Department of Soil Science

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Acidification of sands in citrus orchards fertilized by drip irrigation
(Stellenbosch : Stellenbosch University, 2002-12) Kotze, Theunis Gerhardus; Fey, M. V.; Hoffman, J. E.; Stellenbosch University. Faculty of AgriScience. Dept. of Soil Science.
ENGLISH ABSTRACT: The use of drip irrigation in citrus orchards is becoming increasingly important in the Citrusdal region of the Western Cape. Drip irrigation provides an opportunity to optimize water and fertilizer use by inducing a smaller root zone that can be managed more effectively. Ammoniacal fertilizers are an integral part of any drip fertilization (fertigation) programme. However, a disadvantage is that they generate soil acidity upon nitrification. If insufficient plant uptake of nitrate occurs during the fertigation season, soil acidification may become an important yield-limiting factor. This study investigated the effect of drip fertigation on four sandy soils with different buffer capacities, near Citrusdal. Spatial variation in soil chemical properties below irrigation emitters was investigated to determine the nature and extent of soil acidification from drip fertigation. The response ofthe four sands to acid and base addition (and laboratory incubation) was also studied in order to assess the magnitude and origin of pH buffering. The soil types in the four orchards included two poorly buffered and two moderately buffered soils. Both the poorly buffered soils, at Brakfontein and Swartvlei, were classified in the Kroonstad form and contain less than 5% clay. The two moderately buffered soils, at BoHexrivier and OnderHexrivier, were classified in the Vilafontes and Constantia forms, respectively, and contained more than 10% clay in the subsoil. X-ray diffractometry revealed that kaolinite and quartz dominate the clay fraction of all four soils. Organic carbon content in the topsoils ranged from 0.2-0.98 percent. Drip fertigation for a period of between 4 and 10 years at all four sites has resulted in large decreases in soil pH to depths of up to 1 m below the emitter with a sharp increase in acid saturation at pHKc1 values below 4.5. The Brakfontein and Swartvlei orchard soils were the least acid saturated with levels of about 50-60 percent. At the BoHexrivier site acid saturation values of 70 percent were recorded for the topsoil immediately below the emitter, while the whole soil profile of the OnderHexrivier site had extreme acid saturation levels, exceeding 90 percent in the deeper parts of the profile. It was also revealed that a large proportion of the 1M KCI-extractable acidity \ consisted of A~ although Al saturation showed a poorer relationship to pHKCIthan did acid saturation. The Al component of exchangeable acidity in the OnderHexrivier subsoil was significantly larger [Al = 0.84(acidity)] than in the other soils [AI = 0.69(acidity)]. It was found that most of the wetted soil volume was deficient in exchangeable Ca, Mg, K and enriched with Al. No accumulation of NH/ or N03- was found in any of the soils directly below the emitter, indicating either a sufficient degree of nitrification or the leaching of both NH/ and N03- to greater depths. The mobile anions cr and sol- also appeared at the periphery of the wetting front. Phosphate generally accumulated in the soil just below the emitter, except in two of the soils where P showed some degree of leaching. Spatial variation in soil chemical properties indicated that nitrification and over-irrigation had resulted in a significant volume (between 0.1 and 1.1 rn') of severely acidified soil (pHKcl <4.5) below the emitter at all four study sites. Buffering in these naturally acidic sandy soils from the Citrusdal area is weak as a result of the low clay and organic matter contents. The low content of clay, dominated by kaolinite and quartz, implied that organic carbon plays an important role in pH buffering, especially in the topsoils. Laboratory incubation with acid or base confirmed the fact that CEC becomes increasingly saturated by acidic cations (Ir and Ae+) once soil pHKCIvalues decrease below 4.5. Again Al was found to be the major acidic cation [AI = 0.69(acidity)], especially in the subsoils. This confirmed that, even in these poorly buffered, quartz-rich sandy soils, toxic amounts of Al could enter the soil solution quite rapidly following acidification. Lime requirement calculated from the slope of titration curves following incubation provided a useful way of assessing the magnitude of the acidification problem, even though liming the acidified subsoil may present practical difficulties under field conditions in drip-fertilized irrigation systems. These lime requirement values, ranging from 0.9-10.3 tonnes of CaC03/ha, can be applied to field conditions with some calibration refinements.
Alteration of the soil mantle by strip mining in the Namaqualand Strandveld
(Stellenbosch : University of Stellenbosch, 2005-03) Prinsloo, H. P.; Fey, M. V.; Ellis, F.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.
The purpose of this study was to investigate and identify the occurrence of specific soil properties that may be important for vegetation functioning and the possible effect of the loss of or changes in these properties on rehabilitation success on the sandy coastal plains of the West Coast, South Africa. The study area covered approximately 9 400 ha on the Namaqualand coast in the vicinity of Brand-se-Baai (31º18'S 17º54'E), approximately 350 km north of Cape Town and 70 km north-west of the nearest town, Lutzville. A soil survey was done to reveal the presence of important pedological features. The 20 soil profiles surveyed are situated within six vegetation communities. Pedological features such as surface water repellency, permeable apedal subsurface horizons, subsurface impediments such as cemented (calcrete or dorbank) hardpans and significantly more clayey (cutanic, luvic) horizons were identified. A comparative study between rehabilitated and natural soils indicates that mining operations result in the formation of saline sand tailings, stripped of a large portion of the clay and organic matter fraction. The natural leaching of solutes, over a period of 25 months, is sufficient to lower salinity of the tailings to levels comparable to natural soils. This leaching can also results in lowering of soil fertility. Removal of the dorbank and the dense neocutanic horizon in the western side of the mine, loss of topographical features such as small dune systems and heuweltjies, destruction of natural soil profile morphology and the lowering of organic carbon and clay plus silt fraction can have detrimental effects on attempts at rehabilitation of this area to a natural condition similar to that which preceded the mining operation. Infiltration fingering and deep percolation results in the development of an aquifer below the reach of shallow-rooted desert shrubs. A method of water acquisition by vegetation through water distillation is investigated as a possible solution to the apparent discontinuum between the shallow root systems and deeper-lying aquifer. Volumetric water content measurements indicated that precipitation of 29.5 mm, over a period of 10 days, did not result in any variation at 235 mm, 360 mm and 900 mm depths. An average volumetric water content increase of 0.4 mm per night was measured in the first 23.5 cm of soil surface. This amount is a significant source of water that can explain the shallow root distribution. Water vapour movement due to temperature gradients can explain the diurnal volumetric water content fluctuations observed. Further studies are necessary to determine to what extent the depth of water infiltration influences the capacity of subsurface dew to provide plants with a nocturnal water source. Findings of this study can be summarised into two concepts namely: • Heuweltjies, small dune systems, and variation in depth of cemented hardpans are the main features that contribute to pedosphere variation and possibly to biodiversity. • Pedogenic features such as topsoil hydrophobicity, and cemented dorbank and dense more clayey (cutanic, luvic) subsurface horizons are important components of a soil water distillation process that could be a driving force behind vegetation functioning in this region. Mine activities result in the loss of certain pedogenic features and soil properties that that could be key ingredients to ecosystem functioning. The inability to recognise their significance and ignorance thereof when planning rehabilitation methods might prevent sustainable restoration of the environment.
Annual dynamics of winery wastewater volumes and quality and the impact of disposal on poorly drained duplex soils
(South African Society for Enology and Viticulture, 2018) Mulidzi, A. R.; Clarke, C. E.; Myburgh, P. A.
The composition and volume of winery wastewater change throughout the year. Quality is usually at its worst when vintage operations are dominated by the production of red wines. The objective of this study was to investigate the annual dynamics of winery wastewater volumes and quality, as well as the effect of winery wastewater irrigation on the chemical soil properties of a poorly drained duplex soil. The study was conducted over two and half years. The wastewater contained a high concentration of potassium (K+) and low levels of sodium (Na+). The results of the study confirmed that winery wastewater did not comply with South African national legislation for wastewater irrigation with regard to chemical oxygen demand (COD) and pH throughout the study period, while some prominent spikes were observed in sodium adsorption ratio (SAR) and electrical conductivity (EC). Land application of winery wastewater resulted in the accumulation of high levels of K+ in the soil. It is important to note that this study represents the worst-case scenario, i.e. large amounts of wastewater were disposed of on a small surface, particularly during harvest and in winter. Due to the high volumes of wastewater irrigation plus rainfall, the inevitable over-irrigation leached large amounts of K+ beyond 90 cm soil depth. Unfortunately, the leached elements are bound to end up in natural water resources, resulting in the pollution of the environment.
