Gypsum Surface Research Articles

In situ investigation of growth and dissolution on the (010) surface of gypsum by Scanning Force Microscopy

The kinetics of crystal growth and dissolution on the (010) surface of gypsum were investigated in situ by Scanning Force Microscopy (SFM). It could be shown that growth and dissolution on the (010) surface in aqueous solution is a layer-by-layer process. Monolayer steps parallel to [001], [100], and [101], move with a distinctive anisotropy in velocity. During dissolution experiments, etch pits develop at surface regions, where local structural defects may occur. Therefore, etch pits are getting deeper at the same lateral position. As a consequence of the anisotropy in velocity of step movement, the etch pits are elongated in [001]. Isolated holes remain stable over a long period during growth experiments. Such holes can occur at the same position in several overgrowing monolayers. This kind of memory effect might also be explained by local structural defects. Local surface topography has a strong influence on the velocity of step movement. At surface regions with a high step density, the velocity of monolayer step movement is reduced compared to isolated monolayer steps. Isolated [100] monolayer steps move with a velocity of up to 30.0 nm s −1, whereas [010] steps move with up to 9.5 nm s −1, and [001] steps 2.5 nm s −1, in an undersaturated aqueous solution (9.8 mmol L −1). Imaging monolayer steps with lateral molecular resolution reveals the molecular arrangement at mono-layer steps. Individual kink sites can be observed. The kink site formation energy along [001] monolayer steps is 4.1 ± 0.7 KJ mol −1 in saturated aqueous solution. Observed kink site density agrees with predicted values from Monte Carlo simulations.

Possible microbial origin of phosphorites on Error Seamount, northwestern Arabian Sea

Petrographical, mineralogical and microprobe analysis of phosphorites from Error Seamount, in the northwestern Arabian Sea, demonstrates that microbial processes played an important role in the early diagenetic formation of the phosphorites during subaerial exposure of the seamount. The phosphorites on Error Seamount occur as laminated crusts and massive slabs. Low-magnesium calcite and carbonate fluorapatite are the major minerals of the phosphorites, with goethite being important only in the massive slab phosphorites. Elemental sulphur, pyrrhotite, gypsum and chlorite are the mineral phases present in the acid-insoluble residues of the phosphorites. Elemental sulphur, which is exclusively formed by microbial processes, occurs as submicrometre-sized granules on gypsum surfaces. The laminated crust phosphorites consist of organic-rich and organic-poor laminae interlayered with ghost pellets, index fossils of Oligocene to lower Miocene age, peloids and coated grains. Massive slab phosphorites are fine-grained, and consists of goethitic and non-goethitic zones. Non-goethitic zones comprise mainly microsphorite with Oligocene to lower Miocene fossils and microfossils. Goethitic zones contain abundant Quaternary skeletons. Microprobe analyses of the Error Seamount phosphorites indicate that the geochemistry of these phosphorites is similar to that of the Tertiary seamount phosphorites in the Atlantic and Pacific Ocean and that they are enriched in Ca and depleted in P compared to other seamount and island phosphorites in the Indian Ocean. Microbial activity is evident in both types of phosphorites and occurs in the form of microborings in peloids and on skeletal fragments, encrustation of phosphate crystals on hollow, globular microstructures and phosphate sheaths enclosing organic filaments. The presence of calcite rim cements, microspar-filled patches and recrystallised grains indicates exposure of the seamount under subaerial conditions, during which phosphatisation took place.

An evaluation of dental stones after repeated exposure to spray disinfectants. Part I: Abrasion and compressive strength

This study investigated the effect of repeated applications of spray disinfectants on gypsum surfaces. Types III and IV gypsum products were evaluated in combination with iodophor, acid glutaraldehyde, phenol, and water spray. Results demonstrated greater resistance to abrasion with increasing numbers of water or disinfectant spray applications. Acid glutaraldehyde spray decreased the compressive strength of type III stone by 26%, phenol increased the compressive strength of type IV stone by 18%, and iodophor had no significant effect on either stone relative to compressive strength.

