Anion movement in a structured soil : a thesis for the degree of Doctor of Philosophy in Soil Science

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Massey University
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Anion movement in soil was studied both in the laboratory and in the field, using structured "undisturbed" soil and sieved aggregates. The movement of chloride, bromide and phosphorus was investigated. Chloride and bromide being non-reactive in soil, were used to indicate whether the flow was more uniform or preferential, while phosphorus was used to indicate the behaviour of strongly adsorbed anions. Experiments involving the movement of chloride and phosphorus through columns of 0.5-1 mm soil aggregates provided data on phosphate adsorption during miscible displacement. Chloride breakthrough curves were described well by conventional convective-dispersive theory. For phosphorus, linear adsorption isotherms were determined independently, using solution concentrations and equilibrium times similar to those pertaining in the aggregate columns. Conventional theory using these data predicted reasonably well the early part of the breakthrough curves, but did not predict the observed "tailing" of phosphorus breakthrough curves. The movement of anions through artificial soil channels and planar cracks was studied. The breakthrough curves showed the movement of both chloride and phosphorus was highly preferential through 0.5 mm diameter channels and 0.17 mm wide planar cracks. The results agreed quite well with model predictions. The movement of anions through 2.4 litre "undisturbed" soil cores was also studied. Under saturated conditions, both chloride and phosphorus moved preferentially. Dye studies indicated the major pathways were worm channels, root channels, and soil cracks. Under unsaturated conditions when the pressure potential was maintained at -0.02 bar (at which channels larger than 0.15 mm diameter and cracks wider than 0.07mm would be drained), the breakthrough curves for bromide were much less preferential than under saturated conditions. The experimental set-up for this experiment was designed so that the blockage of natural flow paths was minimized and the effects of porous plates at either end of the cores were avoided. Two field experiments were conducted at a mole-tile drained site on Tokomaru silt loam (a Fragiaqualf). One experiment investigated the movement of chloride and phosphorus solution ponded on the soil surface. The breakthrough curves for both chloride and phosphorus percolating from the surface to the mole-drains indicated the movement was very preferential, both anions reaching the mole-drains located at 400 mm depth within a minute of their application to the soil surface. Dye staining indicated the movement occurred mostly through worm channels and plant root channels associated with planar cracks. The other field experiment investigated the leaching of bromide under both ponded water and natural rainfall conditions. When the same amount of water was considered, leaching by rainfall was more effective than by ponding. However, under both water treatments, relatively large amounts of applied bromide remained unleached near the soil surface, while some bromide moved deep into the soil profile. Interception and stem flow appeared to be important factors causing non-uniform leaching under pasture by natural rainfall. Very considerable variation in bromide concentration between replicate soil samples was found, with a log-normal rather than normal distribution. Quite different leaching patterns were found in soil under pasture and in a soil which had been cultivated and cropped.
Anions, Chloride in soil, Bromide in soil, Phosphorus in soil