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Chloride, Soil Solution Osmotic Potential, and Soil pH Effects on Nitrification¹

ABSTRACT

Laboratory incubation studies designed to measure chloride (Cl^-), soil solution osmotic potential, and soil pH effects on nitrification of added NH⁺₄ were conducted in order to help explain effects of Cl-containing fertilizers and liming on soil NH⁺₄ and NO^-₃ concentrations measured in field experiments. Samples of an Aquultic Argixerolls (pH 5.5, 6.0, 6.4, and 6.7) and a Xeric Haplohumults (pH 4.9, 5.5, and 6.2) were amended with 100 mg NH⁺₄-N kg^–1, maintained at –10-kPa matric potential and incubated at 25°C. Ammonium was added as either NH₄Cl or (NH₄)₂SO₄ salt solutions with four osmotic potential levels ranging from –96 to –692 and –90 to –669 kPa, respectively. Osmotic potential gradients were obtained by adding KCl to the NH₄Cl solutions and K₂SO₄ to the (NH₄)₂SO₄ solutions. At 0, 3, 9, 14, and 28 d, samples were extracted with 2 M KCl and analyzed for NH⁺₄, NO^-₃, and NO^-₂. At 15 d, soil solutions were recovered by centrifugation and analyzed for Al, Ca, Mg, K, Na, Mn, NH₄, NO₃, PO₄, SO₄, Cl, pH, and osmotic potential. Soil solution osmotic potential was lower in soils amended with Cl^- than with SO^2-₄ salt solutions. In both soils, nitrification was inhibited by Cl^- and by decreasing soil solution osmotic potential. Chloride inhibition of nitrification was greatest (30%) at low soil pH (4.9-5.5) but disappeared or decreased markedly in magnitude above pH 6.0 to 6.2. Because of soil solution osmotic potential and pH by Cl^- interaction effects, inhibition of nitrification would be expected to be greatest where Cl-containing fertilizers are applied in a band on moderately acid (pH 5.0-5.5) soils.

¹ Oregon Agric. Exp. Stn. Technical paper no. 7625. Contribution of the Dep. of Soil Science.

² Graduate Research Assistant, Associate Professor, and Professor, respectively, Dep. of Soil Science, Oregon State Univ., Corvallis, OR 97331.

Received for publication August 12, 1985.

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