Sedimentologic and geochemical constraints on the evolution of Bristol Dry Lake Basin, California, U.S.A

Two continuous cores over 400 m in length drilled in Bristol Dry Lake, a large playa in the Mojave Desert, California, U.S.A., provide sedimentologic and isotopic information which can be integrated with surface trenches and groundwater analyses. The combination of these data provide three-dimensional data on the facies distribution and geochemical evolution of the basin for the last 4 Ma. At the surface, the basin exhibits a generalized bulls-eye facies distribution. Low-gradient alluvial fans ring the playa. Extensive calcrete and pedogenic calcite associated with halophyte plants cement the mid-to-distal fan gravels and sands. Basinward of the distal fans, in the playa margin facies, an extensive gypsum zone is over 300 m wide. Within the gypsum zone, celestite forms large decimetre-size nodules which may coalesce into metre-size patches. Basinward of the playa margin facies, halite hopper crystals (up to 0.5 m in diameter) form in water-saturated muds of the saline mud-flat facies. Finally, in the basin-centre a 0.2-m thick chevron halite crust (salt pan) forms from evaporation of ponded ephermeral water. The lateral facies distribution seen at the surface is found throughout the entire length of the cores. The core drilled in the basin-centre exhibits beds of alternating salt pan and saline mud-flat facies, whereas the core drilled in the playa margin exhibits alternating beds of playa margin sediments and saline mud-flat for most of the length of the core. Towards the base of the playa-margin core, distal alluvial fan sediments are also present. Stable oxygen isotopes of basin-centre calcite concretions from the surface and core indicate that the isotopic composition of the water precipitating the concretions has not changed substantially throughout the evolution of the basin. The sulfur isotopic composition of the surface gypsum, anhydrite and celestite show little variation. However, δ 34S values of anhydrite from core are isotopically lighter relative to surface gypsum and anhydrite. This isotopic depletion indicates that bacterial sulfate-reduction is involved in the dehydration of gypsum to anhydrite, and is not caused by evolving isotopic composition of sulfate in the brine. At the surface, gypsum δ 34S values show no statistically significant trends with lateral distribution. Chemical analyses from shallow groundwater wells and basin-centre brines from up to 150 m depth indicate that: (1) the brine is dominated by Na, Cl, and Ca. (2) Na, Ca, Mg, K, and Cl contents increase toward the basin-centre, SO 4 increases from the alluvial fan facies to the playa margin and then sharply decreases in the basin-centre, and Si, and HCO 3 decrease slightly toward the basin-centre. This pattern is consistent with the observed evaporite mineral distribution in the basin. (3) molar Na:Cl and Ca:Cl ratios demonstrate that the simple dissolution of previously deposited halite deposits, as has been suggested for other playa basins, cannot account for the proportions of these ions present in the brine. (4) Chemical budget calculations suggest that Cl can be accounted for by atmospheric input, although hydrothermal sources cannot be discounted. The repititious nature of the alternating shallow brine-pond halite and siliciclastics, the consistency of the carbonate isotopic data from the surface and core, and the agreement of water chemistry data with observed evaporite sequences, indicate a relatively stable brine composition for most of the history of Bristol Dry Lake. All sedimentary structures and primary halite fabrics in the core indicate shallow-water, brine-pond halite alternated with halite-saturated siliciclastic muds in the basin-centre. A delicate balance between subsidence, and mechanical and chemical deposition of minerals was necessary to maintain the largely ephemeral environment of deposition during the deposition of over 500 m of basin fill.

