C. W. van Huyssteen

The relationship between subsoil colour and degree of wetness in a suite of soils in the Grabouw district, Western Cape I. Characterization of colour-defined horizons

Soil colour is an easily identifiable property that is invariably used as a parameter in all soil classification systems including the system used in South Africa. To test the hypothesis that soil colour is a reflection of the soil water regime, the physical, chemical, morphological and hydrological properties were measured for soils on three catenas in the Grabouw district, Western Cape. Significant differences were observed between diagnostic red apedal B, yellow-brown apedal B, yellow E and grey E horizons. Fine silt, silt, clay, sum of cations, cation exchange capacity and free iron and aluminium decreased in the sequence red apedal B > yellow-brown apedal B > yellow E > grey E horizons. Average duration of free water saturation was 1.3% for red apedal B horizons, 18.8% for yellow-brown apedal B horizons, 42.4% for yellow E and 54.2% for grey E horizons. This supports the hypothesis that yellow and grey horizons are formed by a process of reduction and leaching.

The relationship between subsoil colour and degree of wetness in a suite of soils in the Grabouw district, Western Cape II. Predicting duration of water saturation and evaluation of colour definitions for colour-defined horizons

Existing colour indices were evaluated to determine their correlation with duration of water saturation, for a hydrosequence in the Grabouw district, Western Cape. Correlation coefficients ranged between 0.31 and 0.63. More simple colour indices were developed. Dry soil colour is a relatively good indicator (r = 0.77) of duration of free water. For this study the equation: Duration of free water = 2.35 x Huedry + 5.79 x Valuedry − 7.31 Chromadry – 27.89 can be used to predict duration of free water in diagnostic red apedal B, yellow-brown apedal B, yellow E and grey E horizons. It seems that the present colour definitions for diagnostic horizons in the South African soil classification system are sufficiently accurate to distinguish meaningfully between these horizons with respect to duration of free water.

The effect of degree and duration of water saturation on selected redox indicators : pe, Fe2+ and Mn2+

It was previously hypothesised that reduction in the soil will set in at 70% water saturation (S0.7). This study aimed to determine the effect of different degrees and durations of water saturation on reduction in soil. Reduction was measured as a decrease in soil oxidisability (pe) and an increase in soluble iron (Fe2+). A yellow brown apedal B horizon from profile 234 in the Weatherley catchment was used in this study. Soil cores were packed to a bulk density of 1.6 Mg m−3 and saturated to S0.6 (60% of the pores saturated with water), S0.7 (70% of the pores saturated with water), S0.8 (80% of the pores saturated with water), and S0.9 (90% of the pores saturated with water). Analyses started three days after water saturation. Samples were analysed every 3.5 days for the first three months and then once a week for another month. The experiment was terminated after 121 days. There was a good correlation between an increase in degree of water saturation and pe (R2=0.95), Mn2+ (R2=0.99) and Fe2+ (R2=0.99) concentration. The tendency was for pe, pH, Fe2+, and Mn2+ to remain relatively stable at S0.6 and to increase in variability as the water saturation increased, with most variability observed at S0.9. The change in redox activity took place between the S0.7 and S0.8 treatments. For this particular soil the highest rate of reduction took place in the S0.9 treatment.

Development of a simple empirical model for predicting maize yields in a semi-arid area

A yield prediction model is necessary, together with long-term climate data, to calculate the long-term yields needed for making a quantitative productivity evaluation of a crop ecotope in the form of a cumulative probability function (CPF). In this study the development of such a model for maize grown on two semi-arid ecotopes, i.e. the Glen/Hutton-Ventersdorp and Glen/Oakleaf-Dipene ecotopes, located in the Free State Province, is described. A fairly reliable empirical relationship was obtained between the total above ground biomass and a water stress index using the results of 22 field experiments on these ecotopes. The model consists of the regression equation which describes this relationship viz. Yb = 15238 ISI + 1067 (R = 0.69) where Yb is the predicted total above ground biomass in kg ha−1 and ISI is an integrated stress index for a particular growing season. The stress index is based on the ET/Eo relationship during the growing season.

