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Volatilization differences between N sources

By FERTILIZER PRODUCTS, Nitrogen, Nitrogen Products, PLANT & SOIL NUTRITION No Comments

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. 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. 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…

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