Nitrification in high chloride zones near fertilizer bands in a calcareous soil

The effect of high chloride (Cl) on nitrification of ammonium-nitrogen (NH4-N) in zones surrounding applied fertilizer and on nitrogen (N) uptake by sorghum [Sorghum bicolor L. (Moench)] was examined in a calcareous silt loam soil. Fertilizer N was applied at a rate of 120 mg N/kg as ammonium sulfate [(NH4)2SO4] or ammonium chloride (NH4Cl) and supplemented by calcium chloride (CaCl2) to provide a final Cl concentration of 350 or 700 mg Cl/kg to simulate a concentrated point placement. Soil around the point of fertilizer placement and shoot dry matter (if present) were sampled and analyzed at 1, 7, 14, 24, 34, and 44 days after application. Soil pH within 2.0 cm from the fertilizer placement point for the Cl treatments was below the bulk soil pH during the entire study due to the effect of adding CaCl2. Beyond the fertilizer placement zone, soil pH for the ammonium sulfate treatment without Cl increased initially to near pH 8.5 then declined steadily with time as acidification from nitrification increased. Soil Cl diffusion from the fertilizer placement point occurred rapidly and was essentially complete within 14 days. The rate of NH4-N oxidation or nitrate-nitrogen (NO3-N) production was significantly affected by fertilizer applications containing Cl in the early stages of the experiment. In addition, shoot dry matter and N uptake was lower in the NH4Cl treatments compared to the (NH4)2SO4 treatment probably because of the inability of the sorghum roots to grow into the concentrated fertilizer zone with high electrical conductivity and Cl where some fertilizer N resided. These effects were probably related to high osmotic pressure rather than a specific Cl-ion effect. Volatilization of ammonia (NH3) which may occur in the reaction of fertilizer (NH4)2SO4 with soil calcium carbonate (CaCO3) was low.