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Effect of Fertilizer Amendments, Bulk Density, and Moisture on Calcium and Magnesium Diffusion¹

ABSTRACT

The fertility of acid subsoils may be improved by the injection of fertilizer materials into narrow slits that can be cut in subsoils during tillage operations. Little information is available to describe the effects that soil properties and added fertilizers have on the diffusion of Ca²⁺ and Mg²⁺ in acid subsoils. Laboratory studies were conducted with an acid subsoil of a Marvyn sandy loam soil (fine loamy, siliceous, thermic, Typic Hapludult) to determine the effects of fertilizers, bulk density, and volumetric moisture () on the diffusion of Ca²⁺ and Mg²⁺. The soil was treated with either dolomitic limestone, NH₄NO₃, NH₄H₂PO₄, Ca(H₂PO₄)₂, or Ca(NO₃)₂. Limestone was applied at a ratio of 79 g kg^–1, whereas the remaining treatments were applied to provide 2.64 g kg^–1 of either NO^-₃ or H₂PO^-₄. The rates were used to simulate the concentrations that would occur in the zone of injection if the amendments were injected into the subsoil under field conditions. Calcium diffusion coefficients (D_e) measured with a cation exchange resin paper method ranged from 2.4 x 10^–9 cm² s^–1 in the check to 3.9 x 10^–6 cm² s^–1 in the Ca(NO₃)₂ treatment, and counter D_e for Mg²⁺ ranged from 3.6 x 10^–9 cm² s^–1 in the check to 5.1 x 10^–6 cm² s^–1 in the Ca(NO₃)₂ treatment. All treatments increased D_e for Ca²⁺ and Mg²⁺ relative to the check. In general, D_e increased as the concentration of Ca²⁺ and Mg²⁺ in the soil solution increased, and as the proportion of the extractable cation associated with the solid phase decreased. Diffusion coefficients for Ca²⁺ and Mg²⁺ increased as increased from 0.12 to 0.30. Calcium and Mg²⁺ D_e reached a maximum at a bulk density of 1.6 to 1.7 Mg m^–3 and then decreased slightly as bulk density increased to 1.9 Mg m^–3. Bulk density effects were more pronounced at soil-water pressure head values <0.033 MPa.

¹ Contribution of Dep. of Agronomy and Soils, Alabama Agric. Exp. Stn., and USDA-ARS, National Soil Dynamics Laboratory, Soil-Plant Interactions Research Unit, Auburn Univ., AL 36849.

² Assistant Professor, Dep. of Agronomy and Soils, Alabama Agric. Exp. Stn., and Soil Scientist, USDA-ARS, respectively.

Received for publication November 10, 1986.