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Volatilisation differences between N sources

                                                                                                                                                        

Volatilisation of applied nitrogen is primarily in the form of ammonia (NH3), although losses in the form of atmospheric nitrogen (N2 en N2O) may also occur when soils are waterlogged.

Ammonia is released from ammonium (NH4+) containing and forming fertilisers when there is not enough soil water in which the ammonia could dissolve. This will occur when fertilisers are applied and left to remain on or close to the soil surface. Atmospheric nitrogen is formed from nitrate nitrogen (NO3-) when the topsoil is waterlogged and thus deprived of oxygen for long periods. Water scarcity rather than long periods of water logging are far more common in South Africa. This article therefore focusses on ammonia losses from applied fertilisers together with factors affecting this process such as soil pH and temperature.

The efficacies of urease inhibitors which delay the conversion of urea to ammonia together with other possible solutions for the problem of ammonia volatilisation are also discussed.


Soil pH strongly affects Ammonia volatilisation losses. Ammonia losses from urea were increased by 18% over five soils when the pH was increased from 6.5 to 9.1 (Figure 1). Most losses occurred from urea, followed by DAP, Ammonium sulphate, MAP and LAN (Figure 1). The difference in ammonia volatilisation between urea and LAN was 15% at a pH of 9.1 (Figure 1).
The conversion of urea to ammonium and DAP to ammonium are alkaline reactions. This explains why these products would lose more N in the form of ammonia than other products, forming or releasing similar quantities of ammonium with no increase in pH. High rates of application of urea or DAP which would result in high concentrations on the soil surface would increase soil pH more and consequently more ammonia will be formed and lost.

Figure 1. Effect of soil pH, averaged over five soils, on ammonia volatilisation of different N-sources applied on the surface at a rate of 120 kg N/ha under controlled conditions. (Redrawn from Du Preez & Burger 1986)

Nitrogen loss in the form of ammonia could be much higher than indicated in Figure 1. Du Preez & Burger (1986) showed ammonia losses of 55% which resulted from urea applications at a rate of 240 kg N/ha, on a soil containing 50% clay and which had an original pH (H2O) of 7.5. Botha & Pretorius (1988) showed ammonia losses of as much as 61% following urea applications at a rate of 83 kg N/ha on a soil with a clay content of 9.5% and a pH (H2O) of 7.9 after urea applications. Fenn & Miyamoto (1981) showed ammonia losses of 66% following urea surface applications on a soil with a pH (H2O) of 7.8.


Ammonia losses are strongly affected by temperature. As temperatures rise from spring to mid-summer ammonia losses increased tremendously when using urea but also significantly with UAN (Figure 2). Ammonia losses from LAN did however remain very low with increasing temperatures (Figure 2). Hoeft, et.al. (2000) stated that the potential for urease inhibitors to be effective would be best above 10° C.

Figure 2. Effect of seasonal change in temperature on ammonia volatilisation from different N-sources applied on the soil surface in Argentina under field conditions. (Fantanetto, 1995).


Urease inhibitors such as Agrotain, SKW Piesteritz and Hanfeng Evergreen delay the conversion of urea to ammonia and therefore also the release of ammonia. The use of Agrotain resulted in average reductions of ammonia volatilisation losses of 70% (25 to 100%) for urea and 44% (15 to 71%) for UAN (Chambers & Dampney, 2009). Volatilisation from ammonium nitrate is however lower than volatilisation from urea + Agrotain, which partly explains higher yields for ammonium nitrate (Chambers & Dampney, 2009). Schwab & Murdock (2010) showed that when Agrotain was added to urea, maize yield was increased significantly (from 9.4 ton/ha to 10.7 ton/ha), but the ammonium nitrate treatment out-yielded both these treatments (11.6 ton/ha). Urease inhibitors delay the formation of ammonium and therefore also reduce ammonia volatilisation but in the process, it will also delay the formation of nitrate. Since only ammonium and nitrate are effectively utilized by crops the release of N in available forms is also delayed by urease inhibitors. When urea is to be applied together with urease inhibitors timely applications is therefore imperative. Since urea is 100% leachable (Adriaanse, 2012) and as such not utilisable by crops the use of urease inhibitors will enhance the leaching of urea. Urease inhibitors were developed with the objective to reduce ammonia volatilisation from the soil surface but serves no purpose when soil incorporated or washed into the soil together with urea (Hoeft et.al., 2000).