Assessing the influence of DEM source on derived streamline and catchment boundary accuracy
(South African Water Research Commission, 2019) Mashimbye, Zama Eric; De Clercq, Willem Petrus; Van Niekerk, Adriaan
Accurate DEM-derived streamlines and catchment boundaries are essential for hydrological modelling. Due to the popularity of hydrological parameters derived mainly from free DEMs, it is essential to investigate the accuracy of these parameters. This study compared the spatial accuracy of streamlines and catchment boundaries derived from available digital elevation models in South Africa. Two versions of Stellenbosch University DEMs (SUDEM5 and DEMSA2), the second version of the 30 m advanced spaceborne thermal emission and reflection radiometer global digital elevation model (ASTER GDEM2), the 30 and 90 m shuttle radar topography mission (SRTM30 and SRTM90 DEM), and the 90 m Water Research Commission DEM (WRC DEM) were considered. As a reference, a 1 m GEOEYE DEM was generated from GeoEye stereo images. Catchment boundaries and streamlines were extracted from the DEMs using the Arc Hydro module. A reference catchment boundary was generated from the GEOEYE DEM and verified during field visits. Reference streamlines were digitised at a scale of 1:10 000 from the 1 m orthorectified GeoEye images. Visual inspection, as well as quantitative measures such as correctness index, mean absolute error, root mean squares error and figure of merit index were used to validate the results. The study affirmed that high resolution (<30 m) DEMs produce more accurate parameters and that DEM source and resampling techniques also play a role. However, if high resolution DEMs are not available, the 30 m SRTM DEM is recommended as its vertical accuracy was relatively high and the quality of the streamlines and catchment boundary was good. In addition, it was found that the novel Euclidean distance-based MAE and RMSE proposed in this study to compare reference and DEM-extracted raster datasets of different resolutions is a more reliable indicator of geometrical accuracy than the correctness and figure of merit indices.
Assessing the potential of using microcomputerized tomography to determine the physical properties of different textured soils
(Stellenbosch : Stellenbosch University, 2020-12) Hartnick, Emke Kim; Du Plessis, Anton; Lategan, Eugene; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: The physical properties of soil, particularly the structure, texture, and pore geometry affect hydraulic conductivity in soils. The hydraulic conductivity is an important parameter for understanding the flow of water through different soil types for determining irrigation rates, monitoring of groundwater, and runoff. The hydraulic conductivity is a highly variable soil property and there are several methods for determining the hydraulic conductivity in soils. Each has its advantages and limitations. Traditional methods are time-consuming and the application of X-ray micro-computer tomography (microCT) offers fast and effective analysis and results. The additional use of microCT for this purpose might improve the accuracy of laboratory constant hydraulic head tests if these methods are used in combination. The application of microCT technology can visualize the internal pore geometry of a soil sample without destroying the soil sample. Thus, the microCT technology can examine the porosity and the pore connectivity in soils, which influence the hydraulic conductivity. The purpose of this study was to apply microCT technology to examine the range within it can be used to determine the saturated hydraulic conductivity (Ksat) and porosity of homogeneous and heterogeneous soils. This was done in the context of considering if the advantages of this application (fast and efficient analysis) exceed the disadvantages (cost). The investigation of an effective sampling method for soil samples for X-ray microCT scans and associated image-based analysis were examined. The saturated hydraulic conductivity (Ksat) and porosity of five different soil types (homogeneous and heterogeneous soil) will be obtained through standard laboratory methods (constant hydraulic head test and calculating the porosity from the bulk and particle density) and microCT image-based simulations and analysis. Additional calculation of the Ksat based on grain size theoretical equations such as Hazen and Kozenzy & Carman equation will be used. The results showed that the Ksat of the homogeneous soil very coarse, coarse, and medium sand were underestimated by the microCT simulations. The Ksat of the fine and very fine sand determined through microCT simulations and constant hydraulic head tests compared well. MicroCT simulations underestimated the Ksat of the heterogeneous soil. The porosity values for the homogeneous soil were overestimated by microCT image-based analysis. The porosity values of the heterogeneous soil obtained from microCT image-based analysis for the coarse and pure fine sand were higher than the sandy clay loam, sandy loam and sandy clay soils. The sandy clay loam, sandy loam and sandy clay soils porosity were measured inaccurately by microCT due to resolution limitations. The Ksat of the homogeneous soil analysed by microCT simulation, however, fell within the same order of magnitude as the Ksat obtained from the constant hydraulic head test. Fundamentally, the microCT technology demonstrated great capabilities for analysing both the Ksat and porosity of homogeneous soils. The microCT application is best used for soils with larger particle sizes due to image resolution limits. The 2D visualization of the microCT scans can be useful for investigating structural changes within a soil sample caused by laboratory analysis. Such analysis may include constant hydraulic head tests or the examination of soil samples after sample preparations.
An assessment of winery wastewater diluted for irrigation of grapevines in the Breede River Valley with respect to water quality and nutrient load
(South African Society for Enology and Viticulture, 2015) Howell, C. L.; Myburgh, P. A.; Lategan, E. L.; Hoffman, J. E.
Possible re-use of winery wastewater for irrigation was investigated in a field trial with micro-sprinklerirrigated Cabernet Sauvignon/99 Richter in the Breede River Valley region of South Africa. Irrigation with winery wastewater diluted to 100, 250, 500, 1 000, 1 500, 2 000, 2 500 and 3 000 mg/L chemical oxygen demand (COD), respectively, was compared to irrigation with raw river water. Since the pH was lower than 6, the diluted wastewater could cause nutrient toxicity. The diluted winery wastewater did not pose any salinity hazard, as the electrical conductivity was well below 2 dS/m. For the given range of dilutions, the sodium adsorption ratio never exceeded 10, which indicates that the water posed no sodicity hazard. Sodium and Cl- never exceeded 115 and 150 mg/L, the respective upper thresholds for grapevines. With the exception of N, levels of H2 PO4 - , K+ , Na+ , Ca2+, Mg2+, HCO3 - , SO4 2- and B3+ in the diluted wastewater increased with a decrease in dilution level. The N load in diluted winery wastewater appeared to be completely inadequate to supply the grapevine’s requirements. In contrast, the P load in the winery wastewater diluted to 2 500 mg/L COD and higher would supply more than adequate P if the grape yield amounts to 10 t/ha. Likewise, the dilution of winery wastewater to 250 mg/L COD and higher would supply more than adequate K+ if the grape yield amounts to 10 t/ha. However, K+ applied via the wastewater will only be beneficial if it is not leached from the root zone during winter.
Attenuation of ionic pollutants in selected South African soils
(Stellenbosch : University of Stellenbosch, 2007-03) Mwepu, Mireille K. M.; Fey, M. V.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.
Two–thirds of South Africa, including more than 280 towns and settlements are largely dependent on groundwater for their drinking water supply and development. However, groundwater resources in South Africa are limited both in terms of quantity and quality, especially in the semi–arid parts of the country (Sililo et al., 2001, p. i). Therefore, the importance of protecting groundwater resources from pollution has been recognized. The first objective of this research was to investigate the attenuation capacity of a selection of soil horizons and materials representing major types of diagnostic horizons and materials in the South African soil classification in order to validate their chemical attenuation ratings as provisionally specified by Sililo et al. (2001, p. 4.6). The second objective was to assess the pollutant attenuation capacity of South African soil horizons and materials as well as describe the diagnostic value of key chemical properties of soils for conveying information on their contaminant transport/attenuation potential. The third objective was to investigate whether it is possible to apply acid/base priming using H2SO4 and Ca(OH)2 to a bulk quantity of soil in order to reduce the mobility of contaminants.
Baseline concentration of Cd, Co, Cr, Cu, Pb, Ni and Zn in surface soils of South Africa
(Academy of Science for South Africa, 2005) Herselman, J. E.; Steyn, C. E.; Fey, M. V.
