Masters Degrees (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.
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.
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.
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.
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.
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.
Determination of the lime requirement of sandy, organic-rich, and structured, high Mg:Ca ratio soils by the Eksteen method
(Stellenbosch : Stellenbosch University, 2001-03) Smuts, Michiel Nicolaas; Lambrechts, J. J. N.; Saayman, D.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.; Davids, Ricardo
ENGLISH ABSTRACT: The Eksteen method of lime requirement determination, based on the ratio of Ca+Mg:H (R-value), is widely used in the Western Cape and has proven to be extremely accurate for most soils. However, the Eksteen method is known to give erroneous predictions of lime requirement for certain soil groups. These include sandy soils (notably pale coloured sandy soils), organic-rich topsoils and strongly structured, Mg-rich subsoils. The objective of this study was to examine the nature of the Eksteen RpH relationship for these problem soils, and to develop appropriate modifications to the Eksteen method, where possible. A population of both normal and problem soils was selected and analysed, the RpH relationships of the soils were determined, and various relationships were examined. The principal findings with regard to the nature of the Eksteen RpH relationship were as follows: 1. The lime requirements of pale coloured sandy soils can be most accurately determined on an equivalent basis relative to the total extractable acidity, after the total extractable acidity has been corrected for residual lime. 2. The previously observed and acknowledged inaccuracy of the Eksteen method on organic-rich soils stems from the pH-dependant acidity component of the organic matter. The accuracy with which the lime requirement of these soils is predicted by the Eksteen method can be greatly improved by the application of an organic matter correction factor (OMCF) to the total extractable acidity, which is the parameter on which the lime requirement is then based. 3. In structured, Mg-rich subsoils the pH-neutralising capacity of Mg is considerably lower than that of Ca, a factor which is not taken into account by the Eksteen method. The Eksteen method can nevertheless be corrected to overcome this discrepancy. The high levels of Mg in the structured, Mg-rich subsoils cannot be attributed to minerals in these subsoils containing brucite interlayers. 4. Toxic levels of AI may only be expected at soil pH values below approximately 4.7 as determined in 1M KC!. Soil organic matter and CEC have marked affects on AI extractability. In accordance with the objective that appropriate modifications to the Eksteen method should be developed if possible, theoretically-based correction factors were developed which will permit the use of the Eksteen method to be confidently extended to those soil groups (pale coloured sandy soils, organicrich topsoils and strongly structured, Mg-rich subsoils) for which it was formerly considered unreliable.
Determining of optimum irrigation schedules for drip irrigated Shiraz vineyards in the Breede River Valley
(Stellenbosch : Stellenbosch University, 2011-12) Lategan, Eugene Lourens; Hoffman, J. E.; Myburgh, P. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Shiraz/110R grapevines, growing in a fine sandy loam soil in the Breede River Valley, were subjected to ten different drip irrigation strategies during the 2006/07, 2007/08 and 2008/09 seasons. Grapevines of the control treatment (T1) were irrigated at 30% to 40% plant available water (PAW) depletion throughout the growing season. Grapevines of three treatments were irrigated at 70% to 80% PAW depletion from bud break until véraison (i.e. when ca. 95% of grape berries have changed colour), followed by either irrigation at 30% to 40% PAW depletion (T2) or a continuous deficit irrigation (CDI) strategy (T3) or irrigation at 70% to 80% PAW depletion (T4) during ripening. The CDI strategy was obtained by applying ca. half the volume of water that was applied to the control. This allowed the soil to dry out gradually between physiological stages (i.e. bud break and véraison or véraison and harvest). Grapevines of three further treatments were irrigated at ca. 90% PAW depletion from bud break until véraison, followed by irrigation at 30% to 40% PAW depletion (T5) or a CDI strategy (T6) or irrigation at ca. 90% PAW depletion (T7) during ripening. Grapevines of two treatments were irrigated by means of a CDI strategy from bud break until véraison. For both treatments, the soil water content (SWC) was allowed to dry out