Subsoil macropore space of a transitional red-brown earth after either deep tillage, gypsum or both .II. Chemical effects and long term changes

The management of sodic texture-contrast soils of the riverine plains of S.E. Australia for crops other than rice is difficult because improved subsoil structure often deteriorates after a few seasons of irrigated cropping. This research aimed to improve understanding of the processess which cause deterioration of macropore space (volume and permeability of air-filled soil at -10 kPa matric potential) as well as providing some guidelines for improved management. A transitional red-brown earth was ameliorated by eight different treatments and compared with an unameliorated control treatment in a field experiment. The treatments included surface gypsum application, deep ripping after gypsum application, five designs of gypsum-enriched slots and traffic-free beds after addition of gypsum and deep ripping. The macropore space of the subsoil after initial winter cropping and summer fallow is described from measurements on soil cores collected from the experiment. An index of electrochemical stability (EC/ESP) is proposed which explains the loss of macroporosity of cultivated subsoil in the absence of significant soil stressing by wheels. Considerable loss of macropore volume and permeability after the first summer crop could be associated with loss of soluble gypsum, some increases of exchangeable sodium and compression by lateral swelling of the uncultivated soil from between the slots. These changes were compensated by an increase in the number of vertical fissures, directly observed and deduced from measurements of water entry into whole plots. Thus, management of the balance of soluble salts is essential for the long-term productivity and profitability of such systems of amelioration using deep tillage zones and gypsum application. This is in the absence of significant compaction under wheels or opportunities for addition or organic matter and formation of biopores by roots and soil animals. The relief of lateral swelling of the uncultivated B horizon by the loose slots and formation of extra vertical fissures is a process which may provide long-term benefits and deserves further research.

Gypsum as an ameliorant for subsoil hardpans

In many highly weathered soils, subsoil penetration by roots is severely limited by chemical and physical barriers such as Al toxicity, hardpans and dense textural B horizons. Previous work has demonstrated the efficiency of surface applied gypsum in ameliorating such chemical and physical barriers after sufficient time has elapsed to allow the gypsum to leach into the subsoil. Cone index (CI) was measured using a tractor-mounted computerized penetrometer on plots which had received gypsum 2 to 4 years prior to measurement. Water stable aggregates were measured by the wet sieving method. In all cases, surface gypsum application reduced CI values for the subsoil hardpan layers when the crops used were alfalfa, peaches and cotton. The gypsum treatments resulted in significantly improved rooting in the subsoil hardpans. The proportion of large water-stable aggregates in the dense B horizon was also increased. Under laboratory conditions, both CI and gravimetric moisture contens at high matric potential were higher then the soil was wetted with deionized water than saturated gypsum solution.These improvements in subsoil physical condition appear to be due to both a root effect and a direct effect of the gypsum on flocculation and aggregation.

An evaluation of the surface characteristics of a facial prosthetic elastomer. Part II: The surface texture

Silicone elastomers do not have an inherent surface texture but take on the surface texture of the surfaces against which they are processed. This study compares the surface texture of Cosmesil material, a purpose-designed facial prosthetic elastomer, and Molloplast-B material, which is a well-established material for use in the clinic. We used a profilometer to determine the roughness average, maximum roughness, and waviness of the dies, control, gypsum, and silicone test surfaces. Statistical analyses of results involved multivariate analyses of variance and Tukey's procedures to compare roughness average, maximum roughness, and waviness (in all casesρ < 0.05). The results showed that the silicone elastomers, Cosmesil and Molloplast-B, have similar surface characteristics. The only statistically significant difference was in waviness (Molloplast-B,¯x= 10.82 μm Cosmesil,¯x= 12.82 μm). The gypsum surface yielding the most satisfactory result was the surface that had been treated with silicone paste separator. On the basis of surface characteristics, Cosmesil material is likely to cause no more surface trauma of the tissue against which it is placed than the use of Molloplast-B material.

Use of the neutron method in assessing the changes in soil strength of undisturbed and ameliorated transitional red-brown earths during soil drying cycles

