Several types of N fertilizer sources are available. The best one to use in a particular situation will depend on factors such as fertilizer material availability and price, equipment accessibility, time of year, and tillage practices.

Urea is one of the most commonly available N sources. It contains about 46% actual N, that is, 46 kg of N for each 100 kg of fertilizer material. Urea has good handling and storage properties but may absorb moisture; thus, high urea feed supplements increasingly are stored in plastic-lined rather than paper bags. Most blended fertilizers use urea as the source of N.

Urea presents a unique problem, that is, the potential to lose a large percentage of its N content to the atmosphere as ammonia gas (NH3) when used as a fertilizer. This occurs when urea is applied to the soil surface and not incorporated by tillage or rainfall. This loss of ammonia is called volatilization; it is increased by high temperatures, high soil pH, and high rates of application. The amount of N lost through volatilization can increase fourfold as soil temperature increases from 7 to 32°C (45-90°F) (Ernst and Massey, 1960). Urea is much less susceptible to volatilization as a N source for spring fertilization, when temperatures are cool, than in fall, when daytime temperatures can be high.

An alkaline soil pH or the presence of agricultural limestone on the soil surface can increase N loss from surface-applied urea. An 11-fold increase in N loss from urea has been determined as the surface soil pH increased from 5.0 to 7.5 (Ernst and Massey, 1960). This can be avoided by applying urea before a maintenance application of limestone whenever practical and, where the soil pH is not already low (<5.6), allowing rainfall to dissolve the urea-N and allow infiltration into the soil before lime is applied.

Urea fertilizer initially raises the soil pH when applied (Overrein and Moe, 1967). The higher the rate of urea application, the higher the soil pH change in the zone immediately surrounding the urea fertilizer. A surface application of 224 kg N/ha (~500 kg/ha of urea) has been shown to increase soil pH temporarily as much as two pH units within the zone of application. The pH returns to normal levels within 30 d; however, the short-term increase in pH will increase N volatilization.

Ammonium nitrate is the primary non-urea source used in hay and pasture systems. It contains about 34 kg of N per 100 kg of fertilizer material (34% N). The main advantage to using ammonium nitrate is that it has little potential for N loss by volatilization. That is the reason this material is preferred for supplying N when stockpiling tall fescue in the fall (see Stockpiling section in this chapter).

Like urea, ammonium nitrate tends to absorb moisture. Because of this problem, many farm supply stores do not stock it in bulk, especially during the humid summer months; instead they store it in sealed plastic or plastic-lined bags. These bagged materials usually are available, but often are more costly and time-consuming to handle and apply than bulk materials. There also may be some restrictions for purchases of sizeable amounts, since ammonium nitrate can be used as an explosive.

Blended fertilizers, such as 19-19-19 or 10-10-10, are available commonly and are used quite extensively. The three numbers indicate the percentage of N, phosphate (P2O5), and potash (K2O), respectively, in each 100 kg of the bulk fertilizer. The N source for these fertilizers usually is urea, so the blended fertilizers have the same volatilization problems as exists when urea is used alone. Blended fertilizers are satisfactory for soil surface application in spring, but they can result in large N losses in late summer and fall if application is not followed by rainfall or irrigation within a couple of days.


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