Conclusions and Recommendations.
1. Nitrogen sources which are prone to volatilisation should as far as possible be soil incorporated to bring it in contact with soil water.
2. Urea containing fertilizers could also be washed into the soil under irrigation or surface applied just before the rains under dry-land conditions. DAP, MAP, Ammonium Sulphate and LAN, cannot be washed into the soil in this manner.
3. Surface applications of ammonium containing or forming N sources, immediately following lime applications will result in ammonia volatilisation and should therefore be avoided.                                                                                                                                          4. Urea and DAP surface applications during the warmest times of days and seasons will result in high ammonia volatilisation losses and should therefore be avoided. Surface applications of urea plus urease inhibitors could however be considered at high temperatures. Ammonia losses from surface applications of LAN at high temperatures is expected to be very low and therefore this remains the best option.
5. Multiple urea applications at relatively low rates will result in less volatilisation than single urea applications at relatively high rates.
6. Rather use LAN than urea when surface applications are unavoidable, cannot be soil incorporated or washed in. Urea that was treated with urease inhibitors could also be considered but bear in mind that volatilisation from LAN will still be much less. LAN will also result in quicker responses to N deficiencies and most likely also higher yields than urea or urea combined with urease inhibitors.
7. The cost of aerial applications will probably be much higher for LAN compared to urea plus urease inhibitors not justifying the higher expected yield for LAN. Under such conditions timely aerial applications of urea plus urease inhibitors is recommended to allow for the expected delayed response.

Dr Erik Adriaanse, Manager Product Development and Technical Support Sasol Chemicals, Fertiliser Division


References
ADRIAANSE, F. G., 2012. Logingsverskille by stikstofbronne. S.A.Graan/Grain, 11/12.
BOTHA, A.D.P. & PRETORIUS, D.C. 1988. Ammonia volatilization and denitrification losses from commercial fertilizers applied to soil samples. S. Afr. J. Plant Soil, 1988, 5(2), 89-91.
CHAMBERS, B. & DAMPNEY, P, 2009. Nitrogen efficiency and ammonia emissions from urea-based and ammonium nitrate fertilisers. International Fertiliser Society Conference, Cambridge, 10th December 2009. IFA, ISBN 978-0-85310-294-6 (ISSN 1466-1314)
DU PREEZ C C, DU T BURGER, R. 1986. A proposed mechanism for the volatilization of ammonia from fertilized neutral to alkaline soils. S. Afr. J. Plant Soil, 1986, 3(1), 31-34.

FANTANETTO H. 1995., Ciencia del Suelo, INTA, Argentina.
FENN L. B. & MIYAMOTO S., 1981. Ammonia loss and associated reactions of urea in calcareous soils. Soil Sci. Soc. Am J. 45:537-540.
HOEFT. R.G., NAFZIGER. E.D., JOHNSON. R.R.& ALDRICH. S.R., 2000. Modern Corn and Soybeen Production pp 139. MCSP Publications, 1520, Yorkshire Dr, Champaign, IL, USA.
SCHWAB, G.J. AND MURDOCK, L.W. 2010. Enhanced-Efficiency Nitrogen Fertilizer for Corn and Wheat Production. IFA International Conference on Enhanced-Efficiency Fertilizers, Miami, International Fertilizer Industry Association, Paris, France.
For more information contact Sasol Nitro Contact Centre at 087 350 2222 or This email address is being protected from spambots. You need JavaScript enabled to view it.
NB. Consult a qualified agronomist for locality specific applications. The results referred to in this article were obtained under specific conditions and are therefore not generally applicable under all conditions.


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