THE AIM OF THIS STUDY WAS TO DRAW up baseline concentrations for several environmentally important trace elements in South African soils. A major inventory of some 4500 soil profiles has been compiled in South Africa over the past three decades and information on chemical composition can now readily be generated for the country as a whole. Selected soil samples from surface horizons were analysed by inductively coupled plasma-mass spectrometry for total (nitric acid-extractable; EPA method 3050) and available (NH4EDTA-extractable) fractions of Cd, Co, Cr, Cu, Ni, Pb and Zn. A baseline concentration range (defined as 95% of the expected range of background concentrations) was calculated for each element from geometric means and standard deviations after the data set was normalized by log 10 transformation. These supposedly natural, baseline values were used to revise South African guidelines and to set the total investigation level and the total maximum threshold level in each case as follows: Cd 2 and 3; Co 20 and 50; Cr 80 and 350; Cu 100 and 120; Ni 50 and 150; Pb 56 and 100; and Zn 185 and 200 mg kg-1, respectively. Four-fifths of all soils were found to be Zn-deficient, one-third Cu-deficient and one-fifth Co-deficient.
Carbon characteristics and the effect of terrain and cultivation on topsoil carbon and salts of the West Coast heuweltjies
(Stellenbosch : Stellenbosch University, 2023-03) Sakala, Brian Josat; Clarke, Catherine E; Francis, Michele Louise ; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: The 21ˢᵗ century has been marked by global increases in atmospheric carbon dioxide concentrations from annual emissions from land-use conversion and soil cultivation. In addition, the issue of increased salinization of soils is another problem that poses a threat to agricultural production and environmental health. Large (>20 m diameter) regularly spaced earthen mounds called “heuweltjies” (Afrikaans for “little hills”) occur widely in the West Coast region of South Africa with heights between 1 to 2.5 m. These mounds are characterised by significantly higher carbon and salt contents compared to surrounding soils. Therefore, this study was conducted to determine the total carbon characteristics and the effect of terrain and cultivation on topsoil carbon and salts of the West Coast heuweltjies. Firstly, the total carbon characteristics of three previously excavated heuweltjies in three differing rainfall environments in the Buffels river (mean annual rainfall <100 mm/year), Klawer (mean annual rainfall between 100-200 mm/year) and Piketberg (mean annual rainfall between 300- 400 mm/year), were investigated. The analyses focused on soil bulk density, total carbon (total C), and soil organic carbon (SOC) content as well as carbon stocks. Both rainfall and termite foraging activities affected the horizontal and vertical distribution of carbon in all three areas. The vertical distribution of SOC to soil inorganic carbon (SIC) were inversely related, with SOC higher in the upper horizons and SIC increasing with depth. The influence of termite foraging activities on the depth distribution of SOC was observed in the subsoils of the heuweltjies, which was unlike the usual exponential decrease in SOC with depth in a normal soil profile. The subsoils showed localised elevated contents of SOC with depth, attributed to translocation of organic debris by termites and burrowing animals in the mounds. This signified the importance of subsoil studies in accounting for C stocks in termite affected landscapes. Further, the contribution of heuweltjies to total C and SOC stocks was calculated based on the area-weighted land coverage of the mound and inter-mound soils of the Klawer heuweltjie. The amount of total C stored was found to be 14.6 Mg C per mound and SOC was estimated to be 1.1 Mg C per mound. Thus, for the Klawer region, the total carbon stored per hectare is 102 Mg and 7.5 Mg of SOC, suggesting heuweltjies are an important store of atmospheric carbon in global carbon sequestration processes. Secondly, the effects of cultivation on topsoil pH, salinity and carbon were investigated across two rainfall zones in Trawal (mean annual rainfall between 50-200 mm/year) and Koringberg (mean annual rainfall between 300-400 mm/year). The analyses focused on pH, EC, and carbon content in the top 0-10 cm and 0-20 cm at the two study sites, respectively. Adjacent heuweltjies in cultivated and uncultivated land were selected for comparisons. Generally, differences occurred between land use and sampling position. Cultivation practices resulted in an increase in pH of the inter-mound soils by approximately 1.3 units in comparison to the inter-mound soils of uncultivated land. The impact of cultivation on salinity did not provide enough evidence to suggest a lateral distribution of salts within the landscape. The salts were generally higher on than off the mound soils at both study sites, a result consistent in this study. The study also showed that carbon content was negatively affected by cultivation, resulting in approximately 50% loss of total C, SOC, and C stocks in cultivated land. In addition, a lateral uniform distribution of carbon was observed on and off the heuweltjies in cultivated land, suggesting a reduction in termite driven SOM inputs in the mound soils. The carbon stocks in uncultivated soils were found to be generally higher on than off the mounds following the order moderate to low rainfall site. Finally, the effect of terrain on salt load, salt type and distribution of carbon content on and off the heuweltjies across a cultivated hillslope was investigated. The results for the spatial distribution of carbon on and off the heuweltjies showed an inconsistent trend of either higher or lower carbon content with distance downslope. This unclear pattern did not provide enough evidence to support the hypothesis of a reduction in the carbon content on the mounds from crest to footslope. The spatial distribution of salts showed a trend of higher salts on mounds in comparison to inter-mound soils reaffirming earlier results suggesting that cultivation does not have a major influence on salt movement across the landscape. This trend did not support lateral movement of salts downslope. Furthermore, based on the comparison of the heuweltjie salt signature to that of the seawater dilution line, and despite many decades of cultivation and reduced termite activity, the heuweltjies still maintained a marine signature, adding more evidence to the hypothesis suggesting that the salts are derived from marine deposited aerosols. Concentrated hotspots of ions and minerals within the biogenic features of mound soils suggested that burrowing fauna are responsible for accumulating marine-derived salts in heuweltjie soils. The results of this study, based on the difference in SOC concentrations of the heuweltjies in cultivated and uncultivated land, have shown that heuweltjies are an important store of carbon, which when cultivated become a source of atmospheric carbon dioxide (CO2). In addition, the results presented are relevant for land-use change and management strategies that promote carbon storage in drylands. Also, the results showed that tillage has no major influence on salt distribution across the landscape, or that shallow lateral flow is an important contributor to dryland salinity.
Carbonate-bearing eruptives between the Great Karas Mountains and the Bremen igneous complex, South West Africa
(Stellenbosch : Stellenbosch University, 1975-12) Schreuder, C. P.; Verwoerd, W. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT:Minor carbonate-bearing bodies of igneous origin are widely distributed in the Karasberg district, South West Africa. The area in which they occur consists of granites and gneisses of the Namaqualand Metamorphic Complex overlain by relatively flat strata of the Nama and Karoo Groups and intruded by plutonic and hypabyssal rocks of various ages. The latter include a single post-Karoo carbonatite. The largest concentration of carbonate-bearing eruptives is on the farm Garub 266 in the Great Karas Mountains, but they extend approximately 100 km westwards as far as the Bremen Alkaline Complex. The Karas Mountains are now believed to be the result of a series of thrust-faults which may perhaps be associated with the intrusion of plutonic complexes of the Kuboos-Tatasberg-Bremen-Haruchas lineament, to which the Garub eruptives may also be related. The Garub-type pipes, dykes and sills are composed of alkaline-ultrabasic carbonate-bearing breccia, lamprophyric carbonate rock and tuffisite. They intrude rocks of the Namaqualand Metamorphic Complex and the Kuibis, Schwarzrand and lower Fish River Formations of the Nama Group, and are considered to be subvolcanic. The bodies contain between 10 and 20 per cent co2 and about 25 per cent Si02 and are obviously not typical carbonatite. Biotite, pyroxene, amphibole and ilmenite form both phenocrysts and fine-grained ccy.stals in a groundmass of ankerite. Minute ankeritised lath-shaped crystals (either melilite or feldspar originally}, are almost invariably present in the lamprophyric carbona~e rock. Interstitial quartz and feldspar occur sporadically, whereas inclusions of wall-rock, where present, are usually abraded and rounded. Fenitisation has been observed at two localities, where quartz and feldspar in the wall-rock have been replaced by soda amphibole. Fluidisation appears to provide a satisfactory mechanism for the emplacement of these bodies. Strong evidence in favour of this interpretation are the intrusive contacts, abraded and rounded inclusions, nondilational veins in the wall-rock, accretionary pisolites, upward and downward movement of inclusions in the bodies, carbonated inclusions and matrices . and the absence of contact or pyrometamorphic effects. Chemically the carbonate rocks bear similarities to kimberlite and olivine-melilitite, but are most akin to alnoite. Carbonatite, olivine-melilitite, kimberlite, alnoite, damkjernite and the Garub rocks are all considered to have the same magmatic affinities.· It is tentatively suggested that the Garub suite is genetically related to an unexposed alkaline complex of the Fen type, and that the composition of the carbonate-bearing rocks approaches that of the parent magma of the plutonic complexes along the Kuboos lineament.