The relationships between penetrometer resistance (qp) and volumetric moisture content (�v) measured using the neutron method in an undisturbed transitional red-brown earth and after an~elioration by application of surface gypsum and slotted gypsum were examined. A very highly significant (P &lt; 0.001) negative correlation was obtained between qp and �v in all treatments. The low r2 values of the regressions were attributed to heterogeneity in strength characteristics of the soil matrix, due to presence of cracks and macropores and the associated wetting patterns. The qp at any given e, was significantly reduced in the slots with lower bulk density compared to the undisturbed soil. The differences in qp- �v relationship of the undisturbed part of the soil under different ameliorative practices were attributed to changes in the swelling characteristic, and hence in the bulk density at any given �v of the undisturbed soil, caused by the presence of gypsum and the slots. Regression equations between qp and neutron count rate (n) for the undisturbed soil and for the slots were developed by combining the qp on �v relationships with the neutron meter calibration for �v measurements. The use of these regression equations and measured n values to predict changes in soil strength profiles during a wheat crop drying cycle in an undisturbed and ameliorated transitional red-brown earth was evaluated on another experimental site. There were no significant differences between the predicted and measured qp values in the non-ameliorated soil and the gypsum-slotted soil. Significant differences were observed between the predicted and measured qp values in the surface gypsum applied soil. The study shows the potential for using the neutron method as a convenient in-situ field technique to predict qp profile changes, preferably using qp on n relationships developed at the experimental site.

Exchange‐induced dissolution of gypsum and the reclamation of sodic soils

SUMMARYA study was undertaken to define the dissolution kinetics of gypsum in the presence of ion exchange resins and to study sodic soil reclamation in laboratory soil columns.Gypsum pellets were prepared at 1.5 MPa pressure and the time course of their dissolution followed by measuring the electrical conductivity of the solution. Dissolution experiments were carried out in distilled water with and without Na‐ and Cl‐saturated exchange resins or a combination of the two. The results indicate that in the presence of resin the reaction is first order as calcium and/or sulphate are removed from solution by the resin driving the reaction to completion. In water alone the reaction follows second‐order kinetics and depends on the rate of mixing as the reaction is transport controlled.The effect of gypsum placement (uniformly mixed with the entire soil, mixed with the top third of the soil, applied to the soil surface or applied as a saturated gypsum solution) on the efficiency of exchangeable sodium removal, leaching of soluble salts and soil hydraulic conductivity was studied. In addition, the effect of various flow rates on reclamation efficiency was investigated.The amount of leachate required for reclamation was found to be dependent on gypsum placement, tending to decrease in the order mixed &lt; top third mixed &lt; saturated gypsum solution &lt; gypsum surface application &lt; water. Soil hydraulic conductivity was much higher in the mixed gypsum column than in the gypsum applied on the surface; a result of the higher effective gypsum solubility. Sodic soil electrical conductivity in the presence of solid‐phase gypsum is linearly related to the total amount of exchangeable sodium expressed in mol dm−3.

Effects of amelioration on infiltration characteristics of a transitional red-brown earth

Abstract Infiltration into a transitional red-brown earth was analysed for the following ameliorative treatments: no treatment (C), surface gypsum at 8 t/ha (G), 8 t/ha of gypsum concentrated into slots 0.4 m deep and 0.66 m apart (SGI) or 1.0 m apart (SG2). Analysis consisted of fitting the infiltration data to the following equations: At short time, i = i*i + S* t ½ + K* t ; at long time, i = i*i + i*0 + K*t where i is the cumulative infiltration, i*i is a short-time storage term to account for cracks, i*0 is a long-time storage term, K* is the steady-state infiltration rate, t is time, and S* is the sorptivity. S* was similar for all treatments. The increase in i*0 for the slotted treatments resulted from the increased surface area of B horizon exposed to the wetting front. K* increased with treatment in the order C<G<SG2<SG1. The higher values of K* for the slotted treatments were attributed to the increased rate of flow through the slot into the surrounding soil. An equation which predicts K* for ame...

