Many different chemical and physical forms of nitrogen (N) fertilizers exist. Some of the more common fertilizer nitrogen sources are given in Table: "Description of fertilizer materials". The nitrogen in most farm-grade fertilizers is readily available. Some fertilizers, such as “turf-grade” fertilizers, release nitrogen very slowly. Plants can use nitrogen in one of two forms: ammonium nitrogen (NH4+) or nitrate nitrogen (NO3-).
|Fertilizer||Total N, %||Available phosphoric acid, %||Soluable potash, %||Equivalent acidity ¹||Salt index ²||Comments|
|Anhydrous ammonia—NH3||82||0||0||1.48||47||A high-pressure liquid that turns into a gas when released. Must be injected 6–8 inches deep on friable, moist soil. N loss by volatilization can occur if not properly injected, or if soil is too wet or too dry at application.|
|Urea—NH2-CO-NH2||46||0||0||0.84||75||A dry material in granular or prilled form, urea-N rapidly hydrolyzes to NH4+. Can be used for direct application, in mixed fertilizers, and in liquid nitrogen. N at application is present as urea-N. Within 1 day after application, about 66% of urea-N is hydrolyzed to ammonia-N; all within 1 week. When not incorporated, significant N loss by volatilization can occur until approximately 0.5 inch of rain has fallen. Not recommended for starter use. Broadcast (incorporated) or sidedress.|
|Ammonium nitrate—NH4NO3||33–34||0||0||0.63||105||A dry material in granular or prilled form, in which half of the N is as nitrate and half is as ammonium. Used for direct application and in the production of nitrogen solutions (see below). Broadcast or sidedress. Can be left on surface or incorporated into soil. Ammonium nitrate is a good fertilizer but it can be very difficult to get because it is used as an explosive.|
|Nitrogen solutions (UAN)—Urea+NH4NO3+Water||28–32 (mostly 30 in PA)
||0||0||0.54||74||A mixture of ammonium nitrate, urea, and water. Urea supplies about half of the N that may be subject to volatilization loss—read comments above for urea. The other half of N is supplied by ammonium nitrate—read comments above for ammonium nitrate. Once applied, nitrogen solution behaves exactly like dry urea and ammonium nitrate. To minimize N loss, incorporate into soil as soon as possible after application. Use caution when spraying, as leaf burn can occur. To minimize injury, do not spray on vegetation. For postemergence application, use a directed spray or dribble between the rows.|
|Ammonium sulfate—(NH4)2SO4||21||0||0||1.12||69||A dry crystalline material in which the nitrogen is all in the ammonium form. Produced by two methods—by-product and synthetic. Used for direct application and blended complete fertilizers. Broadcast or sidedress. Can be left on surface or incorporated into soil. Contains 24% sulfur. Good starter N source.|
|Diammonium phosphate (DAP)—(NH4)2HPO4||18||46||0||0.74||34||A dry granular or crystalline material. Common analysis is 18-46-0. Used for direct application and in blended fertilizers. Starter fertilizers containing DAP should be used with caution; be sure to band at least 2 inches to the side and 2 inches below seed.|
|Monoammonium phosphate (MAP)—NH4H2PO4||11||52||0||0.65||30||A dry granular material. Common analysis 11-52-0. Used for direct application and in blended fertilizers. Makes an excellent starter fertilizer, either alone or with a small amount of potash.|
|Ammonium polyphosphate||10||34||0||0.53||-||A liquid solution (10-34-0). The agronomic effectiveness of APP is similar to that of MAP. Sequesters some micronutrients and impurities in fluid fertilizers, keeping them in solution.|
|Triple superphosphate—Ca(H2PO4)2||0||46||0||0||10||Dry granular material. Used for direct application and in blended fertilizers.|
|Muriate of potash—KCl||0||0||60–62||0||116||Dry granular material. Used for direct application and in blended fertilizers.|
|Potassium sulfate—K2SO4||0||0||50||0||46||Dry crystalline material. A specialty fertilizer used for direct application and in blended fertilizers.|
|Potassium nitrate—KNO3||13||0||45||-0.26||74||Dry crystalline material. A specialty fertilizer used for direct application and in blended fertilizers.|
|Potassium hydroxide—KOH||0||0||70||-0.89||-||Crystalline material usually used in liquid fertilizers. Basic nature of this material allows production of neutral liquid fertilizers. Primarily used in liquid starter fertilizers.|
|Sulfate of potash magnesia— Sul-Po-Mag or K-Mag||0||0||22||-||-||Crystalline material made from langbeinite. Contains 22% sulfur and 11% magnesium.|
Ammonium nitrogen (NH4+) carries a positive charge and is adsorbed onto soil particles. In this chemical form, leaching of nitrogen does not occur; however, NH4+ is changed to the NO3- form by bacteria. This process occurs rapidly (beginning within 2 to 3 days) as the soil temperature climbs above 50°F. Complete conversion from NH4+ to NO3- occurs within about a month of application.
Nitrate nitrogen (NO3-) carries a negative charge and is not adsorbed onto soil particles; it is free to be leached from the soil. Nitrate nitrogen also can be lost to the atmosphere through denitrification when soils become water saturated. The nitrogen fertilizers listed in Tables "Description of fertilizer materials" and "Nitrogen conversion — approximate pounds of materials required per acre to supply rates of nitrogen recommended per acre" contain nitrogen in either or both of these forms. Products called nitrification inhibitors can inhibit the conversion of N from the nonmobile ammonium form to the very mobile nitrate form. This can reduce the loss of N. This effect is greatest under the following conditions: N applied long before crop uptake; N applied to very coarse-textured soils, especially when significant rain is expected before crop uptake; and N applied to poorly drained soils, again especially when significant rain is expected before crop uptake. Generally, even though the products may work perfectly, there is less benefit for N applied at planting or at sidedressing time.
