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

Abstract

Competition among cations for exchange sites in soil may influence NH 4 concentrations in soil solution and consequently affect NH 4 availability to plants. Laboratory, greenhouse, and field experiments were conducted to determine whether Ca additions enhance NH 4 availability, thereby increasing the potential for NH 4 absorption by plants. A Shano silt loam soil (coarse-silty, mixed, mesic Andic Mollic Camborthid) was equilibrated with NH 4 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 NH 4 -N L -1 in solution than the distilled water matrix at an exchangeable NH 4 concentration of 100 mg N kg -1 soil. Calcium also reduced the potential maximum quantity of NH 4 adsorbed by the soil as well as the soil buffer capacity for NH 4 . Combining Ca with NH 4 fertilizers (1 :2 Ca/N molar ratio) increased NH 4 concentrations in the soil solution in subsequent greenhouse and field experiments. The influence of Ca on soil solution NH 4 concentrations is attributed to preferential adsorption of Ca and the displacement of NH 4 from soil exchange sites. Mechanistic model predictions of NH 4 uptake were 22% higher for rice (Oryza sativa L.) and 7.5% higher for corn (Zea mays L.) when Ca was combined with NH 4 , as a result of an increase in soil solution NH 4 concentrations and the effective diffusion coefficient for NH 4 . These experimental results and model simulations indicate that in NH 4 -fertilized soils, the application of Ca will increase NH 4 availability and absorption by plants.