Desert gypsum crusts as palaeoenvironmental indicators: A micropetrographic study of crusts from southern Tunisia and the central Namib Desert

Desert gypsum crusts are of three main types: shallow-water evaporites, which are characterized by size-graded beds; subsurface crusts, which are either macrocrystalline groundwater evaporites, or mesocrystalline illuvial accretions; and surface crusts—excluding the bedded type—which are exhumed illuvial crusts. Pedogenic processes involving the leaching of gypsum-rich surface deposits and subsequent displacive gypsum crystallization in the lower soil zone account for the formation of the illuvial crusts. The various forms of surface gypsum crust—columnar, powdery, and cobble—represent different stages in the degradation of exhumed crusts. Dissolution and leaching of gypsum transform columnar crusts to cobbles which deteriorate leaving a powdery residuum. Examination of thin sections of gypsum crusts from southern Tunisia and the central Namib Desert reveals that the exposure and degradation of illuvial, pedogenic crusts are characterized by distinct diagenetic features. The subsurface, illuvial crusts are composed predominantly of mesocrystalline (50 μm to 1·0 mm in diameter), lenticular gypsum. Crystals larger than 1·0 mm sometimes have poikilitic inclusions, though these features are more common in the macrocrystalline groundwater crusts. Rarely, fibrous gypsum crystals—indicative of displacive crystallization—are found in the illuvial crusts. Syndepositional and early diagenetic features include biogenic structures and calcite pseudomorphs after lenticular gypsum. Columnar surface crusts are composed predominantly of albastrine gypsum (crystallites less than 50 p.m in diameter). This texture develops through rapid gypsum crystallization following partial dissolution of lenticular crystals by infiltrating meteoric water. When large crystals occur as isolated remnants (porphyroblasts) in the surface crusts, they typically exhibit dissolution features and, occasionally, alabastrine overgrowths. The recognition of these characteristic structural and textural features in relict gypsum crusts can provide detailed information about the geomorphic history and palaeoenvironments of many arid regions. Radiometric dating of gypsum palaeosols is problematic because of their illuvial origin. Nevertheless, evidence suggests that the gypsum crusts of southern Tunisia developed in the early Holocene following desiccation of the chotts which had been inundated during the late Pleistocene. Evaporation of the lake waters precipitated gypsum which was then deflated and redeposited on the surrounding landscape. From here it was leached into the lower soil zone and illuvial gypsum crusts accreted. Subsequently, these pedogenic crusts were exhumed and the gypsum is currently being leached into the lower soil zone once again. In the central Namib Desert, the ages of the gypsum crusts are uncertain. Notwithstanding the difficulty in obtaining radiometric dates, studies of the gypsum's isotopic composition have provided valuable information on palaeoenvironments, and are a promising area of future research. Some of these studies have suggested that the crusts in the Namib are recent features. However, several 14C dates and also calculations of the theoretical rates of accretion indicate that the crusts may have formed during the late Pleistocene. If this is the case, their preservation supports the view that the Namib Desert has remained arid throughout the late Pleistocene and Holocene.

スラリー粒子の凝集状態と成形体の充てん構造

Generally a slip-casting process is closely related to the flocculation state of particles. The present investigation was undertaken in order to understand the relationship between the flocculation state of the particles and the packing structure on the surface of gypsum mold.The flocculation state of particles in an Al2O3 -slurry was discussed on the basis of rheological and sedimentation behavior as a function of dispersant concentration. It was found that these properties could be classified into three modes.On the slurries, the rate and pressure in the absorption of water, the increase in the packing layer thickness, and the porosity and permeability of the packing layer were measured in order to explain the packing structure on the surface of mold.From these measurements, the relationship between the flocculation state of the particles and the packing structure could be modelled.

鋳込み成形の基礎的研究

Generally, a slip casting process is closely related to the flocculation state of particles. The present investigation was undertaken in order to understand the relationship between the flocculation state of particles and the packing structure on the surface of gypsum mold.The flocculation state of particles in Al2O3-slurry was discussed on the basis of rheological behavior as a function of dispersant concentration. As the result, it was found that the type of rheological properties was classified into three modes.For these slurries, the absorbing rate and pressure of water and the increase of packing layer thickness were measured in order to explain the packing structure on the surface of mold. From these results, it was shown that the most suitable condition for slip casting could be prepared by using a weakly flocculated slurry.

