Urea v AN

Making the most of Urea

Summary

  • Current fertiliser prices indicate that cost savings can be made by using urea over ammonium nitrate (AN), however, urea must be used in optimum conditions to make the most of this price differential
  • Urea is most effective in spring in low temperature conditions when rainfall is present
  • At current fertiliser prices (January 2014) a growth response to urea relative to AN of less than 0.8 is unlikely to be cost effective.

Urea is often viewed as a cheaper source of nitrogen (N), indeed, average January 2014 prices for urea and ammonium nitrate (AN) show a 24% difference in price between 1kg N as urea and 1kg AN (Table 1).

 

Table 1: Relative prices of urea and ammonium nitrate fertiliser at January 2014

Urea 1

Source: ©FARM BRIEF

*Prices are spot deals for payment within 28 days after month of delivery to farm for orders of 20 tonne plus full loads. Prices also assume 500kg bag delivery.

However, to take advantage of this price differential, it is important to ensure N losses to the atmosphere (through ammonia volatilisation) are minimised when spreading urea. The two main factors which govern N losses following fertiliser application are: the amount of rainfall after application and the average air temperature in the three days following application.

Urea is most effective in low temperatures and when rainfall is present in the three days following application (as a result, spring time applications of urea can be particularly effective).

Rainfall will ensure the urea will be washed into soil where nitrifying bacteria convert the urea to ammonium-N and the ammonium-N to nitrate, which is then available for uptake by the plant. In contrast, applying AN in similar conditions can result in higher levels of N loss through leaching.

However, at high temperatures and at low levels of rainfall, high gaseous losses of ammonia from urea will occur, with the result that grass growth response to urea will be below that of AN.

Table 2 shows the growth response to urea N, relative to AN, depending on the rainfall and temperature following application. In optimum conditions, the grass growth response to urea can significantly outperform AN (+33%; see Table 2), however, if used in sub-optimal conditions grass growth response can be reduced by as much as 32%.

Table 2. Relative response (%) of grass growth to application of urea N compared to AN depending on rainfall and temperature conditions (values above 100% indicates a grass growth advantage in urea N; values below 100% indicate an advantage to AN).

Can v Urea

Source: Grassland soils and fertilisers: digging out the answer.

But at current fertiliser prices (January 2014), what level of relative grass growth response is required to make is cost-effective to use urea over AN? Figure 1, below shows the average fertiliser cost per tonne of grass DM produced at different fertiliser application rates.

The green area relates to the fertiliser cost per tonne of grass DM produced at different application rates of N as ammonium nitrate. The graph shows that if the relative grass growth response of urea compared to AN is 0.8 (80%; purple line) or less, it is more cost effective to sow ammonium nitrate than urea. In contrast, at relative grass growth responses greater than 80% it is more cost effective to sow urea. 

Urea graph

Figure 1: The fertiliser cost associated with growing 1 tonne grass DM at varying application rates and at different grass growth responses of urea relative to ammonium nitrate. At a relative grass growth response >80% of urea compared with ammonium nitrate (based on January 2014 fertiliser prices) it is more cost effective to spread urea than ammonium nitrate.

The relative grass growth response of urea compared to ammonium nitrate, however, can be altered by urease inhibitors. These slow down the conversion of urea to ammonia resulting in lower gaseous losses of ammonia. These may improve grass growth response to urea in high temperature, low rainfall conditions.

Full information on this subject, is contained in the BGS booklet Grassland soil and fertilisers: digging out the answers. The booklet is free to dairy farmers and available to download from the BGS website.

For up-to-date information visit the DairyCo Market Information page on fertiliser prices on fertiliser prices