CALCIUM EFFECTS ON QUANTITY-INTENSITY RELATIONSHIPS AND PLANT AVAILABILITY OF AMMONIUM

Competition among cations for exchange sites in soil may influence NH4 concentrations in soil solution and consequently affect NH4 availabil ity to plants. Laboratory, greenhouse, and held experiments were condu cted to determine whether Ca additions enhance NH4 availability, there by increasing the potential for NH4 absorption by plants. A Shano silt loam soil (coarse-silty, mixed, mesic Andic Mollic Camborthid) was eq uilibrated 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 t he 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 NH 4, Combining Ca with NH4 fertilizers (1:2 Ca/N molar ratio) increased NH4 concentrations in the soil solution in subsequent greenhouse and f ield experiments. The influence of Ca on soil solution NH4 concentrati ons is attributed to preferential adsorption of Ca and the displacemen t of NH? from soil exchange sites. Mechanistic model predictions of NH 4 uptake were 22% higher for rice (Oryza sativa L,) and 7.5% higher fo r corn (Zea mays L.) when Ca was combined with NH4, as a result of an increase in soil solution NH4 concentrations and the effective diffusi on coefficient for NH4, These experimental results and model simulatio ns indicate that in NH4-fertilized soils, the application of Ca will i ncrease NH4 availability and absorption by plants.