium nitrate (UAN) 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 N comes from the urea solution and half from the ammonium nitrate solution. UAN is made in batches in some facilities or in a continual process in others. No emissions or waste products occur during mixing. 

Since UAN is a concentrated N solution, its solubility increases as the temperature rises. To prevent the N components from precipitating as crystals, UAN solutions are made more dilute in regions with cold winter temperatures. Therefore, the N concentration in commercial UAN fertilizers will vary from 28% N to 32% N depending on geography. A corrosion inhibitor is usually added to the final solution to protect the steel in storage tanks.

UAN fluid fertilizer

 Chemical Properties

Properties:                              28% N   30% N   32% N

Composition (% by weight)

Ammonium Nitrate                40            42            44

Urea                                        30            33            35 

Water                                      30             25           20

Salt-out temp (ºC)                 -18           -10           -2

(Crystal precipitation temperature)

Solution pH                        - - - - approximately 7 - - - -

 Agricultural Use

Solutions of UAN are widely used as a source of N for plant nutrition. The NO₃^- portion (25% of the total N) is immediately available for plant uptake. The NH₄⁺ fraction (25% of the total N) can also be assimilated directly by most plants, but is rapidly oxidized by soil bacteria to form NO₃^-. The remaining urea portion (50% of the total N) is hydrolyzed by soil enzymes to form NH₄⁺, which is subsequently transformed to NO₃^- in most soil conditions. 

Fluid fertilizer application

Solutions of UAN are extremely versatile as a source of plant nutrition. Due to its chemical properties, UAN is compatible with many other nutrients and agricultural chemicals, and is frequently mixed with solutions containing P, K, and other essen­tial plant nutrients. Fluid fertilizers can be blended to precisely meet the specific needs of a soil or crop. 

     

UAN solutions are commonly injected into the soil beneath the surface, sprayed onto the soil surface, dribbled as a band onto the surface, added to irrigation water, or sprayed onto plant leaves as a source of foliar nutrition. However, UAN may dam­age foliage if sprayed directly on some plants, so dilution with water may be needed.

UAN applied with fertigation

Management Practices

UAN makes an excellent source of N nutrition for plants. However, since half of the total N is present as urea, extra manage­ment may be required to avoid volatile losses. When UAN remains on the surface of the soil for extended periods (a few days), soil enzymes will convert the urea to NH4+, a portion of which can be lost as ammonia gas. Therefore, UAN should not remain on the soil surface for more than a few days in order to avoid significant loss. Inhibitors that slow these N transformations are sometimes added. When UAN is first applied to soil, the urea and the NO3- molecules will move freely with water in the soil. The NH4+ will be retained in the soil where it first contacts cation exchange sites on clay or organic matter. Within 2 to 10 days, most of the urea will be converted to NH4+ and no longer be mobile. The originally added NH4+ plus the NH4+ coming from urea will eventually be converted to NO3- by soil microorganisms.