gradually until ca. 90% PAW depletion was reached. After véraison, the SWC of the one treatment was maintained at ca. 90% PAW depletion by applying only four small irrigations of three hours each during ripening (T8). The soil of the other treatment, received an irrigation at véraison to refill the SWC to field capacity (T9) followed by the CDI strategy during ripening. Grapevines of the tenth treatment were irrigated at ca. 90% PAW depletion between bud break and véraison followed by a partial profile refill (PPR) strategy during ripening (T10). In order to obtain the PPR strategy, SWC was only maintained between 40% and 60% PAW depletion. Evapotranspiration varied between 3.5 mm/day and 0.1 mm/day for driest and wettest treatments, respectively, during the period between December and February. This was substantially less than the volumes required for full surface irrigation. For irrigations applied at 30% to 40% PAW depletion (T1), 70% to 80% PAW depletion (T4) and ca. 90% PAW depletion (T7) levels throughout the season, crop coefficients for the Penman-Monteith reference evapotranspiration (ETo) were 0.4, 0.2 and 0.1, respectively. Under the given conditions, the different irrigation strategies did not have any effect on root distribution and density. Shoot growth of grapevines exposed to high to severe water deficits in the pre-véraison period stopped before mid December. Shoots of grapevines that were exposed to high or severe water deficits before véraison followed by more frequent irrigation during ripening showed active re-growth. These trends occurred during all the seasons. The level of PAW depletion reflected strongly in the plant water potential in the grapevines. Leaf water potential was influenced by the prevailing atmospheric conditions, whereas stem water potential was less sensitive to atmospheric conditions, but responded more directly to soil water availability. Due to the good relationships between pre-dawn leaf, mid-day leaf, mid-day stem and total diurnal water potential, it was possible to re-classify the water status in terms of previous classifications for these water potentials based on pre-dawn measurements. Water constraints in T1, T2 and T5 grapevines were classed as experiencing no stress, whereas the T7 and T8 ones experienced strong to severe water constraints before harvest. High frequency irrigation strategies during ripening delayed sugar accumulation due to dilution of sugar in the larger berries. Except for the wettest strategy, and where grapevines were subjected to the CDI strategy throughout the season, berry mass increased during ripening, i.e. from véraison to harvest. Water deficits had a negative effect on berry mass, bunch size and yield. Where higher soil water depletion levels were allowed, irrigation strategies had a positive effect on the irrigation water productivity of grapevines compared to the frequently irrigated or CDI strategies. Higher water constraints in grapevines, particularly during ripening, improved sensorial wine colour and enhanced some of the more prominent wine aromas, e.g. spicy and berry. Grapevines that were irrigated at a high frequency during ripening produced wines with diluted character flavours and aromas and inferior overall quality. Under the given conditions, sensorial wine colour and spicy character were the dominant factors in determining overall sensorial wine quality.
Development, optimization and use of a reduced-sample, water dispersible clay extraction technique for taxonomic horizon discrimination
(Stellenbosch : Stellenbosch University, 2018-03) Dinwa, Siziphiwe; Clarke, Catherine E.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Water dispersible clay (WDC) is defined as the colloid fraction which disperses in water without removal of cementing compounds or the use of dispersing agents. It is a commonly determined parameter and is used in many erosion models and is a proxy for aggregate stability and clay dispersivity. There is no standard method for determining WDC, and although modified particle size analysis (PSA) is the most common technique, numerous other methods are also employed to save time, bench space and reduce sample size. These methods have not been tested against the benchmark PSA method and vary in terms of agitation (time and type), extraction, measurement and expression of WDC. This makes comparison between these methods very difficult. This study aims to develop, test and optimise a simple, reduced sample centrifuge method for determining WDC in order to allow analysis of archive samples and assess the use of WDC as a soil classification discriminator on a limited number of soils. A reliable and calibrated, reduced sample size method will be of value for measuring WDC in sample collections, such as the national profile