P. A.L. le Roux

Soil indicators of hillslope hydrology in the Bedford catchments, South Africa.

Water plays a primary role in soil genesis and soil strongly influences hydrological processes (flowpaths, residence times and storage). Morphological soil properties serve as indicators of hillslope hydrological behaviour and can facilitate hydrological predictions. Three catchments in the Bedford district (B3, B4 and B5) were surveyed for hydropedological purposes and the observed soil indicators and related geological, topographical and vegetation features were interpreted. I n B4 & B5 shallow soils are the dominant factor governing overland flow promoting short residence times. Deeper soils and fractured bedrock in B3 facilitate bedrock flow and recharge of regional and phreatic water tables. The presence of lime and mottles in the subsoils of valley bottom soils confirm flow in the phreatic zone.

Indications of ferrolysis and structure degradation in an Estcourt soil and possible relationships with plinthite formation

Geographic associations of duplex soils, like the Estcourt form, and plinthic soils, like the Avalon and Westleigh forms, such as are found in the Eastern Free State, are recognised worldwide. These soils share redox morphology. In duplex soils, the redox morphology is dominated by a contrast in clay content between the A or E horizon and the B horizon, indicating that degradation of silicate clay minerals dominates. In plinthic soils, mottling due to redistributed Fe-Mn dominates. Both features can be the result of ferrolysis. In order to research this relationship, an Estcourt profile with degrading peds was sampled and analysed. Undisturbed peds of the B- horizon were subdivided for analysis, together with disturbed samples of the A, E and C horizons. The spatial variation of the colours, Fe, Mn, clay content and mineralogy in the profile indicates that both ferrolysis and redistribution of Fe-Mn are active during short periods of reduction in the E and interpedal pores of the B horizon. Degradation of the silicate clay minerals results in ped degradation visible as rounded ped tops and grey material interfingering the B horizon. In this landscape, in soils of the Avalon and Westleigh forms, degraded peds are also present in the lower soft plinthic B horizons and underlying material with the degree of ped degradation visibly associated with the degree of redistribution of Fe-Mn in mottles and nodules. In this landscape, a transitional horizon with properties of both plinthic and degrading structured B horizons covers large areas. This indicates that plinthic and/or E horizons can develop from structured horizons under the current climate.

Duration of water saturation in selected soils of Weatherley, South Africa

Soil water contents were measured weekly for six years (1997–2005) in the Weatherley catchment in the northern Eastern Cape Province of South Africa, and used to calculate average duration of water saturation above 70% of porosity (ADs>0.7). Data were used to determine wetness in soils, representative of midslopes, foot-slopes and valley bottoms. Hutton soils (Typic Ustorthents), representative of midslopes, had ADs>07 = 17 days year−1 in the subsoil. Westleigh soils (Aerie Endoaquents), representative of footslopes, had ADs>07 = 175 days year−1 in the subsoil. Katspruit soils (Typic Endoaqualfs), representative of valley bottoms, had ADs>07 = 334 days year−1 in the subsoil. These differences were highly significant. It was hypothesised that Hutton soils drained fastest (within half a month), and would contribute to interflow; Westleigh and Katspruit soils would drain slower (over a period of 6 and 11 months respectively) and would not contribute to interflow, but would rather contribute to peak flow during rainfall events. This hypothesis was tested against flow data from the Weatherley and Cathedral Peak VI catchments, during a rain-free period following prolonged rain. Total outflow during the rain-free periods was 4 362 m3 (2.7 mm) for Weatherley and 52 093 m3 (76.9 mm) for Cathedral Peak VI. The difference in outflow was attributed to the larger water storage capacity, from a larger area of midslope soils in Cathedral Peak catchment VI, compared to the relatively small area of midslope soils in the Weatherley catchment.