A long-term effect of all ammonium-based nitrogen fertilizers is to lower soil pH. Anhydrous ammonia, urea, diammonium phosphate, and nitrogen solutions, when first applied, greatly but temporarily increase soil pH in the zone of application. Ammonia is released and can “burn” germinating seeds or seedling roots in the area of fertilizer placement. In the eventual conversion of NH4+ to NO3-, however, an acid residue is formed. The residual acidity from the common N sources is given in Table: "Description of fertilizer materials".
Acidity is a particular problem under no-till and minimum-till conditions, because the nitrogen is concentrated on the soil surface. An “acid roof” can form; the pH in the upper 1 to 2 inches may be 0.5 to 1.0 pH units lower than at deeper depths. This greatly decreases the efficiency of triazine herbicides and can negatively impact root growth. Therefore, the upper 2 inches of soil should be tested for pH regularly. For details on taking this sample, see the discussion on sampling no-till fields earlier in this chapter.
If the normal soil sample does not indicate a need for limestone, check the surface pH. If the surface pH is below 6.2, apply 2,000 pounds of calcium carbonate equivalent per acre. This should be sufficient to take care of the acidity caused by nitrogen fertilizer. As a rule of thumb, 6 pounds of limestone are required for each pound of nitrogen applied as ammonium sulfate, and 3 pounds of limestone are required for each pound of nitrogen applied as anhydrous ammonia, urea, ammonium nitrate, or nitrogen solution. The effect of manure nitrogen on soil pH is very variable. The ammonium nitrogen in all manure can lower soil pH. However, some manures, particularly poultry layer manure, may contain significant calcium carbonate which can actually increase pH over time.
The nitrogen in urea is completely water-soluble. Upon application, urea nitrogen changes rapidly to NH4-N. Urea nitrogen therefore is readily available to plants on application to the soil. Urea presents another problem, in that when it is surface-applied, significant quantities of nitrogen as ammonia may be lost through volatilization. These losses happen very rapidly, with most occurring within the first day or two following application and can account for over one-third of the urea N being lost within a week after application. Losses are accelerated by warm moist soils, high pH, and surface organic matter. Losses are higher on low cation exchange capacity (CEC) or sandy soils than on soils with a high CEC, a heavy clay content, or a high organic matter content. Thus, urea or nitrogen solutions (which are approximately 50 percent urea) should be incorporated into the soil by mechanical mixing or by water movement. Light tillage or one-half inch of rain usually is adequate. Non-urea N sources, such as ammonium nitrate and ammonium sulfate, are not subject to volatilization under Pennsylvania conditions.
Research also has shown that volatilization losses from nitrogen solution can be reduced significantly by dribbling the solution in a band on the surface, rather than spraying it over the entire soil surface. This can be accomplished by using drop tubes on a conventional sprayer. Urease inhibitors can also be used to effectively reduce volatilization from surface application. These are only effective on urea-containing fertilizers and will only provide a benefit if the fertilizer is not incorporated immediately by tillage or rainfall.
Urea and urea-blended fertilizers are not recommended as starter fertilizers because of possible ammonia toxicity to germinating seeds, which results in reduced plant stand.
When the different nitrogen fertilizer materials are applied properly, they give the same results per unit of nitrogen applied. For equivalent conversions of the different materials, see Table: "Nitrogen conversion — approximate pounds of materials required per acre to supply rates of nitrogen recommended per acre".
To compare nitrogen costs for these materials, convert the prices to a per-unit-of-actual-nitrogen basis by dividing the price per ton of fertilizer by the pounds of actual nitrogen in a ton of each material. This figure is given at the bottom of the table. Example:
Compare urea at $500/T with ammonium sulfate at $300/T.
Urea: $500/920 lb N = $0.54/lb N
Ammonium sulfate: $300/420 lb N = $0.71/lb N
In this example, urea is the best buy. Other factors such as suitability of the material, convenience, method of application, and other fertilizer properties also should be considered.
Actual N lb/T = 2,000 × N analysis (as a decimal)
Example: Calculation of lb N/T UAN of 30% liquid nitrogen (30-0-0)
Actual N lb/T = 2,000 × 0.30 = 600 lb N/T of UAN
Also, note that the analysis of fertilizer is always on a weight basis. Therefore when applying a liquid fertilizer like UAN in gallons per acre, this must be converted to a weight to determine the amount of actual N applied.
Example: Actual N applied in a 20 gal of UAN/A application.
UAN weighs 10.85 lbs/gal, therefore 20 gal/A = 217 lbs UAN/A
Since UAN is 30% N by weight, the actual N applied would be 217 lbs UAN/A x 0.30 = 65 lbs N/A
|For a nitrogen recommendation of||Material to supply rate of N per acre (pounds per acre)
|Anhydrous ammonia (82% N)||Ammonium nitrate (33.5% N)||Ammonium sulfate (21% N)||Liquid nitrogen (UAN)(30% N)||Urea (46% N)|
|Actual N (lbs/T)||1640||670||420||600||920|
Reference: The Agronomy Guide - Section 2: Soil Fertility Management
Table 1.2-11 and Table 1.2-12 from the Agronomy Guide