Measurement and modelling of chloride and sulphate leaching from a mole-drained soil : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
A study of the leaching of sulphate, chloride, nitrate and the associated cations was carried out over two winter periods on three mole-drained paddocks on a yellow-grey earth (Tokomaru silt loam). The paddocks were occasionally grazed by sheep. At the beginning of each drainage season potassium chloride (KCl) and sulphur fertilizer, as either single superphosphate (SSP) or elemental sulphur (S°), were applied to two of the paddocks, while the third served as a control. The amount of KCl applied was 200 kg ha-1 yr-1, while sulphur was applied at 50 kg S ha-1 in 1988 and 30 kg S ha-1 1989. Drain flow from the fertilized paddocks was measured with V-notch weirs, and sampled using proportional samplers. Large differences in the total drainage flow were measured in the two years, with values of approximately 280 mm in 1988 and 110 mm in 1989. Significant amounts of the chloride added through both fertilizer and rainfall were leached, amounting to approximately 105 kg Cl ha-1 yr-1. The leaching of sulphate-S depended on the form of S fertilizer applied, the quantity of drainage, and the rate of mineralization. Leaching losses of 17 and 3.4 kg S ha-1 were measured from the SSP and S° fertilized paddocks, respectively, in 1988, and 9.4 and 3.5 kg S ha-1 in 1989. The study showed that applying SSP just before the drainage season increased leaching losses of sulphate-S substantially. Although the particle sizes of S° used in both years were much bigger than those specified by the Ministry of Agriculture and Fisheries of New Zealand, there appeared to be no difference in the yield response to S applied either as SSP or as S°. The reduced leaching of S when applied as S° resulted from the slow oxidation of S° to sulphate-S. Relatively little nitrate-N was leached, the amount ranging from 11 to 17 kg N ha-1 yr-1. Losses of potassium were less than 10 kg K ha-1 yr-1, despite the large quantity applied as KCl fertilizer immediately before winter. However a large amount of calcium was leached, between 31 and 52 kg ha-1 yr-1. The amount of magnesium leached was between 9 and 15 kg ha-1 yr-1. A considerable quantity of sodium was leached, around 58 kg ha-1 in 1988, and 31 kg Na ha-1 in 1989, roughly equal to the input in rainfall. Good mass balances were obtained for both chloride and sulphate. The measured moles of negative and positive charge from cations and anions in the leachate were also almost equal. Three models were developed for the leaching of chloride and sulphate-S. An approach dividing the soil water into mobile-immobile phases, and using a fairly detailed soil water flow model was developed. The model was able to simulate the concentration of chloride in the drainage closely, even just after the application of chloride fertilizer, and during highly preferential flow conditions induced by heavy rain. A transfer function model, assuming a log-normal probability density function (pdf) of solute travel pathway lengths, was able to simulate the leaching of chloride reasonably well in 1988. The prediction was less satisfactory in 1989 and during highly preferential flow. By adapting the pdf for chloride to sulphate and by taking adsorption of sulphate into account, the model could predict the sulphate concentration in the drainage quite successfully. A simpler model, using the idea that the two-dimensional flow geometry to the mole drain implies that the drainage concentration approximately equals the average soil solution concentration, was also developed. Despite its simplicity it was able to simulate the leaching of chloride and sulphate as well as the transfer function model. Irrespective of the model being used, the net mineralization rate of soil organic sulphur emerged as an important factor in predicting the leaching of sulphate.
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Scotter, D. R., Heng, L. K., Horne, D. J., & White, R. E. (1990). A simplified analysis of soil water flow to a mole drain. Journal of Soil Science, 41(2), 189-198.