Effect of changes in moisture profiles of a transitional red-brown earth with surface and slotted gypsum applications on the development and yield of a wheat crop

The low productivity of transitional red-brown earths for flood irrigated upland cropping is associated with their low infiltration rates and inadequate aeration of the root zone. The effect of the measured changes in the moisture and aeration profiles with surface and slotted gypsum applications on growth and development of a wheat crop was evaluated in a preliminary field study, on non-replicated plots.The patterns of changes in moisture profiles in the gypsum slotted plots were similar to those observed in the previous season, namely, deeper preferential wetting and faster internal drainage through the slots. This resulted in lower volumetric moisture contents in the slots and in the surface and subsurface layers between slots, compared to the non-slotted plots.The critical moisture contents were defined for each soil depth at which an air-filled porosity of 0.08 mm3 mm-3 was reached. For the transitional red-brown earth used in this study, air-filled porosity needs to be larger than 0.08 mm3 mm-3 to provide a soil pathway for oxygen flow to roots. The moisture profiles in the no-gypsum, surface gypsum and slotted gypsum plots measured throughout the cropping season indicated the period of time when oxygen flow through different soil layers was likely to be restricted. The moisture contents were higher than the critical value in the surface and subsurface layers of the non-slotted plots, particularly in the plot with no gypsum applications, during a period in winter with prolonged rainfall and low evapotranspiration rates. This resulted in reductions in the rates of phasic development, tillering, canopy closure and dry matter production and finally lower yields in the non-slotted plots, especially in the plot without gypsum. Differences in grain yields were mainly due to differences in the number of spikes m-2.During the second half of the growing season higher potential evapotranspiration, lower rainfall and accurate irrigation scheduling resulted in the moisture contents being maintained below the critical limits at all depths in all plots. Consequently, the two yield components which were determined during this period, namely, the number of kernels per spike and kernel weight, showed only slight variations between plots.

Adsorption of phosphonates on gypsum crystals

The absorption of phosphonate inhibitors at the crystal surface of gypsum has been determined from the decrease in phosphonate concentration of the bulk solution. The phosphonate was therefore oxidized to orthophosphate and spectrophotometrically determined as a molybdovanadophosphoric acid complex. The adsorption isotherms of AMDP at pH 5 and HEDP at pH values of 5 and 7 were measured at 25°C. From a comparison with the inhibitor effectiveness of both compounds during suspension growth experiments under similar conditions the surface coverage needed for growth inhibition was concluded to be 4–5%. The growth process appears to be blocked when all active growth sites provided by the steps upon the crystal surface, are occupied by adsorbed phosphonate ions. The phosphonate adsorption process to be irreversible and almost instantaneously completed. Differences in inhibitor performance of AMDP and HEDP at pH 5, or of HEDP at pH 5 and 7, can be explained from a different surface coverage of the gypsum crystals.

Origin and geochemical evolution of saline groundwater in the Brisbane coastal plain, Australia

Fresh, brackish and saline groundwaters of variable acidity and redox potentials occur in a coastal floodplain area, north of the Brisbane River in Southeast Queensland, Australia. The complicated spatial and vertical distribution of these groundwaters, and their tendency towards a seawater chemistry, reflect that the local groundwater system has undergone repeated episodes of hydrological adjustments in response to Quaternary sea-level fluctuations and coastal landscape modifications. Since the last major Holocene transgression, topographic changes have permitted repetition of evaporative concentration, aeration and acidification processes in near-surface groundwaters. The surface and subsurface occurrences of gypsum, pyrite and jarosite minerals, in the Brisbane coastal plain, testify to the intrinsic relationship between sea-level and topographic variations, and hydrogeochemistry of the local groundwater system.