Chemical characterisation of the soils of East Central Namibia
(Stellenbosch : University of Stellenbosch, 2009-03) Coetzee, Marina Elda; Ellis, F.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: A number of chemical and physical features of Namibian soils in a 22 790 km2, two degree-square block between 17 – 19 oE and 22 – 23 oS in eastern central Namibia, had been investigated, and the fertility status established. In 80 % of samples the nitrate, nitrite, and sulfate concentrations of the saturated paste extract are 0 – 54.6 mg l-1 NO3 -, 0 – 24.7 mg l-1 NO2 -, and 5.4 – 20.9 mg l-1 SO4 2- respectively. In 90 % of samples the plantavailable phosphorus is below 12 mg kg-1, which is low for a soil under natural grassland, but in line with the prevailing semi-arid climate and low biomass production of the study area. In 80 % of samples the extractable calcium, magnesium, potassium, and sodium concentrations are 0.61 – 5.73 cmolckg-1 Ca (122 – 1 146 mg kg-1 Ca), 0.12 – 2.28 cmolckg-1 Mg (15 – 278 mg kg-1 Mg), 0.13 – 0.54 cmolckg-1 K (51 – 213 mg kg-1 K) and 0.05 – 0.38 cmolckg-1 Na (11 – 87 mg kg-1 Na) respectively, while the exchangeable calcium, magnesium, potassium, and sodium concentrations are 0.21 – 6.02 cmolckg-1 Ca, 0.12 – 2.01 cmolckg-1 Mg, 0.12 – 0.49 cmolckg-1 K and 0 – 0.13 cmolckg-1 Na respectively. The mean ± 1 standard deviation is 3.57 ± 3.57 cmolckg-1 for cation exchange capacity, 3.48 ± 3.61 for sum of exchangeable bases and 4.53 ± 4.39 for sum of extractable bases. The cation exchange capacity and the sum of exchangeable bases are virtually identical, which indicate the almost complete absence of exchangeable H+ and Al3+ in the soils of the study area, as expected from a semi-arid climate. None of the profiles were classified as saline or sodic. In 80 % of samples the plant-available iron, manganese, zinc and copper concentrations are 7.2 – 32.8 mg kg-1 Fe, 13.6 – 207.5 mg kg-1 Mn, 0 – 1.80 mg kg-1 Zn and 0 – 4.0 mg kg-1 Cu respectively. Soil organic matter content of the study area soils ranges between 0.05 – 2.00 %, with most (80 % of samples) containing 0.25 – 1.20 % organic matter. This is considerably lower than values reported in literature, even for other southern African countries. The reason lies with the hot, semi-arid climate. The pH distribution is close to normal, with 80 % of samples having pH (H2O) of 5.54 – 8.18, namely moderately acid to moderately alkaline. Sand, silt and clay content of most (80 %) samples varies between 60.3 – 89.7 % sand, 4.6 – 25.2 % silt and 3.5 – 19.1 % clay. The soils of the study area are mainly sandy, sandy loam and loamy sand. In 80 % of samples the coarse sand fraction ranges from 3.5 – 34.5 %, the medium sand fraction from 20.5 – 37.3 %, the fine sand fraction from 38.7 – 54.5 % and the very fine sand fraction from 0.0 – 12.9 % of all sand. The fine sand fraction, thus, dominates, with very fine sand being least abundant. The topsoil contains relatively more coarse sand and less very fine sand than the subsoil. Instances of sealing, crusting and hardening occur sporadically in the study area. Cracking is only found in pans, while self-mulching is not evident. No highly instable soils were encountered in the study area. The water-holding capacity is generally low, with depth limitations in the western highlands, the Khomas Hochland, and texture limitations in the eastern Kalahari sands. The central area has soils with a somewhat better water-holding capacity, but it is still very low when compared to arable soils of temperate, sub-humid and humid zones elsewhere in southern Africa. Soil characteristics are perceptibly correlated with climate, parent material, topography, degree of dissection of the landscape and position in the landscape. The most obvious differences are between soils formed in schistose parent material of the Khomas Hochland in the west and those of the Kalahari sands in the east. The soils of the study area are unsuitable to marginally suitable for rainfed crop production, due to low iv fertility. The study area is climatologically unsuited for rainfed crop production, so the present major land use is extensive livestock production on large farms. The natural vegetation is well adapted to the prevailing conditions. The methodology followed to delineate terrain units, with a combination of procedures involving digital elevation data and satellite imagery, seems to work well in the Namibian landscape. This study thus served as a successful proof-of-concept for the methodology, which can in future be rolled out for the remainder of the country. The site and analytical information is available in digital format as spreadsheets and in a geographical information system, as well as in a variety of digital and printed maps.
Comparison of calcium ameliorants and coal ash in alleviating the effects of subsoil acidity on maize root development near Middelburg, Mpumalanga
(Stellenbosch : University of Stellenbosch, 2010-03) Awkes, Meryl Mandy; Hoffman, J. E.; Fey, M. V.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Acidic soils are a major limitation to agriculture worldwide. The Highveld in South Africa has many acidic soils and several coal burning power stations. These coal burning power stations generate alkaline fly ash as a waste material and it can thus serve as an ameliorant to the surrounding acidic soils. A two year field trial was undertaken to compare fly ash and other calcium ameliorants to alleviate the effects of subsoil acidity on maize root development. The field trail was established on Beestepan Farm in Middelburg, Mpumalanga. It consisted of 24 treatments, each done in triplicate, rendering a total of 72 plots. The materials used were unweathered fly ash (CCE 10%), calcitic lime (CCE 77%) and Calmasil (a calcium silicate slag, CCE 99%). Calmasil and lime were applied at rates of 0-, 1-, 2-, and 4t/ha, while fly ash was applied at 0-, 7-, 14- and 28t/ha. These treatments were applied to an acidic sandy loam soils in the presence or absence of 4t/ha gypsum. Beans were harvested after the first season following the application of amendments and maize was harvested in the second season. Yield, root length, leaf and soil analysis was undertaken to evaluate the effectiveness of the different liming materials. The effect of the treatments on fertility indicators such as pH, exchangeable acidity, Ca and Mg was investigated. Results indicated that all liming materials increased topsoil pH, soil nutrient and base status and crop yield in both seasons. Calmasil was the superior liming material in all respects. Fly ash increased pH minimally but reduced exchangeable acidity by 12% and 24% in the first and second seasons, respectively. Fly ash increased topsoil Ca levels from 74 to 102mg/kg and subsoil Ca from 61 to 114mg/kg. Topsoil Mg levels were increased from 7.3 to 16mg/kg and subsoil Mg was increased from 9.4 to 13mg/kg. The consequence of these increased nutrients was the subsequent increased foliar uptake of Ca and Mg. The substantial increase in bean yield from 958 to 1724kg/ha and maize yield from 5569 to 7553kg/ha following ash application compared well with results obtained from lime and Calmasil application. This may partly be due to the presence of additional plant nutrients such as P and K in the fly ash. Dissolution behaviour of fly ash indicates that upon exposure to acidity the release of micronutrients like B, Co, Cu, Fe, Mo, Mn and Zn occurs, and preliminary data shows that there is comparatively little concern regarding heavy metal accumulation in crops. The application of 4t/ha gypsum had no effect on pH and decreased subsoil acidity only minimally however, subsoil Ca status and acid saturation levels were considerably improved which would possibly account for the overall beneficial effect on maize yield, increasing by an average of 1071kg/ha. It was not possible to make any conclusions relating treatment application and maize root length. This field trial has confirmed that fly ash can be used as an efficient liming material and that it compares well with traditional liming materials.