Principles of soil classification and the future of the South African system

Humans classify their environment to create order, make it understandable, aid recollection and to communicate. The nature of these classifications is not always understood, because they are learnt from an early age. Building on these principles provides a sound basis for any scientific classification. This paper explores these principles, those of the USDA Soil Taxonomy, the World Reference Base for soil resources, and the South African Soil Taxonomy. Knowledge should be ultimate aim of soil classification. A hierarchical system with four levels is proposed for the South African Soil Taxonomy. This can easily be achieved by adding a higher level, proposed to be called a Soil Group, to the current three levels (form, family, and phase). The South African Soil Taxonomy must guard against too many taxa, because humans have a limited ability to comprehend numerous taxa. The distinguishing criteria between taxa should be more clearly defined, while at the same time guarding against becoming too data hungry. The classification should not shy away from intergrades. The object being classified (soils) is a natural system and intergrades will necessarily occur. It is proposed that these should be classified as intergrades, rather than trying to artificially separate natural soil bodies.

Soil water variability in the Weatherley grassland catchment, South Africa: II. Soil water content

Abstract Soil water content is a major factor that affects the hydrological response of a hillslope or catchment. It is therefore important to have reliable soil water content data to estimate catchment water yield. Daily soil water content (θ) data was calculated based on weekly measured and other data for the Weatherley grassland catchment in South Africa. A modelling procedure, based on the soil water balance equation and the interpretation of the physical properties of soils was used to calculate daily θ for all 28 sites for the six-year period. A statistical model performance indicated that the mean index of agreement was 0.88, root mean square error (RMSE) was 6.8 mm water per 300 mm soil and mean unsystematic RMSE to total RMSE was 93%. These results indicated that the calculated soil water contents agreed well with the measured values and could therefore be used with reasonable confidence to fill data gaps. The proposed procedure therefore affords the possibility to increase the resolution of irregular measured soil water content data. This would significantly advance the usability of such data, because the influence of rainfall events on soil water content is frequently missed by manual soil water content measurements.

A review of advances in hydropedology for application in South Africa : review article

Abstract Hydropedology is internationally a new and fast-growing science. It is therefore pertinent to take heed of developments in this regard to guide current and future research in South Africa. This paper aims to discuss the initiation and advances in the interdisciplinary science of hydropedology. Hydropedology has been defined to interlink pedology, soil physics, and hydrology to bridge scales and to transform soil survey data into soil hydraulic information. As such hydropedology could contribute to the understanding of various environmental and ecological issues. In this regard pedotransfer functions relate simple soil characteristics (e.g. morphological features) to more complex parameters (e.g. soil hydraulic properties) that are relatively difficult to measure. The hydrology of soil types (HOST), published in the United Kingdom to aid hydrological studies and analyses, is a good working example of a pedotransfer function. HOST is based on three conceptual models of water flow processes in the soil: soil on a permeable substrate with a deep aquifer (>2 m); soil on permeable substrate with a shallow water table (<2 m); and soil with an impermeable or semi permeable layer within 1 m of the surface. The three conceptual models are further subdivided into 11 models, which can be subdivided into 29 HOST classes. A useful tool for the summation of long-term soil water content data is the temporal stability of spatially measured soil water contents which is defined as the time invariant average of spatially measured soil water contents and offers a valuable method to study the relationship between soil and water. It has been used to reduce the number of measurements needed in catchment characterization and is therefore also a valuable tool in site selection. Understanding redox reactions in soil and the influence thereof on the development of redoxi-morphic colour patterns, are vital in discerning the relationship between soil water regime and soil morphology. Redoximorphic colour patterns are manifested as redox depletions and/or redox accumulations. The former is evident as grey matrices, cutans, mottles, or pore linings, while the latter is evident as Fe masses, pore linings, nodules, or concretions.

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.