Soil Solute Movement Research Articles

The Chemistry of the Reclamation of Sodic Soils with Gypsum and Lime

Sodic soil reclamation was theoretically evaluated assuming equilibrium chemistry and piston movement of soil solution. The effective solubility of gypsum when mixed with a sodic soil is increased because the exchange phase acts as a sink for Ca2+ until both the gypsum dissolution and exchange reactions reach equilibrium. The electrical conductivity of a soil solution in equilibrium with both gypsum and an exchangeable sodium fraction (ENa) of 0.0 and 0.43 is 2.3 and 14 dS m−1. Thus, mixing gypsum into the soil hastens reclamation and provides higher solution concentrations for the maintenance or improvement of the soil hydraulic conductivity. The amount of gypsum dissolved, expressed in moles of charge (equivalents), is a linear function of the moles of exchangeable Na+ replaced; r2 values typically exceeded 0.98. The slope of the regression line decreased with increasing final ENa. Typical values were 1.40, 1.27, and 1.20 moles of charge gypsum dissolved per mole of exchangeable sodium replaced at final ENa's of 0.05, 0.10, and 0.15. The inclusion of lime equilibrium reduces these slopes by 3, 6, and 9% for PCO2's of 1, 4, and 10 kPa (1, 4, and 10% CO2). Gypsum requirements for calcareous, sodic soils based on quantitative replacement of exchangeable sodium should be increased by factors of 1.3 to 1.1 depending on the desired final levels of exchangeable sodium.

A SIMPLE COMPUTER MODEL FOR LEACHING IN STRUCTURED SOILS

SummaryA layer model for the movement of solutes in soil has been developed in which, as a working approximation, the soil solution is partitioned into mobile and retained phases. Only the mobile solution is displaced during water movement, and equilibrium is assumed to be established between the mobile and retained phases when flow ceases, giving holdback of solute. The model permits the displacement of the mobile solution through an indefinite number of layers when large amounts of rain fall. It can be assumed either that water and solute are stored in the bottom layer and can be withdrawn up the profile by evaporation or that they drain from the bottom layer. Predicted soil nitrate concentrations agreed reasonably well with those measured in a field experiment and indicated some sensitivity to layer thickness. The capacity of the model to predict concentrations in drainage is demonstrated.

Water and Solute Movement in Svea Loam for Two Water Management Regimes

AbstractSoil water and soil solute movement were investigated in a sub‐humid environment over a 3‐month period for two water regimes. Ammonium nitrate and KCl were broadcast on duplicate 3.7‐m square plots. Each plot was irrigated with 6 to 10 cm of water at 7‐ to 14‐day intervals. One plot was covered to prevent exchange of water at the soil surface (covered plot) while the second remained bare and was thus subjected to the prevailing environmental conditions. Soil water flux and unsaturated hydraulic conductivity values were calculated using soil water pressure and water content data from the covered plot. Analyses of soil solution collected from 30‐, 61‐, 91‐, 122‐, and 152‐cm depths for NO3‐N and Cl revealed that, for the environmental conditions encountered (35 cm pan evaporation and 15 cm rainfall), downward movement of ions was nearly 1.5 times greater for the covered compared with the bare plot. The solute did not move with the soil water in a one to one relationship for either plot.

INFLUENCE OF SALTS IN ASSOCIATION WITH MONOCALCIUM AND DIAMMONIUM PHOSPHATES ON THE CHEMICAL CHARACTERISTICS AND MOVEMENT OF SOIL SOLUTION

Various salts thoroughly mixed with monocalcium phosphate monohydrate (MCP) and diammonium phosphate (DP) were made to react with moist Lima silt loam soil arranged in 5-mm layers and separated by filter papers. After a reaction period of 3 weeks, soil moisture, pH, hot nitric acid-soluble phosphorus, and sodium bicarbonate-soluble phosphorus were determined in the different layers. As compared with MCP, the more soluble salts in association with MCP enhanced soil solution movement away from the fertilizer layer, and there was considerably greater movement of soil solution from the fertilizer layer with DP compared with MCP. The movement of soil solution with DP was attendant with greater movement of phosphorus into soil layers than for MCP, with the result that phosphorus was distributed into a greater volume of soil. When calcium sulphate was associated with DP, movement of soil solution and the diffusion of phosphorus into soil layers was markedly restricted because of massive precipitation reactions. At the soil layer closest to the MCP–fertilizer layer, a pH of 2.8 units developed. These very acid conditions were in every case ameliorated when MCP was associated with different soils. No changes in the natural soil pH were occasioned by the use of DP.By sampling soil solutions with filter papers at increasing distances from a fertilizer layer containing MCP, considerable amounts of Al, Fe, Mn, and Ca were detected. When various salts were associated with MCP, the amounts of soluble constituents moved to greater distances than with pure MCP, thereby effecting a greater diffusion of potentially toxic elements into a greater volume of soil. Chloride salts were particularly effective in causing the diffusion of manganese away from the fertilizer layer. No measurable amounts of Al, Fe, or Mn were found in soil solutions when DP reacted in soil.