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Calcium Effects on Quantity-Intensity Relationships and Plant Availability of Ammonium

Dep. of Plants, Soils, and Biometeorology, Utah State Univ., Logan, UT 84322-4820

W. L. Pan^*

Dep. of Crop and Soil Sciences, Washington State Univ., Pullman, WA 99164-6420

*Corresponding author (wlpan{at}wsu.edu).

ABSTRACT

Competition among cations for exchange sites in soil may influence NH₄ concentrations in soil solution adn consequently affect NH₄ availability to plants. Laboratory, greenhouse, and field experiments were conducted to determine whether Ca additions enhance NH₄ availability, thereby increasing the potential for NH₄ absorption by plants. A Shano silt loam soil (coarse-silty, mixed, mesic Andic Mollic Camborthid) was equilibrated with NH₄ solutions ranging in concentration from 0 to 70 mg N L^-1 with either distilled water or a 400 mg Ca L^-1 matrix. The Ca matrix resulted in 20 mg (167%) more NH₄-N L^-1 in solution than the distilled water matrix at an exchangeable NH₄ concentration of 100 mg N kg^-1 soil. Calcium also reduced the potential maximum quantity of NH₄ adsorbed by the soil as well as the soil buffer capacity for NH₄. Combining Ca with NH₄ fertillzers (1:2 Ca/N molar ratio) increased NH₄ concentrations in the soil solution in subsequent greenhouse and field experiments. The influence of Ca on soil solution NH₄ concentration is attributed to preferential adsorption of Ca and the displacement of NH₄ from soil exchange sites. Mechanistic model predictions of NH₄ uptake were 22% higher for rice (Oryza sativa L.) and 7.5% higher for corn (Zea mays L.) when Ca was combined with NH₄, as a result of an increase in soil solution NH₄ concentrations and the effective diffusion coefficient for NH₄. These experimental results and model simulations indicate that in NH₄-fertilized soils, the applicaiton of Ca will increase NH₄ availability and absorption by plants.

Contribution from the College of Agriculture and Home Economics, Dep. of Crop and Soil Sciences, Washington State Univ., Pullman. Departmental paper no. 9512-38. Received 15 Feb. 1995. This research was funded in part by the Tennessee Valley Authority, National Fertilizer and Environmental Research Center.

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