Comparison of different methods by means of which water holding capacity of soil is determined and the prediction of water holding capacity from soil texture in coarse-textured soil
(Stellenbosch : Stellenbosch University, 2004-12) Howell, C. L. (Carolyn Louise); Hoffman, J. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Irrigation scheduling is one of the most important cultural practices in irrigated vineyards. Water holding capacity of soil is arguably therefore one of the most important characteristics of a soil as it determines how much water can be made available to the plant. The measurement of water holding capacity of soils is time consuming and costly. In situ determinations are often impractical to determine. For routine determinations, water holding capacity is therefore determined on disturbed samples. Such a method for example is the rubber ring method. A great deal of criticism surrounds this rubber ring method and results are often questioned. The objectives of this study were therefore to determine what the relationship was between undisturbed and disturbed samples and to determine whether compacted samples could give a more accurate representation of the water holding capacity of soil. Soil textural factors influencing the volumetric water content of undisturbed, rubber ring and compacted samples at 5, 10 and 100 kPa were investigated. In addition, soil textural properties influencing water holding capacity of the respective samples between 5 and 100 kPa and 10 and 100 kPa were investigated. The final objective of the study was to develop simple models to predict the volumetric water content and water holding capacity of soil. Undisturbed and disturbed soil samples were taken at various localities to ensure a wide range of textures. Water holding capacity of undisturbed and disturbed samples was determined at ARC Infruitec-Nietvoorbij using the standard air pressure and ceramic plate technique and the routine rubber ring method respectively. Soil samples were also compacted to a bulk density of approximately 1.5 g.cm-3 as a further treatment for determination of water holding capacity using the air pressure and ceramic plate technique. To investigate aspects of soil texture that could possibly influence volumetric water content of the soil, correlations were done between different texture components and volumetric water content of undisturbed, rubber ring and compacted samples at 5, 10 and 100 kPa. In order to determine the effect of texture on water holding capacity of the soil, correlations were drawn between texture components and water holding capacity of undisturbed, rubber ring and compacted samples between matric potential ranges 5 and 100 kPa and 10 and 100 kPa. The results from this study were used to develop models to predict volumetric soil water content and water holding capacity of soils for a range of soils. Volumetric water content of rubber ring samples at 5 kPa was more than the volumetric water content of undisturbed samples at 5 kPa. The volumetric water content of rubber ring samples at 5 kPa and the volumetric water content of undisturbed samples at 5 kPa was correlated by 87%. Volumetric water content of compacted samples at 5 kPa had a 85% degree of correlation with the volumetric water content of undisturbed samples. At 10 kPa, the correlation between volumetric water content determined using rubber ring samples and undisturbed samples was 77%. This was identical to the correlation between volumetric water content of compacted samples at 10 kPa and undisturbed samples. At 100 kPa, most of the rubber ring samples' volumetric water content fell below the 1:1 line of volumetric water content of undisturbed samples. The volumetric water content of all the compacted samples was higher than that of the undisturbed samples. Water holding capacity of all the rubber ring samples between 5 and 100 kPa was greater than the water holding capacity of the undisturbed samples between 5 and 100 kPa. Rubber ring samples therefore generally overestimated the water holding capacity of the soil. The water holding capacity of most of the rubber ring samples between 10 and 100 kPa was greater than the water holding capacity of the undisturbed samples. In contrast, the water holding capacity of compacted samples between 5 and 100 kPa was less than the water holding capacity of undisturbed samples between 5 and 100 kPa. Water holding capacity of compacted samples was therefore underestimated. The results from this study confirmed that the influence of clay and silt content on volumetric water content of undisturbed, rubber ring and compacted samples increased as the suction on the respective samples is increased. The influence of fine sand content on volumetric water content of undisturbed, rubber ring and compacted samples decreased with an increase in matric potential to 100 kPa. Medium sand content of undisturbed, rubber ring and compacted samples had the greatest influence of all the textural components on the volumetric water content of the respective samples at 5 kPa and 10 kPa. Water holding capacity of undisturbed, rubber ring and compacted samples between 5 and 100 kPa was greatly influenced by the fine sand content of the samples. Medium sand content of the samples also had an influence on the water holding capacity thereof. To predict the volumetric water content of undisturbed samples at 5, 10 and 100 kPa, the independent variables were fine sand content, square root of medium sand content and In of medium sand content. In the case of models to predict the volumetric water content of rubber ring samples at 5, 10 and 100 kPa, the same variables were used as independent variables. Additional variables such as silt content, the In of silt content, square root of clay plus silt content and the medium sand content. To predict the volumetric water content of compacted samples at 5, 10 and 100 kPa the terms used were silt content, clay plus silt content, the e-clay plus silt content. medium sand content and the square root of medium sand content. The models to predict volumetric water content of rubber ring samples gave the best correlation with the actual volumetric water content of rubber ring samples. The final models to predict the water holding capacity of all the samples between 5 and 100 kPa and 10 and 100 kPa used only fine and medium sand parameters as independent variables. Soil textural components do play an important role in determining the volumetric water content of undisturbed, rubber ring and compacted samples at 5, 10 and 100 kPa. The magnitude of the water holding capacity between 5 and 100 kPa and 10 and 100 kPa is also influenced by soil texture. The models developed to predict the volumetric water content of samples at 5, 10 and 100 kPa and the magnitude of the water holding capacity between 5 and 100 kPa and 10 and 100 kPa could be very useful. Both time and money can potentially be saved. Models that can be highly recommended are the models generated for the undisturbed samples. These are: At 5 kPa, VWCu = 0.47259 - 0.04712 medium sando.s At 10 kPa, VWCu = 0.41292 - 0.04221 medium sandos At 100 kPa, VWCu = 0.48080 - 0.00254 fine sand - 0.0865 In medium sand Between 5 and 100 kPa, WHCu = -29.523 + 3.394 fine sand Between 10 and 100 kPa, WHCu = -891.794 + 232.326 In fine sand + 38.006 In medium sand
Comparison of nutrient use efficiencies in citrus under different fertilization and irrigation management practices
(Stellenbosch : Stellenbosch University, 2024-03) Parker, Imaan; Hardie-Pieters, Ailsa G.; Raath, Pieter J.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Excessive applications of nitrogen and phosphorus can lead to soil acidification, runoff, leaching and groundwater contamination. Additionally, the production of synthetic nitrogen fertilisers is an energy- intensive process, while phosphorus fertilisers are derived from non-renewable sources. This underscores the need for the citrus industry to adopt more sustainable management practices, with focus on improving nutrient use efficiency (NUE). Despite extensive research on the NUE of annual crops such as cereal grains and maize in South Africa, little research has been conducted on perennial crops such as citrus. Additionally, there is a need for NUE benchmarks that producers can use to compare their own NUE values with to. The main aim of this study was therefore to gain an understanding of the factors affecting nutrient use efficiency (NUE) of citrus, and to quantitively compare the NUE of different production systems currently used within the citrus industry. Various NUE indices were evaluated over two production seasons, in four of the main citrus production regions of South Africa. Within each region, three to four different irrigation-fertiliser production systems were selected, namely, conventional drip [1.6 L h-1] with fertigation (Drip), low-flow continuous drip [< 1 L h-1] with fertigation (LowFlow), micro-sprinkler irrigation with fertigation (MicroFert) and micro-sprinkler irrigation with granular fertiliser (MicroGran). There were significant differences between production systems in terms of partial factor productivity (PFP), partial nutrient balance (PNB) and internal utilization efficiency (IE). The MicroGran treatment consistently had higher PFP in both seasons of the study. Furthermore, the results showed that production systems (treatments) that applied excessive fertiliser rates or had low yields, exhibited significantly lower PFP and PNB values, this was seen in the LowFlow production systems (treatments) during both seasons. In the 2021/22 season the MicroGran treatment exhibited the highest average yield (62 ton ha-1), while in the 2022/23 season the MicroFert treatment obtained the highest average yield (81 ton ha-1). No significant differences were found between PFP and PNB values among the Drip, MicroFert and MicroGran production systems (treatments), when looking at the main effects of production systems (treatments). When yields between 50-80 ton ha-1 are obtained, producers can expect to achieve PFPN of 406- 424 kg kg-1, and PNBN values 0.68-0.76 kg kg-1, when using Drip, MicroFert and MicroGran production systems. Whereas for LowFlow production systems (treatments) can expect PFPN and PNBN values of 178 and 0.30 kg kg-1. Additionally, the study calculated indices that make use of data from a control plot which received no fertiliser. These indices include agronomic efficiency (AE), crop recovery efficiency (RE) and physiological efficiency (PE). These indices showed the crop response specifically to the fertiliser applied. Results highlighted the significance of interpreting leaf analysis and NUE along with soil analysis, as there were some significant correlations between soil properties and these factors. Soil analysis is especially important for interpreting PNB values. Furthermore, the study showed that NUE is not only dependent on the fertiliser applied but also influenced by mineralizable nutrients in the soil and the remobilization of nutrients stored within the plant biomass. To achieve optimal NUE, it is important to consider all these factors.
