Salt and water dynamics in saline and sodic clay soils
Salt and water dynamics in saline and sodic clay soils
Salt and water movement in saline-sodic clays is studied during leaching of soil columns under simulated and natural rainfall events. The significance of soil structure is evaluated using restructured soil fractions, to simulate cultivations, and undisturbed samples. By varying storm size, intensity, and time between storms, the influence of rainfall pattern on leaching is assessed. Salt loss decreases as (i) aggregate size, (ii) average storm size, and (iii) rainfall intensity, increases. Two salt movement mechanisms, diffusion within aggregates (immobile water) and convective transport in inter-aggregate pores (mobile water), interact in a complex manner to determine leaching efficiency. The concentration-dependency of the apparent diffusion coefficient (DA) of salts in saline-sodic aggregates exerts a significant influence in retarding diffusion of salts to mobile water as leaching progresses. A model developed by simulating rainfall patterns in the laboratory successfully predicts the leaching response under natural rainfall from saline-sodic topsoils when a surface mulch is present. Salt loss is significantly over-estimated if the soil surface is exposed to raindrop impact. Formation of a cracking surface crust and uneven infiltration have important consequences for modelling salt movement. Three calcium amendments (phosphogypsum, rock gypsum, saturated gypsum solution) are compared for solubility, exchangeable sodium replacement and inhibition of clay dispersion. Although phosphogypsum is the most efficient amendment, at least 75% of added calcium is exchanged in all treatments. Gypsum applications, equivalent to 10 tonnes ha-1, reduce exchangeable sodium (ESP) levels from 19.6% to 8-10.7%. Dispersed clay re-appears in column effluents after complete dissolution of added gypsum. In naturally structured topsoils and subsoils, the large ped size and small fraction of cracks active in transporting leaching water severely restricts salt release during leaching. Artificial restructuring is necessary to provide adequately small aggregates and sufficient numbers of inter-connected macro-pores to allow rapid leaching of saline clay subsoils. A new field reclamation technique, leaching salts horizontally through saturated, restructured clay, is successfully tested in southern Turkey. Eighty-five percent of leachable salts are removed in 16 days. (DX89583)
University of Southampton
Armstrong, Andrew Shaw Braidwood
1989
Armstrong, Andrew Shaw Braidwood
Armstrong, Andrew Shaw Braidwood
(1989)
Salt and water dynamics in saline and sodic clay soils.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Salt and water movement in saline-sodic clays is studied during leaching of soil columns under simulated and natural rainfall events. The significance of soil structure is evaluated using restructured soil fractions, to simulate cultivations, and undisturbed samples. By varying storm size, intensity, and time between storms, the influence of rainfall pattern on leaching is assessed. Salt loss decreases as (i) aggregate size, (ii) average storm size, and (iii) rainfall intensity, increases. Two salt movement mechanisms, diffusion within aggregates (immobile water) and convective transport in inter-aggregate pores (mobile water), interact in a complex manner to determine leaching efficiency. The concentration-dependency of the apparent diffusion coefficient (DA) of salts in saline-sodic aggregates exerts a significant influence in retarding diffusion of salts to mobile water as leaching progresses. A model developed by simulating rainfall patterns in the laboratory successfully predicts the leaching response under natural rainfall from saline-sodic topsoils when a surface mulch is present. Salt loss is significantly over-estimated if the soil surface is exposed to raindrop impact. Formation of a cracking surface crust and uneven infiltration have important consequences for modelling salt movement. Three calcium amendments (phosphogypsum, rock gypsum, saturated gypsum solution) are compared for solubility, exchangeable sodium replacement and inhibition of clay dispersion. Although phosphogypsum is the most efficient amendment, at least 75% of added calcium is exchanged in all treatments. Gypsum applications, equivalent to 10 tonnes ha-1, reduce exchangeable sodium (ESP) levels from 19.6% to 8-10.7%. Dispersed clay re-appears in column effluents after complete dissolution of added gypsum. In naturally structured topsoils and subsoils, the large ped size and small fraction of cracks active in transporting leaching water severely restricts salt release during leaching. Artificial restructuring is necessary to provide adequately small aggregates and sufficient numbers of inter-connected macro-pores to allow rapid leaching of saline clay subsoils. A new field reclamation technique, leaching salts horizontally through saturated, restructured clay, is successfully tested in southern Turkey. Eighty-five percent of leachable salts are removed in 16 days. (DX89583)
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Published date: 1989
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Local EPrints ID: 461438
URI: http://eprints.soton.ac.uk/id/eprint/461438
PURE UUID: 8a5fbcb9-017f-42b5-b78b-fa407b6e0234
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Date deposited: 04 Jul 2022 18:46
Last modified: 04 Jul 2022 18:46
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Author:
Andrew Shaw Braidwood Armstrong
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