soil collection housed at the Institute for Soil Climate and Water. This would allow for these valuable collections to be included in erosion models. Archived samples of neocutanic B, yellow-brown and red apedal B horizons and borderline neocutanics/red apedal B horizons were selected for this study. Two reduced sample centrifuge methods (using pipetting and decanting to remove the clay suspension) were examined and their efficiency and accuracy was measured with respect to the sedimentation particle size analysis (PSA) method. For both the centrifuging methods the WDC and chemically dispersed clay, a mixture of sodium hexametaphosphate and sodium carbonate, called CDC were determined. This is the chemically dispersed clay without the removal of organic matter or cementing agents. The effect of ultrasonication and shaking time on WDC was assessed for the centrifuge-pipette method by physically agitating the soil with or without prior sonication, and increasing the initial shaking time incrementally from 1 to 30 hours. X-ray diffraction (XRD) analysis was carried out on the WDC and CDC extracts from the benchmark sedimentation method to establish if the mineralogy of these two fractions differed. The WDC and CDC was measured gravimetrically and by turbidity readings. Water dispersible clay correlated poorly with total clay across all samples. The relationship between CDC and total clay was better, but the extraction efficiency of CDC to total clay was only 54%. The extraction efficiency of WDC is highly dependent on the physical agitation energy exerted on the samples. Increasing the headspace in the centrifuge tube increased the WDC extraction efficiency by 32% (absolute). Shaking time has a major influence on WDC extraction efficiency, with a minimum shaking time of 22 hours required to get maximum extraction. This demonstrates the need to standardise the method as numerous extraction techniques use less than 16 hours shaking time for WDC extraction. Sonication prior to shaking for 22 hours results in a WDC extraction efficiency of 94% for the new centrifuge method compared to the traditional PSA method. The centrifuge-pipette method was shown to be effective in selectively separating the < 2 μm phase, thus reducing the need for sedimentation. Turbidity is not a reliable technique to measure clay in a suspension, due to the clay mineralogy affecting turbidity. Model kaolinite and smectite did not give uniform turbidity readings. This means the gravimetric method cannot be replaced, but centrifugation has both a time saving and sample reducing benefit. Neocutanic horizons tended to have WDCh (the WDC fraction expressed as a function of CDC) content higher than the yellow-brown and red apedal horizons, and were distinguishable from red apedal horizons at a 95% confidence level. However, WDC cannot be used to distinguish neocutanic B from yellow-brown apedals horizons. This supported the tacit knowledge that neocutanic horizons have a less stable clay phase than red apedal horizons, but the distinction is not clear in the case of yellow brown apedals. Borderline neocutanic/red apedal horizons and typical neocutanic proved to have similar WDCh content. Given the importance of clay stability in red apedal horizons, it was recommended they are classified as neocutanics rather than red apedals and a tentative threshold of 47% WDCh be used to differentiate between horizons. The new centrifuge technique for the extraction of WDC is a viable alternative to the PSA method and has the benefits of reducing sample size and extraction time and increasing the number of samples that can be analysed at one time. Standardisation of WDC is important due to the effects of agitation type and duration on the extraction efficiency. Furthermore, WDCh shows promise as a classification aid and should be investigated further.
Die geskiedenis van graanbou aan die Kaap, 1795-1826
(1967-12) van Zyl, D. J.
Daar is alreeds twee M,A,-skripsies oor die geskiedenis van graanbou aan die Kaap geskryf, t.w. die van A.J. du Plessis, getitel: "Die geskiedenis van die graankultuur in Suid-Afrika tydens die eerste eeu, 1652-1752 en die van J.H.D. Schreuder, getitel: "Die geskiedenis van ons graanbou2 1752-1795. Hulle dek dus die Kompanjiestydperk. In oreenstemming met sy merkantilistiese beleid, het die Kompanjie allerlei beperkinge op graanbou geplaas. Gedurende hierdie tydperk het daar geen vryhandel in graan bestaan nie en is pryse deur die owerheid vasgestel. Graanbou is slegs aan gemoedig in soverre dit die belange van die Kompanjie kon dien. Dit was juis die aspek van graanbou, t.w. die graanhandel, wat in besonder my belangstelling geprikkel het om ‘n verdere studie van die onderwerp te maak tot die einde van lord Charles Somerset se bewind, d.w.s. 1826.
