general information and practices in urea fertilizer application

The urea fertilizer, also popularly called forty six zero zero (46-0-0), is a simple or straight (single-element) fertilizer that supplies the major essential element nitrogen in ammonic form (NH₄⁺). The positively charged ammonium ion (NH₄⁺) is nonvolatile and is one of the two forms of nitrogen that can be absorbed by plants, the other being nitrate (NO₃^-).

Urea is the richest source of nitrogen among the common dry fertilizers. Anhydrous ammonia (NH₃), which contains 82% nitrogen, is a pressurized liquid that transforms into gas when released (liquified gas).  According to Jones et al. (2012), urea  ranks as the most preferred dry nitrogenous fertilizer in the United States due to advantages such as high nutrient analysis, easy handling, and reasonable price per unit of nitrogen.

Urea is likewise the most widely used fertilizer worlwide. In 1961 the world consumption of urea was only 2 million tons as against 12 million tons for ammonium sulfate (21-0-0). But the former has continued to grow and, as of 1998, has surpassed the latter by a large margin (86.13 against 12.5 million tons) (Soh 2006).

According to FAO (2011),  there would be a steady rise in the world demand for the fertilizer nutrient nitrogen from 105.3 million tons in 2011 to 112.9 million tons in 2015 at the rate of 1.7% per annum. This forecast is consistent with the predicted increase in the production of major food crops which is necessitated by a continuing population surge. Consequently,  58 more urea plants will become operational, seventeen of which are to be located in China.

Urea Fertilizer: 46-0-0

The alternative name 46-0-0 of urea fertilizer stands for its NPK content (actually N, P₂O₅ and K₂0 or nitrogen, phosphate and potash, respectively). It means that it contains 46% nitrogen (N), zero phosphorus, and zero potassium. Thus 100 kg of granular urea supplies 46 kg N with the remainder consisting of carriers or fillers.

The 46% N as shown on packaging is a nutritional content of the fertilizer that is warranted by the manufacturer. However, it is not exactly the same as the computed value as shown below:

Chemical formula of urea = CO(NH₂)₂ = CH₄N₂O

• C₁ = 12.01 x 1 = 12.01
•  H₄ = 1.01 x 4 = 4.04
•  N₂ = 14.01 x 2 = 28.02
•  O₁ = 16.0 x 1 = 16.0
•  Total = 12.01 + 4.04 + 28.02 + 16.0 = 60.07
•  Percent nitrogen = (28.02/60.07) x 100 = 46.6%

Applying Urea Fertilizer

Urea can be applied in the soil in the form of solid granules or prills, or pellets. The most dominant formulation is the white, crystalline granule. The prills used to be the primary form of urea, but they have been surpassed by granules which are larger, harder, and more stable under high humidity. Urea can also be dissolved in water and used as a soil drench or otherwise distributed with irrigation water or applied as foliar spray.

When urea fertilizer is applied to the soil, it combines with water (hydrolysis) to form ammonium carbonate [(NH₄)₂CO₃] through the catalytic action of urease. The enzyme urease is present in the soil, resulting from the decomposition of organic matter by microorganisms.

Ammonium carbonate is unstable. It decomposes into gaseous ammonia (NH3), carbon dioxide, and water. When incorporated to the soil, NH3 is converted to ammonium (NH₄⁺) with hydrogen ion (H⁺) coming from soil solution or from soil particles. The positively charged ammonium ions are then fixed into the negatively charged soil particles where they remain until absorbed by plant through the roots or used by bacteria as source of energy and converted to nitrate (NO₃^-) in the process of nitrification (Thorup 1984).

A summary reaction for the hydrolysis of urea [CO(NH₂)₂] leading to the formation of ammonium ion (NH₄⁺), a form of N the plants can absorb, is given below (modified from Jones et al. 2012):

CO(NH₂)₂ + H⁺ + 2H₂O + urease ------> 2NH₄⁺ + HCO₃^-

Here are some more information when applying urea fertilizer:

1. Apply urea by soil incorporation. As a general rule, urea should not be applied on the soil surface  or top dressed on sods or crop residues without immediate incorporation. When applied on the soil surface, NH₃, a product of urea hydrolysis, will escape into the air being a gas. This is called ammonia volatilization.

Volatilization loss from nitrogenous fertilizers ammonium nitrate (NH4NO3) and ammonium sulfate (or ammoniun sulphate, (NH₄) ₂SO₄) can be negligible below soil pH of about 7.2, but large losses from urea can occur in both acidic and basic soils. Within 24 hours after surface application, the extent of loss can account for 50 percent.

A substantial loss of nitrogen from urea can be reduced or eliminated by soil incorporation. This can be done by tillage, such as plowing under or by disking, or by irrigation. Being highly soluble to water, the urea fertilizer will be carried into the soil and there behave just like other nitrogen fertilizers. Rainfall can substitute for irrigation (Thorup 1984).

2. Apply singly or mix with the right fertilizers. The urea fertilizer can be applied alone or mixed with some other selected fertilizer materials. However, some blends should be immediately applied. Moreover,  it should not be mixed with some fertilizers because a reaction will occur that will render one of the nutrients useless. Mixing of strongly basic materials with urea will result to loss of nitrogen as ammonia. Here’s a general guide from Xuan and Ross (1976):

• Fertilizers that can be mixed with urea: calcium cyanamide, sulphate of potash, and sulphate of potash magnesia.

• Fertilizers that can be mixed  with urea but not stored in excess of 2-3 days: Chilean nitrate, sulphate of ammonia, nitrogen magnesia, diammonium phosphate, basic slag, rock phosphate, and muriate of potash.

• Fertilizers that cannot be mixed with urea: calcium nitrate, calcium ammonium nitrate or limestone ammonium nitrate, ammonium sulphate nitrate, nitropotash or potash ammonium nitrate, superphosphate, and triple superphosphate.

REFERENCES

1. CAREY FA. 1992. Organic Chemistry. 2nd ed. New York, NY: McGraw-Hill, Inc. p.
2. [FAO] FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. 2011. Current world fertilizer trends and outlook to 2015. Retrieved Jan. 5, 2013 from  ftp://ftp.fao.org/ag/agp/docs/cwfto15.pdf.
3. Jones CA, Koenig RT, Jason WE,   Brown BD, and Jackson GD. 2012. Management of urea fertilizer to minimize volatilization. Retrieved Jan. 6, 2013 from http://www.extension.uidaho.edu/swidaho/nutrient%20management/pnwureamanagement.pdf.
4. SOH KG. 2006. A review of the global fertilizer use by product. Retrieved Jan. 5, 2013 from http://espere.mpch-mainz.mpg.de/documents/ACCENT/Edition08/texts%20material/english/fertilizer_usage_2001.pdf.
5. TANDON HLS. 2012. A short history of fertilizers. Retrieved Jan. 5, 2013 from http://www.tandontech.net/fertilisers.html.
6. THORUP RM (ed.). 1984. Ortho Agronomy Handbook: A Practical Guide to Soil Fertility and Fertilizer Use. San Francisco, CA: Chevron Chemical Company. 454 p.
7. XUAN V, ROSS VE. 1976. Training Manual for Rice Production. The International Rice Research Institute. p. 48

(Ben G. Bareja January 2013)

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