The concentration of selected trace metals in South African soils
(Stellenbosch : University of Stellenbosch, 2007-12) Herselman, Jacoba Elizabeth; Fey, M. V.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.
Trace elements occur naturally in soils, usually at low concentrations (<0.1% or <1000 mg kg-1 of the earth’s crust), as a result of weathering and pedogenic processes acting on the rock fragments from which soil develops (parent material). Since about 98% of human food is produced on land, soil is the primary source supplying these elements to the food chain. Although cases of trace element deficiency and toxicity have been documented in many parts of South Africa, no comprehensive description of trace element concentration has yet been attempted for South Africa as a whole. The Natural Resources Land Type mapping project, initiated in the mid-1970s, has provided a collection of samples (approximately 4500) from soil profiles selected to represent the main soil forms in each land type and therefore to provide representative coverage of most of the soils of South Africa. These archived samples have now been analysed for a spectrum of trace elements, in terms of both available and total concentrations as well as other soil properties. Although detailed information is available on a wide range of trace metals, the seven trace metals considered to be of most interest in a South African context due to natural geological occurrences were selected for this study, including Cd, Co, Cr, Cu, Pb, Ni and Zn. This data was used to: • determine baseline concentrations in SA soils; • determining threshold values for South African agricultural soils receiving sewage sludge at agronomic rates; • determining the influence of certain soil properties on the baseline concentrations of these trace elements in SA soils; and • development of a bioavailable trace element distribution map for SA. The range, the mean and standard deviation (both arithmetic and geometric), and the median were used to summarize the data statistically. The baseline concentration range was calculated using the quotient and product of the geometric mean and the square of the geometric standard deviation, including data below the instrument detection limit. The upper limit of the baseline concentration range was set at the 0.975 percentile value of the population in order to minimize the influence of contamination and the lower limit at the 0.025 percentile value to minimize problems that might be associated with analytical uncertainty near the lower limit of detection. The quantile regression statistical approach was followed to illustrate the relationship between soil properties and trace element concentrations in soils. The soil properties that showed the strongest relation were CEC, clay content, pH (H2O) and S value (base status). The soils were then divided into different classes according to these soil properties and baseline concentrations were derived for the different classes. Soils with low clay contents have lower trace element concentrations than soils with higher clay contents, soils with low or high pH levels have lower trace element contents than soils with intermediate pH values and mesotrophic soils have higher trace element concentrations than dystrophic soils. This information is useful for the compilation of trace element distribution maps for South Africa where different soil forms and series/families could be classified into different classes to determine areas of potential deficiencies as well as toxicities. South Africa, with its diverse geology, has areas of both trace element toxicities and deficiencies and for decision-making purposes it is necessary to identify these areas. Mapping of trace element levels based on soil samples would provide valuable information, which cannot be obtained from geological or geographical maps. Statistical analyses of the data (clay %, base status, pH (H2O) and NH4EDTA extractable trace element concentrations) indicated that soils could be divided into five trace element classes based on their clay content, pH and base status (dystrophic, mesotrophic and eutrophic). The soil series according to the binomial soil classification system for South Africa were then divided into these different classes. The geometric means for each clay class were determined and the baseline concentration range for each class was calculated. The land type maps were used as basis for the distribution maps. A general trace element distribution map for South Africa was derived from this data as well as Cu and Zn distribution maps. A random selection of 500 soil samples across the country was used to verify the accuracy of the distribution map. The general trace element distribution map indicate, with a confidence level between 89 and 96%, where the potentially available trace element content of South African soils are low (deficient) too moderately high, excluding rocky areas and areas with limited soil. The Cu and Zn maps indicate the distribution and expected baseline concentrations of these specific elements in South African soils. The same methodology could be applied to derive risk maps for all the individual trace elements to indicate the distribution and expected baseline concentrations of the elements in South Africa. This presentation of baseline concentrations, reflecting likely natural ranges in South African soils, is the first quantitative report on the spatial extent and intensity of Zn, Cu and Co deficiency in South African soils. The proposal of new threshold values for trace elements in agricultural soils will be valuable in setting more realistic norms for environmental contamination that accommodate the geochemical peculiarities of the region, one example being rather high Cr and Ni concentrations with low bio-availability. This information should be of value not only in environmental pollution studies but also in health, agriculture, forestry and wildlife management. The following recommendations are made: • The baseline concentrations could be used to determine site specific threshold values based on soil properties and soil type. Soils with lower pH, clay content and CEC would require more protection than soils with high pH, clay content and CEC and therefore the threshold levels for these soils should be lower. • Although the distribution maps can be used to indicate broad areas of trace element deficiencies and toxicities, more detailed investigations are recommended for areas where problems are experienced. The same methodology could be applied on smaller scale to increase the value of the map and to add more value on a regional scale. The maps could be used for regional soil quality assessment especially in areas where trace element deficiencies or toxicities could result in negative effects on plants and animals.
The contribution of indigenous vegetables to food security and nutrition within selected sites in South Africa
(Stellenbosch : Stellenbosch University, 2013-12) Mavengahama, Sydney; De Clercq, W. P.; McLachlan, Milla; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: South Africa is rich in biodiversity among which are semi-domesticated vegetable species which are known as wild or indigenous vegetables. These wild indigenous vegetables have been reported to be good in nutritional qualities such as macro and micronutrients. However, there is still a high prevalence of malnutrition; especially micronutrient deficiencies among low or marginal income bracket of the population. The use of indigenous vegetables has been proposed as part of the solutions to the problems of micronutrient malnutrition among these populations. Indigenous vegetables are an important source of food in the maize based subsistence farming sector of rural South Africa. Their main role is as relish as they are used as an accompaniment for staple cereal based diets. They are also generally reported to be rich in micronutrients. Although they may be consumed in small quantities, they influence the intake of cereal staples, manage hunger and play a central role in household food security for the poorer rural groups. Mixing several indigenous vegetables species in one meal contributes to dietary diversity in terms of more vegetable types as well as in terms of choice of relish. For some very poor families indigenous vegetables are substitutes for some food crops. The seasonal occurrence of these vegetables leaves many families without a food source during the off-season. Indigenous vegetables increase agro-biodiversity at the household level. This agro-biodiversity helps in buffering against the accumulation and multiplication of pests and diseases and provides important cover for the soil. Further research on agronomic, social and economic dimensions is required to understand the roles of IV in subsistence farming systems in South Africa. The survey study revealed that indigenous vegetables were important in the diets of most rural people in the study area. They were consumed as relish although they were not being cultivated. Their method of acquisition was gathering from homesteads and the wild. These vegetables were also believed to be medicinal. The local naming of wild vegetables varied among villages in the same district such that a vegetable in one village was assigned to a different species of vegetable in another village. They were reportedly abundant during summer and there was a decrease in availability off-season leaving vulnerable people who rely on them with a food shortage. The utilisation of wild vegetables among South Africans is reported to be declining due to over reliance on introduced temperate species. Efforts to domesticate and cultivate wild vegetables could be hampered by several factors including seed dormancy and premature flowering. In this present study dormancy was observed in C. olitorius. The response of wild genotypes of C. olitorius with different seed sizes to various dry heat and hot water treatments was evaluated. Steeping seeds in boiling water (95oC) for ten seconds and soaking seeds in a hot water bath at 80oC for ten minutes resulted in the highest response to germination in this species. The study also recorded significant interactions between heat treatment and seed sizes. We concluded that C. olitorius seeds of different sizes require