Effects of gypsum‐slotting on infiltration rates and moisture storage in a swelling clay soil

Abstract. Previous studies have shown marked increases in wheat yields on a swelling clay soil due to gypsumslotting compared to no‐gypsum and surface gypsum applications, largely through improved aeration in the surface layers. In the present study, steady infiltration rates indicated 2‐fold increases due to surface gypsum applications and 4‐ to 6‐fold increases due to slotted gypsum. This should provide increased moisture storage and reduced soil erosion hazards during prolonged heavy rainfall periods, provided that a crust does not form under the impact of raindrops. However, gypsum‐slotted lands should not be used in crop rotations which include ponded rice, due to increased potential water use and risk of rising watertables and salinization.The effect of the ‘throttle’ in the upper B horizon which restricts moisture storage in the lower soil layers during short‐term and prolonged ponding was reduced, but not eliminated, by surface and slotted gypsum applications. Thus the moisture contents of the lower depths in both the non‐ameliorated and ameliorated soils were less than the moisture content at saturation or at ‐10 kPa potential even after flooding for 11 days.

The chemical lithotresis of calcium salt crystal in EDTA solution by ultrasonic irradiation.

Local chemical dissolution, i.e. chemical lithotresis, of calcium salt crystal (gypsum) in EDTA solution was made by irradiating the ultrasonic wave locally on the surface of gypsum. The dissolution, however, was hindered by the macrobubbles (0.1 mm order in diameter) on the surface of gypsum. It seems that the macrobubbles contained in the solution and crystal have grown up into the macrobubbles on the surface of gypsum byirradiating the ultrasonic wave. These macrobubbles gathered to the surface of solution when the ultrasonic irradiation stopped, so that the intermittent ultrasonic wave was used in this study. It takes about 0.5 s in order to remove the macrobubbles almost completely from the surface of gypsum. The rate of the chemical lithotresis is proportional to the power of ultrasonic wave under this experimental condition unless the macrobubbles exist on the surface of gypsum.

The effects of gypsum-enriched slots on moisture movement and aeration in an irrigated swelling clay

The low yields of flood irrigated row crops on transitional red-brown earths have been attributed to low infiltration rates and poor aeration. A new method for soil amelioration using gypsum-enriched slots to overcome these limitations was investigated in a field experiment. Gypsum-enriched slots were formed by excavating 0.4 m deep and 0.15 m wide parallel slots, mixing the excavated soil with gypsum and loosely refilling the slots. Moisture and aeration profiles were regularly monitored using a neutron moisture meter during the cropping season on field plots with gypsum enriched slots, and on plots with surface gypsum application and without gypsum. A sorghum crop was grown on all plots. Owing to faster flow through the slots during irrigation, the slotted plots showed deeper wetting than the no gypsum and surface gypsum treatments. Consequently the surface soil layers between the slots were subjected to less wetting and better aeration than in the other plots. The slots, which except immediately after an irrigation, showed high air-filled porosity values due to faster redistribution rates, should provide pathways for oxygen flow to the soil between slots and to lower depths. Infiltration rates measured using infiltration rings showed a twofold increase with addition of surface gypsum and a further two- to three-fold increase through the gypsum-enriched slots. The implications of the changes in the moisture and air filled porosity profiles due to slotting with gypsum for irrigated and non-irrigated cropping on these soils are briefly discussed.

Effect of exchangeable sodium and phosphogypsum on the hydraulic properties of several South African soils

The susceptibility of soils to the deleterious effects of sodicity was assessed for samples which varied with respect to physical, mineralogical and other properties. For this purpose soil samples were collected from different locations in South Africa. The effect of electrolyte concentration and sodium adsorption ratio (SAR) on hydraulic conductivity (HC) was determined with permeameters, while the combined effects of raindrop impact energy and low electrolyte concentration of rain on infiltration rate (IR) were assessed using a rainfall simulator. Soils proved to be either stable or susceptible to the effects of sodicity as indicated by modulus of rupture, air-water permeability ratio, HC and IR measurements. Sesquioxides and organic matter promoted stability. Final IR was always less than HC due to crust formation even at low exchangeable sodium percentage (ESP) levels as a result of the dispersive effects of both mechanical impact (physical disruption) of rain drops and low electrolyte concentration (chemical dispersion). When the chemical effect was reduced (ESP>1 or higher electrolyte concentration as a result of surface gypsum application), final IR was higher. S. Afr. J. Plant Soil 1985, 2: 179–186