Interpretation of digital soil photographs using spatial analysis: I. Methodology

Systems for the evaluation of soil wetness use soil colour extensively. The determination of soil colour normally relies on the users perception of colour and usually employs a colour matching system, e.g. Munsell Soil Color Charts. The South African soil classification system distinguishes between diagnostic grey, yellow-brown and red colours. This paper proposes a computerised methodology for the quantitative determination of diagnostic soil colour using spatial analysis. To achieve this, soil colour definitions had to be converted from Munsell colour notation to the RGB colour notation employed in digital cameras. The conversion also aided in the mathematical manipulation of colour data during spatial analysis. In this study the relationships between photographed and calculated RGB values were as follows: Calculated Red = 0.9238 ¥ Photographed Red −37.24 (R2 = 0.99); Calculated Green = 0.9975 ¥ Photographed Green −38.96 (R2 = 0.99); Calculated Blue = 0.9841 ¥ Photographed Blue −35.74 (R2 = 0.99). The following equations can be used to differentiate between diagnostic grey, yellow and red, as defined in Soil Classification—A Taxonomic System for South Africa, using the RGB values obtained from digital photographs and after correction using the equations given above: Between grey and yellow: Green = 0.88 ¥ Red 5; Between yellow and red: Green = 0.79 ¥-11. The methodology showed a 19% misclassification when tested against photographed Munsell sheets. It was, however, a huge improvement when compared to visual methods. The number of chips classified as grey, yellow-brown or red were equal to the number of chips defined as grey, yellow-brown or red. The misclassification was attributed to the discrete nature of the diagnostic colour definitions in contrast to the continuous nature of the differentiating equations.

The relationship between soil morphology and soil water regime: preliminary results in the Weatherley catchment

Soil water contents were measured for five years at Weatherley, an intensively instrumented catchment in the north eastern part of the Eastern Cape Province. Soil water contents are expressed in terms of average duration of saturation with water above 0.7 of porosity (ADS>07 in days per year) and is related to soil morphology. Preliminary results indicate that different orthic A horizons can have ADS>0.7 from 3 to 356 days per year, depending on the nature of the underlying horizons. The E and soft plinthic B horizons in the Longlands profile had ADS>0.7 of 21 and 92 days per year, respectively. In the Kroonstad profile ADS>0.7 is 351 days in the E and 330 days per year in the G horizon. The red apedal B in the Bloemdal profile had ADS>0.7. = 0, while the underlying unspecified material with signs of wetness had ADS>0.7 of 203 days per year. Initial indications are that ADS>0.7 less than 10 days per year does not lead to mottling, while ADS>0.7 larger than 13 days per year results in mottling.

Redox conditions related to interflow in a soil of the Kroonstad form in the Weatherley catchment

Abstract The Kroonstad soil form, in the Weatherley catchment, Eastern Cape Province consists of an orthic A / E / G horizon sequence, and is a typical gleyed soil of South Africa. The profile has an uncommon diffuse E / G transition. Water contents from weekly neutron water meter readings and redox response as indicated by dissolved Fe2+ concentration were correlated with daily rainfall data from automated weather stations. Results showed that reducing conditions were more pronounced in the E horizon than in the orthic A or G horizon as indicated by the Fe2+ concentration. This is contradictory to what is normally expected. Response in soluble iron concentration in the orthic A horizon was largely associated with rainfall events. Responses in the E horizon could not be linked to current rain events. A time lag was found to exist between a response in the A and the E horizon. The G horizon responded to seasonal changes. The inability to relate current rainfall events to redox conditions as indicated by soluble Fe2+ response in the E horizon could be attributed to water entering the E horizon laterally through interflow. This would also explain the observed time lag. This is typical of perched water tables. The lack of response in the G-horizon is an indication of a year round flow pattern controlled by seasonal changes and is typical of phreatic water tables. It is postulated that the non-abrupt E / G transition is an indication of the dominant role of the phreatic water table during pedogenesis, with upwards and lateral movement of water into the E horizon. It is further postulated that the nature of the E / G transition could serve as a valuable criterion to distinguish between the wetter and drier soils of the Kroonstad form.