Distribution and stability of soil carbon in spekboom thicket, Eastern Cape, South Africa
(Stellenbosch : Stellenbosch University, 2012-03) Mchunu, Sinethemba Euginia; Hardie-Pieters, Ailsa G.; Mills, A. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Soils of semi-arid regions generally have low organic matter contents, primarily attributed to prevailing climatic conditions. Yet, the spekboom thicket, located in the semi-arid region of the Eastern Cape (South Africa) has been distinguished to accumulate large amounts of soil carbon. To further understand this remarkable accumulation, a detailed investigation was carried out to evaluate the distribution, speciation and stability of the soil carbon from directly beneath spekboom (Portulacaria afra) and other “nonspekboom” vegetation within the intact thicket, and from adjacent, degraded (overgrazed), open thicket sites. In the first experimental chapter, the distribution of organic and inorganic C, as well as general soil properties (pH, EC, exchangeable and water soluble cations, hydrophobicity, water stable aggregates) were investigated in the intact (spekboom and nonspekboom) and degraded (open) thicket soils. There were no significant differences in the C content (organic or inorganic) between the spekboom and nonspekboom sites in the intact thicket. Soil organic carbon (SOC) content however was greatly influenced by thicket degradation, whereas, inorganic C remained unaffected. Mean organic C (0-50 cm) ranged from 19-34, 22-62, and 19-58 g C kg-1 for open, spekboom, and nonspekboom sites, respectively. Whereas, mean inorganic C ranged from 2-9 g C kg-1 irrespective of sites, representing 4-22 % of total soil C. However, no significant differences were detected between degraded and intact sites for all (total, organic and inorganic) C stocks, attributed to variation in bulk density. Spekboom soils contained significantly higher concentrations of exchangeable and water soluble Mg, Na, and K, and consequently had higher pH and EC compared to the other sites. Soils tended to be hydrophobic especially at shallower depths in the intact thicket irrespective of vegetation type, whereas soils from the adjacent open sites were not significantly hydrophobic. Macro-aggregates were considerably more stable under intact sites compared to open sites. The second experimental chapter investigated the effects of degradation and vegetation type on the stability and structural chemistry of SOC. This involved partitioning the soil organic matter into particulate and mineral-bound (stable) fractions, and then examining the relationships between the stable organic C fraction and various soil properties as an attempt to elucidate the stabilization mechanism(s). Particulate organic C ranged from 7.0, 9.3, and 14.4 g C kg-1 for open, spekboom, and nonspekboom respectively; representing 22-34 % of total SOC. Stable (mineral-bound) organic C accounted for the largest fraction ca. 60-66 % of total SOC; 28.0 and 26.2 g C kg-1 for spekboom and non-spekboom respectively, versus 17.2 g C kg-1 for open sites. It was concluded that SOC stability was influenced by the inherent residue quality (recalcitrance) rather than soil properties, attributed to the aliphatic (lipids and waxes) and highly aromatic nature of the spekboom litter substrate and particulate OM. These research findings are of fundamental significance in understanding soil organic matter stabilization in semi-arid environments.