diverse durations of exposure to heat treatment methods to break dormancy caused by an impermeable seed coat. Cleome gynandra is another species that is consumed as a vegetable in various parts of the world including Africa. The plant is also used as a medicinal herb for the treatment of various human diseases. Among the wild vegetables, C. gynandra has been reported to flower prematurely, a phenomenon known as bolting and common in many vegetable crops. Premature flowering (bolting) can be as a response to temperature extremes and photoperiod and affects many other leafy vegetables such as lettuce (Lactuca sativa), spinach (Spinacea oleracea) and mustard rape (Brassica juncea). Bolting leads to production losses in leaf vegetable crops as they flower before they have produced an economic yield. The removal of flowers and nitrogen application resulted in significant increases in the fresh and dry weight of cleome leaves. Removal of flowers resulted in a 46% increase in fresh weight of leaves. The observed positive response of leaf yield to removal of flowers offers a possible way to deal with the problem of bolting. The continuous removal of the flowers leads to increased utilisable leaf yield. The application of incremental amounts of nitrogen top dressing results in increased leaf yield in C. gynandra. The response of selected indigenous vegetables (Corchorus olitorius and Amaranthus cruentus) to micronutrients added to the soil was compared with the response of a reference crop; Swiss chard (Beta vulgaris var. cicla). For all the levels of micronutrients applied, Swiss chard accumulated Cu, Zn and Mn in the leaves at significantly (p<0.01) higher concentrations than the wild vegetables. Variations between the vegetables in the micronutrients were greater for Zn (72–363 ppm) and Mn (97.9–285.9 ppm) for Cu (8.8–14 ppm). C. olitorius had the least capacity to concentrate Mn and Zn in the leaf, which suggested that this vegetable is a less attractive candidate for agronomic bio-fortification of these elements. However, C. olitorius accumulated Fe at a significantly higher concentration (327 ppm) in the leaves than did Amaranthus (222 ppm) or Swiss chard (295 ppm). Sulphur as a macronutrient varied little in the plant species tested. The mean S concentration in the leaves ranged from 0.26% in C. olitorius to 0.34% in Amaranthus cruentus and Swiss chard. We concluded that the different vegetables have different abilities to take up Cu and Zn in the order Swiss chard > Amaranthus > Corchorus, and that they responded to micronutrients added to the soil but only up to certain limits of supplementation. The results from this current study seem to contradict the belief that wild vegetables have the inherent ability to concentrate mineral micronutrients in their tissue. Factors such as environment, anti-nutrients, dietary diversity, plant parts, plant age, and varieties result in differences in reported nutritional composition of indigenous vegetables. Post-harvest handling, storage, cooking and preservation also alter the composition. The need to optimise protocols for each vegetable type and for different laboratories makes analysis expensive. Equipment and methods of analysis are varied and may not be comparable, making it difficult to generalise on the composition of these vegetables. The Agricultural Research Council of South Africa and other stake holders are conducting studies on some aspects of these vegetables. There are still many information gaps regarding many aspects of these vegetables which require research attention. These include; the selection and improvement of genotypes, seed biology and germination studies, agronomic (population, fertiliser, crop mixtures) studies and phyto-chemical evaluation of these important species in order to encourage the overall use of these important indigenous resources. Finally, there is need to promote their increased utilisation.
Conversion of soil pH values and exchangeable base cation quantities determined by different methods
(Stellenbosch : Stellenbosch University, 2021-12) Nel, Teneille; Hardie-Pieters, Ailsa G.; Clarke, Catherine E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Soil management recommendations based on chemical test results rely on comparisons with norm values developed using specific analytical methods and soil taxonomic classes. Conversions of soil pH and exchangeable base cation concentration between different methods are required for the standardization of data from scientific literature, soil test reports and soil databases. The combined effects of soil physicochemical properties on the accuracy and parsimony of soil pH and cation concentration conversions are unknown. Rapid soil analyses can be obtained by infrared (IR) spectroscopy, but few studies have evaluated the accuracy of IR spectral-based models to determine soil pH and exchangeable base cations in Western Cape soils. The aim of this study was to develop equations for conversion of soil pH values and exchangeable base cation concentrations between different methods of measurement. An additional aim was to develop IR spectral-based models for determination of soil pH and exchangeable base cation content of Western Cape soils. Soil pH was measured in different matrices (water, 0.1M KCl and 0.01M CaCl2) of varying soil: solution ratios (1:1, 1:2.5, 1:4 and 1:5). The exchangeable base cation content was measured according the ammonium acetate (1M, pH = 7), compulsive exchange, ammonium acetate (0.2M, pH = 7), Ambic- I, Mehlich-III, Bray-II and Citric acid (1%) methods. Models converting soil pH and cation content between values obtained by different methods were developed using simple linear regression (SLR). In addition, multivariate linear regression (MLR) equations incorporating soil physicochemical properties as parameters were developed. Spectral measurements of the soil samples were taken with three spectroscopic instruments in the near- to mid-IR range. IR spectral-based models predicting exchangeable base cation content and soil pH were calibrated using partial least squares regression. SLR equations for predicting soil pH (RMSE = 0.12-0.32) were more accurate than the conversion factors. MLR equations constructed by stepwise regression to convert exchangeable base cation content (RMSE = 1.4-1.5 cmolc kg-1, 0.4 cmolc kg-1, 16-20 mg kg-1 and 27-39 mg kg-1 for Ca, Mg, Na and K, respectively) were more accurate than SLR models. Exchangeable K content was accurately converted between the most extraction methods. Models converting base cation content may be chosen based on the purpose of application and relative size of the prediction error. IR spectroscopy is recommended to determine exchangeable Ca and Mg content in Western Cape soils (RMSE = 1.2-1.4 cmolc kg-1 and 0.37 cmolc kg-1 for Ca and Mg, respectively). Subsetting soil sample sets by similarities in texture and organic carbon content led to more accurate predictions of soil pH and exchangeable Ca content (RMSE decreased by up to 0.19 pH units and 0.79 cmolc kg- 1, respectively). In summary, use of the developed SLR as well as MLR models are recommended to convert soil pH and exchangeable base cation concentrations of Western Cape soils. Models should only be as complex as practical context requires. Moreover, IR spectroscopy may be used for determination of exchangeable Ca and Mg content in Western Cape soils.
Decontamination of soils by activation with acids and bases
(Stellenbosch : Stellenbosch University, 2004-03) Hardie, Ailsa Ghillaine; Fey, M. V.; Ellis, F.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: New, more receptive surfaces can be generated in soils by a partial dissolution of existing, crystalline solids followed by re-precipitation as poorly crystalline colloids with a larger capacity to adsorb ionic and molecular contaminants. This priming process can be carried out by treating the soil with strong acid or base and then neutral ising it again. The aim of this study was to investigate the effectiveness of acid and base treatments in reducing inorganic contaminant availability in different soil types. The first study involved investigating the change in cation (cadmium[II], copper[II]) and anion (phosphate) sorption of four different soils before and after priming. Hydrochloric acid and KOH were used to adjust the pH of soils to below pH 2 or above pH 12 in the dissolution stage of the priming treatment. After neutralisation it was found that base priming resulted in an increase in metal cation adsorption in all the soils, most notably in the sesquioxidic (increase from 19.5 to 73.5 mmol Cd.kg-1 soil) and kaolinitic soils (from 16.9 to 38.3 mmol Cd.kg-1 soil), whereas acid priming decreased it or had little effect on cation sorption. However, acid priming increased anion sorption in all soil types, to a greater extent than base priming, most notably in the organic soil (from 6.3 to 14.7 mmol P04.kg-1 soil). This can be attributed to the differences in the nature of the precipitate (more aluminous or alumino-siliceous) depending on whether the dissolution was carried out in acid or basic conditions, and the final pH of the soil solution, as the hydroxyaluminium and hydroxyaluminosilicate precipitates which form are known to enhance pH-dependent sorption of metals. In the second study, the soil was suspended in Cd or Cu solutions and then the pH was adjusted to below 2 or above 12 using HCI or KOH. After 5 days of shaking the pH was adjusted to a neutral pH again. The availability of the Cd and Cu was determined at each of the stages in the treatment, and it was found that both the acid and base treatments were effective in removing Cu from solution, whereas only the base treatment was effective in removing Cd. Determinations were also carried out using H2S04 and Ca(OH)2 and it was found that they were equally effective. It can be proposed that this acid-base pair would be of most practical importance as the salt generated is gypsum which is generally considered benign, and can actually help to improve the soil structure. Activation of soils by acid or base conditioning could have some useful applications in decontaminating soils or decreasing the mobility of inorganic contaminants in soils. Primed soils could also be used as cheap absorbents for decontaminating water.