Nature and distribution of South African plinthic soils: Conditions for occurrence of soft and hard plinthic soils

The relevance of using redox morphology and irreversible hardened material to classify soft and hard plinthic soil forms is often questioned as the paleoclimate of South Africa varied significantly. Since the origin of the Soil Classification System of South Africa in 1965, enough soils have been classified to test the ability of the classification system to distinguish between different entities and the relevance of the criteria applied to classify plinthic soils. Data of 203 plinthic profiles, classified by various workers, were plotted in phase diagrams and interpreted. Results indicate that the Soil Classification System of South Africa is user friendly and can be applied with success. Soft and hard plinthic soils occur in different phases of environmental conditions. Both the dominant factors of soil formation currently controlling the soil water regime and the soil properties expected to relate to the conditions of soil formation differ between the plinthic soil forms. The soils, therefore, formed u...The relevance of using redox morphology and irreversible hardened material to classify soft and hard plinthic soil forms is often questioned as the paleoclimate of South Africa varied significantly. Since the origin of the Soil Classification System of South Africa in 1965, enough soils have been classified to test the ability of the classification system to distinguish between different entities and the relevance of the criteria applied to classify plinthic soils. Data of 203 plinthic profiles, classified by various workers, were plotted in phase diagrams and interpreted. Results indicate that the Soil Classification System of South Africa is user friendly and can be applied with success. Soft and hard plinthic soils occur in different phases of environmental conditions. Both the dominant factors of soil formation currently controlling the soil water regime and the soil properties expected to relate to the conditions of soil formation differ between the plinthic soil forms. The soils, therefore, formed under conditions similar or related to present day conditions.

Interpretation of digital soil photographs using spatial analysis: II. Application

Colour variation in digital photographs of soils was quantified and classified meaningfully, using ArcView Spatial Analyst. This procedure holds great promise for the unbiased and quantitative determination of soil colour properties and statistical correlation with related soil hydrological conditions. The analysis of digital photographs from ten profiles in the Weatherley catchment indicated that diagnostic grey and yellow-brown colours were dominant in all the profiles described for this study. The occurrence of grey increased with depth, at the expense of yellow-brown. This pointed to a wetter subsoil water regime. Diagnostic red dominated in the Bloemdal profile. The occurrence of diagnostic red in profiles of the Longlands, Tukulu, Kroonstad and Katspruit soil forms was associated with the occurrence of Fe oxide mottles. Black classification was used mainly for the elimination of shaded areas from the photographs.

Advances in pedology in South Africa

The ability of soil to serve as a unique buffer of water, organic carbon, nutrients, pH, redox and temperature is recognised in environmental research. A detailed study on hardpan carbonate and dorbank horizons in the commonly occurring ‘heuweltjies’ of the arid Namaqualand coastal region revealed a ‘sepiolitic’ hardpan carbonate centre through ‘sepiolitic’ / ‘petrosepiolitic’ (hardened) to the dorbank horizon on the edges. Footprints of ferrolysis manifests in various properties of duplex and plinthic soils of the semi-arid eastern Free State as (1) acidity, (2) colour, (3) Fe-Mn, (4) mottles and concretions, (5) CEC and (6) abrupt textural differentiation. Soils with melanic A horizons cover 2% of South Africa and are confined to regions with annual rainfall between 550 and 800 mm (aridity index of 0.2–0.5). The understanding of soil formation is applied to divide a land type into soilscapes with a more homogeneous soil distribution pattern. The quantification of the water regime of South African soils showed that ‘freely drained’ soils may have significant periods of drainable water.

Evaluation of the Aardvark constant head soil permeameter to predict saturated hydraulic conductivity.

The saturated hydraulic conductivity (Ksat) is a key parameter for analyzing or modelling water flow and chemical transport in subsurface soil. Several methods were developed over the last thirty years to measure Ksat which considered unsaturated and/or saturated flow of water around the holes where measurements are made. Most methods make use of devices to measure the infiltration rate into the soil and in combination with numerical models predict the Ksat parameter. If maximum infiltration rate exceeds the infiltration capacity, runoff will be the consequence. Therefore maintaining a constant head means the rate of water supplied corresponds to the infiltration capacity. Commonly used devices for infiltration capacity measurements are infiltrometers, disc permeameters, sprinkler infiltrometers and different types of constant head permeameters. This paper evaluates the theory and practical application of the Aardvark constant head soil permeameter (ACHSP) in combination with the Glover equation for field measurement of Ksat above the water table. The Glover equation has been criticized during the eighties because only the saturated flow around the hole was considered in its development. Several papers published since 2002, however, demonstrated that the Glover equation results are relatively close to the results obtained by other models for most practical applications and therefore the use of the Glover equation is justified.