The effect of biochar application to sandy soil on nitrogen nutrition of maize (Zea mays)
(Stellenbosch : Stellenbosch University, 2023-03) Kemp, Bernitia; Rozanov, Andrei Borisivich; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Sustainable agricultural development requires a holistic systems approach to address environmental, economic, and social challenges associated with agricultural productivity. Sustainable management of sandy soil in agricultural systems requires fertilization strategies that maintain and enhance soil physio-chemical properties while sustaining optimum crop yield for the long run. Biochar production and utilization are linked directly to agriculture, the environment, and sustainable development. Biochar application is widely considered a sustainable management strategy to sequester carbon in the soil and improve soil properties. Reservations have been expressed in the literature regarding this soil amendment, particularly regarding the possible adverse effects of biochar application to soils on nitrogen nutrition of crops. A pot trial was conducted to understand the effects of biochar application on the nitrogen nutrition of maize, by investigating the impact of locally produced biochar application on the growth response (vigour) of maize and its foliar N content. This study compared the effect of biochar addition to soil with other soil amendments commonly used in agricultural systems. To clarify, this study compared biochar with compost and nitrogen fertilizer, applied independently and in combinations. Biochar was derived from black wattle pyrolyzed at 730 ° C. This study has shown that the compound application of biochar with the recommended dosage of nitrogen fertilizer had a clear positive synergistic effect on the performance of maize. This positive synergistic effect may be attributed to various improvements in soil quality typically observed in biochar-amended soils. Applying biochar had a liming effect on the acidic sandy soil when used on its own and in combination with organic/ inorganic fertilizers. The potentially mineralizable N in the soil did not differ for any treatments relative to the control. Biochar may serve as a long-term soil management strategy in agricultural systems to effectively manage nutrients, enhance the nutrient use efficiency of cropping systems, reduce N loss and subsequently limit environmental pollution, and reuse and recycle waste, while increasing agricultural production. Biochar should be designed according to the context of any given agricultural system and modified to optimally perform specific functions. Subsequently, the production and utilization of biochar will be not only efficient and profitable but also sustainable. In addition, this study emphasized that excessive N fertilization is both unnecessary and an unsustainable soil management strategy. Thus, balanced fertilization schemes may be highly beneficial for the productivity of maize cropping systems. Additional studies are required to enhance future biochar development. Especially since biochar production can significantly contribute to the sustainable development of economies, society, agricultural systems, and the environment.
Effect of biochar on chemistry, nutrient uptake and fertilizer mobility in sandy soil
(Stellenbosch : Stellenbosch University, 2012-03) Sika, Makhosazana Princess; Hardie-Pieters, Ailsa G.; Hoffman, J. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Biochar is a carbon-rich solid material produced during pyrolysis, which is the thermal degradation of biomass under oxygen limited conditions. Biochar can be used as a soil amendment to increase the agronomic productivity of low potential soils. The aim of this study was to investigate the effect of applying locally-produced biochar on the fertility of low-nutrient holding, sandy soil from the Western Cape, and to determine the optimum biochar application level. Furthermore, this study investigates the effect of biochar on the leaching of an inorganic nitrogen fertilizer and a multi-element fertilizer from the sandy soil. The biochar used in this study was produced from pinewood sawmill waste using slow pyrolysis (450 °C). The soil used was a leached, acidic, sandy soil from Brackenfell, Western Cape. In the first study, the sandy soil mixed with five different levels of biochar (0, 0.05, 0.5, 0.5 and 10.0 % w/w) was chemically characterised. Total carbon and nitrogen, pH, CEC and plant-available nutrients and toxins were determined. The application of biochar resulted in a significant increase in soil pH, exchangeable basic cations, phosphorus and water holding capacity. A wheat pot trial using the biochar-amended soil was carried out for 12 weeks and to maturity (reached at 22 weeks). The trial was conducted with and without the addition of a water-soluble broad spectrum fertilizer. Results showed that biochar improved wheat biomass production when added at low levels. The optimum biochar application level in the wheat pot trial was 0.5 % (approximately 10 t ha-1 to a depth of 15 cm) for the fertilized treatments (21 % biomass increase), and 2.5 % (approximately 50 t ha-1 to a depth of 15 cm) for unfertilized treatments (29 % biomass increase). Since most biochars are alkaline and have a high C:N ratio, caution should be taken when applying it on poorly buffered sandy soil or without the addition of sufficient nitrogen to prevent nutrient deficiencies. In the second study, leaching columns packed with sandy soil and biochar (0, 0.5, 2.5 and 10.0 % w/w) were set up to determine the effect of biochar on inorganic nitrogen fertilizer leaching over a period of 6 weeks. It was found that biochar (0.5, 2.5, and 10.0 % w/w) significantly reduced the leaching of ammonium (12, 50 and 86 % respectively) and nitrate (26, 42 and 95 % respectively) fertilizer from the sandy soil. Moreover, biochar (0.5 %) significantly reduced the leaching of basic cations, phosphorus and certain micronutrients. This study demonstrated the potential of biochar as an amendment of acidic, sandy soils. Our findings suggest that an application rate of 10 t ha-1 should not be exceeded when applying biochar on these soils. Furthermore, biochar application can significantly reduce nutrient leaching in sandy agricultural soils.

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