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Dr Erik Adriaanse

Afrikaans Version: Verskille in vervlugtiging tussen Stikstofbronne

Volatilization of applied nitrogen (N) is primarily in the form of ammonia (NH3), although losses in the form of atmospheric N (N2 and N2O) may also occur when soils are waterlogged. Ammonia is released from ammonium (NH4+) containing and forming fertilisers when there is insufficient soil water present in which the ammonia can dissolve. This will also occur when fertilisers are applied and left remaining on or close to the soil surface. Atmospheric nitrogen is formed from nitrate nitrogen (NO3) when the topsoil is waterlogged and 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 combined 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 volatilization are also discussed.

Soil pH significantly affects Ammonia volatilization 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 volatilization between urea and LAN was 15% at a pH of 9.1 (Figure 1).

The conversion of urea to ammonium and also DAP to ammonium are alkaline reactions. This explains why these products will lose more N in the form of ammonia than other products, forming or releasing similar quantities of ammonium with no increase in pH. High application rates of urea or DAP which would result in high concentrations on the soil surface will 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 volatilization 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 significantly affected by temperature. As temperatures increased from spring to mid-summer ammonia losses increased tremendously when using urea but also significantly with UAN (Figure 2). Ammonia losses from LAN however remained very low with increasing temperatures (Figure 2). Hoeft, (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 volatilization 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 volatilization losses of 70% (25 to 100%) for urea and 44% (15 to 71%) for UAN (Chambers & Dampney, 2009). Volatilization from ammonium nitrate is however lower than volatilization from urea + Agrotain, which partly explains the higher yields obtained from using 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 volatilization but in the process, 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 volatilization from the soil surface but serves no purpose when soil incorporated or washed into the soil together with urea (Hoeft, 2000).

Conclusions and Recommendations.

  1. Nitrogen sources which are prone to volatilization should as far as possible be soil incorporated to bring them 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.
  3. Surface applications of ammonium containing or forming N sources, immediately following lime applications will result in ammonia volatilization and should therefore be avoided.
  4. Urea and DAP surface applications during the warmest times of day and seasons will result in higher ammonia volatilization 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 volatilization than single urea applications at relatively higher rates.
  6. Rather use LAN than urea when surface applications are unavoidable and cannot be soil incorporated or washed in. Urea that is treated with urease inhibitors can also be considered but bear in mind that volatilization from LAN will still be much lower. The use of 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.

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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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.
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