Deficit irrigation and canopy management practices to improve water use efficiency and profitability of wine grapes
(Stellenbosch : Stellenbosch University, 2024-03) Lategan, Eugene Lourens; Hoffman, Josias Eduard; Myburgh, Philipus Albertus; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Grapevines irrigated at low plant available water (PAW) depletion levels required more than double the irrigation volumes compared to those irrigated at high depletion levels. The accelerated sugar accumulation of sprawling grapevine canopies resulted in earlier harvest dates, reducing pre-harvest irrigation requirements. Diﬀerent canopy manipulations did not aﬀect total leaf area (LA) per grapevine within an irrigation strategy, but negatively aﬀected LA as less water was applied. Non-suckered grapevines produced more shoots and more vigorous shoot growth, while non-suckered vertical shoot positioned (VSP) grapevines tended to produce lower cane mass. The LA distribution provides a good indication of canopy orientation, and non-destructive measurements of primary and secondary shoots can estimate winter pruned cane mass. This would allow viticulturists, producers, or irrigation consultants to estimate the maximum cane mass and use the VINET© model to predict grapevine water requirements in real-time throughout the season, as the LA is estimated using cane mass. Grapevines with sprawling canopies had lower mid-day leaf water (ΨL) and stem water (Ψs) potentials compared to those with VSP canopies. Grapevines experiencing severe water constraints ripened more rapidly than those without or with medium water constraints. Low frequency irrigation increased water constraints compared to high frequency irrigation. Diurnal ΨL cycles showed that grapevines with sprawling canopies had lower ΨL after 18:00 and throughout the night, suggesting that their water status could not recover as fast as VSP grapevines. High irrigation frequencies led to higher grapevine row evapotranspiration (ETGR) losses, with losses from sprawling grapevines, particularly those irrigated at ca. 30% plant available water (PAW) depletion, being higher in January and February than those with VSP canopies. Seasonal full surface evapotranspiration was more sensitive to irrigation frequency than to canopy manipulations. Grapevines irrigated at ca. 30% PAW depletion had higher mean full surface crop coeﬃcient (Kc) values compared to other strategies, with those irrigated at ca. 90% PAW depletion having the lowest Kc values. The mean peak Kc was generally obtained in February for grapevines irrigated at frequencies, while the lowest Kc was found during the same period for low frequency irrigation applications. The fraction of soil wetted during irrigation applications under grapevine row (Kc,GR) could be a more realistic coeﬃcient than Kc for producers and consultants to use in irrigation scheduling requirements. Irrigation frequency had a more signiﬁcant impact on yield than canopy manipulation. Higher rainfall in 2013/14 increased vegetative growth and yield, with low frequency irrigations resulting in higher production water use eﬃciency compared to medium and high frequency irrigations. The incidence of grey rot was higher during the wetter season, with grapevines with sprawling canopies experiencing higher yield losses due to sun burn and less frequent irrigation. The highest incidences and yield loss to grey rot were found in grapevines left un-suckered and irrigated at ca. 30 PAW depletion, while irrigation at around 90 PAW depletion resulted in the absence of grey rot. Grapes were harvested near the target total soluble solids level of 24ºB, with severe water constraints enhancing berry maturation. Non-suckered VSP grapevines produced poorer quality at lower levels (30% and 60% depletion levels), with the highest overall wine quality obtained when irrigated at ca. 90% PAW depletion. Less frequent irrigations reduced summer canopy management requirements, but grapevines with more shoots required higher labour inputs at harvest. Pruning labour input requirements were aﬀected by the number of shoots produced per grapevine and the mass per individual shoot. Sprawling canopy grapevines generally required lower labour costs, and pump costs were aﬀected by the frequency of irrigation applications. During low to normal rainfall seasons, grapevines with sprawling canopies irrigated at ca. 60% PAW depletion produced the highest gross margins incomes, followed by box pruned grapevines irrigated at ca. 90% PAW depletion. In high summer rainfall seasons, box pruned grapevines and non-suckered VSP canopies had the highest gross margins. Grapevines with sprawling canopies, particularly those irrigated at ca. 60% PAW depletion, produced the best balance between yield and quality, ensuring the best gross margin incomes. The gross margin water use eﬃciency (WUEGM) increased with an increase in PAW depletion level irrigation, with box pruned grapevines consistently having the highest WUEGM. The study found that grapevines with sprawling canopies experienced lower diurnal and cumulative evaporation losses compared to VSP grapevines, regardless of PAW depletion levels. The higher mean leaf area per grapevine resulted in denser canopies, and treatments irrigated at approximately 30% PAW depletion were always within stage 1 of evaporation. Grapevines irrigated at around 60% PAW depletion occasionally went into stage 2, particularly in sprawling canopies. The water content of soil under grapevines irrigated at around 90% PAW depletion spent most of the season in stage 2. The vegetation coeﬃcient (Kv) of sprawling canopies was lower than VSP grapevines, irrespective of PAW depletion. The VINET© model generally underestimated transpiration rates in wet soil regimes and overestimated them during dry soil regimes. Adjusting the model by addition of Kv and adapted transpiration water predictions can be done using two multilinear regressions after a few grapevine canopy measurements inputs have been considered.
The degradation of atrazine by soil minerals : effects of drying mineral surfaces
(Stellenbosch : Stellenbosch University, 2014-04) Adams, Adrian Richard; Clarke, Catherine E.; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: The herbicide atrazine (ATZ, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) has been identified as an environmental endocrine disruptor and possible human carcinogen. The presence of atrazine, along with its degradation products, in soils and water supplies therefore raises concern. Atrazine biodegradation in soils is well-covered to date, however, atrazine degradation by abiotic mineral surfaces, and the chemical mechanism by which it occurs, is not fully understood. Furthermore, with a changing global climate, the effects of wetting and drying cycles on soil processes (e.g. atrazine degradation) is largely unknown, but increasing in importance. This study therefore investigated atrazine degradation on six common soil mineral surfaces, namely birnessite, goethite, ferrihydrite, gibbsite, Al3+-saturated smectite and quartz, as well as the effects that drying these surfaces has on atrazine degradation. In the first part, a comparison was conducted between the reactivity of fully hydrated and drying mineral surfaces toward atrazine, by reacting atrazine-mineral mixtures under both moist and ambient drying conditions, in parallel, for 14 days. Under moist conditions, none of the mineral surfaces degraded atrazine, but under drying, birnessite and goethite degraded atrazine to non-phytotoxic hydroxyatrazine (ATZ-OH, 2-hydroxy-4-ethylamino-6-isopropylamino-1,3,5-triazine) as major product and phytotoxic deethylatrazine (DEA, 2-chloro-4-amino-6-isopropylamino-1,3,5-triazine) as minor product. The mineral surface reactivity was birnessite (66% degradation) > goethite (18% degradation) >> other mineral surfaces (negligible degradation), indicating possible atrazine oxidation. In the second part, the effects of drying rate were investigated on birnessite only (the most reactive surface), by conducting the drying (1) gradually at ambient rates, (2) rapidly under an air stream, and (3) gradually in the absence of water using only organic solvent. After 30 days of ambient drying, 90% of the atrazine was degraded to ATZ-OH and DEA, but the same extent of degradation was achieved after only 4 days of rapid drying with an air stream. Thirty days of gradual drying using only organic solvent did not increase atrazine degradation compared to the water-moist drying surface. In each case, degradation initiated at a critical moisture content of 10% of the original moisture content. In the third part, the degradation mechanism was further investigated. To test for the possible oxidation of atrazine by the birnessite surface, moist atrazine-birnessite mixtures were dried under a nitrogen (N2) stream to eliminate possible oxidation by atmospheric oxygen (O2). Dissolved Mn2+ was extracted at the end of the experiment to observe any reduction of birnessite. Under N2, the same products were formed as before, with no appreciable Mn2+ production, indicating non-oxidative atrazine degradation by birnessite. The final part investigated the effects ultraviolet (UV) radiation has on the degradation of atrazine by drying mineral surfaces. The UV-radiation enhanced the degradation of atrazine, but no other degradation products were formed. It was therefore concluded that atrazine degradation on redox-active soil mineral surfaces is enhanced by drying, via a net non-oxidative mechanism. Furthermore, this drying-induced degradation is an atrazine detoxification mechanism which could be easily applied through agricultural practices such as windrowing, ploughing and any other practice that (rapidly) dries a Mn- or Fe-oxide rich agricultural soil.

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