Nitrogen losses from urea, UAN and LAN when applied to four different soils

Nitrogen transformations and losses from three nitrogenous fertilizers, urea, urea ammonium nitrate (UAN) and limestone ammonium nitrate (LAN), when applied to four South African soils, were compared in a laboratory study. The highest losses on all these soils were experienced when urea was applied, followed by UAN solution and then LAN. It also appears that a relationship exists between N volatilization and the change in pH of the soil. The lower the buffer capacity or clay content of the soil, the greater the pH change and the nitrogen losses. In the case of LAN, the pH did not rise to above 7 in any of the four soils and no ammonia volatilization was detected. Explanations for the nitrogen losses are proposed and supported with chemical reactions. High soil surface temperatures may result in accelerated mass flow and ammonia volatilization. On the other hand, it was shown that nitrites or nitrous acid promoted denitrification, especially when LAN was applied.