Masters Degrees (Soil Science)
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- ItemThe effect of gypsum form and source on soil amelioration(Stellenbosch : Stellenbosch University, 2024-03) Du Plessis, M; Hardie-Pieters, Ailsa G.; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.ENGLISH ABSTRACT: Studies evaluating the effectiveness of South African gypsum sources are scarce or outdated, as new products, such as granulated and liquid gypsums, have been introduced. The efficacy of calcium (Ca) nitrate and micro-fine (MF) calcitic limes in comparison to multiple gypsum sources has also never been evaluated. Consequently, the main aim of the study was to characterise and compare the effectiveness of various South African gypsum sources for use in crop production. A controlled laboratory study was performed to compare the effectiveness of mined and by-product gypsums and other Ca sources in terms of soil sodicity remediation and saturated hydraulic conductivity (Ksat) improvement. Twelve mined and by-product gypsums sources, MF calcitic limes and calcium nitrate were surface-applied to a sodic, sandy loam soil to achieve an exchangeable sodium percentage (ESP) of 5%. The soils received 300 mm simulated rainfall. Finer by-product gypsum sources (particularly liquid and flue gas desulphurisation (FGD) gypsums) exhibited faster dissolution rates and superior performance in lowering soil ESP compared to Class A mined gypsums. Only the liquid, phospho-, FGD gypsums and calcium nitrate significantly increased soil hydraulic conductivity compared to the control. The MF calcitic limes had no effect on the sodic soil properties. In a separate controlled laboratory study, 7 gypsum sources (reagent grade gypsum, phosphogypsum, granulated phosphogypsum, mine water, FGD, mined gypsums), two calcitic lime products (granulated MF and agricultural lime), hydrated agricultural lime, and a 1:1 combination of mined agricultural gypsum and calcitic lime were surface-applied to two Western Cape acid (sandy and sandy loam) soils and compared in terms of soil acidity remediation in 50 mm increments to a depth of 400 mm. The soils received 1000 mm simulated rainfall. Gypsum could not substitute lime in terms of acidity neutralisation but did show capability in displacing acidity away from the topsoil. The 1:1 combination of mined Class A gypsum and calcitic lime proved to be a viable substitute for lime alone. In a greenhouse pot trial, 6 gypsum products (FGD gypsum, mine water gypsum, phosphogypsum, granulated phosphogypsum, mined recrystallised gypsum and mined gypsum) were surfaced applied to an acidic, sandy soil (5.1% clay) to raise soil Ca levels to 400 mg kg⁻¹. The objectives of this study were to investigate the effect of different sources and forms of gypsum on the chemical properties of an acid, sandy soil and assess their effects on canola growth and nutrient uptake. Canola (Blazer TT cultivar) plants were grown in the pots and harvested after 7 weeks. Shoot and root biomass, foliar nutrient content soil chemical properties were then determined. In the short- term, granulated phosphogypsum and mined recrystallised gypsum were superior in terms of correcting acidity and improving root growth compared to the other gypsums, but all gypsum sources adequately addressed Canola Ca and S deficiency.
- ItemComparison 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.
- ItemThe effect of invasive alien plants (IAPs) biochar properties on soil quality and crop production(Stellenbosch : Stellenbosch University, 2024-03) Mncedi, Sipho; Hardie-Pieters, Ailsa G.; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.ENGLISH ABSTRACT: Woody invasive alien plants (IAPs), such as black wattle (Acacia mearnsii) and eucalypt (Eucalyptus camaldulensis), are a major problem in South Africa. IAPs can decrease surface water runoff and groundwater recharge and are actively being cleared on a large scale. The woody biomass generated from the clearing of IAPs can serve as feedstock for biochar production. By sequestering carbon and improving soil functions, biochar has shown promise for enhancing agricultural practices. The efficacy of biochar in enhancing soil quality and crop production varies widely depending on its properties which are influenced by the type of feedstock and pyrolysis conditions used. According to the literature, biochar can contain certain toxic compounds such as heavy metals and volatile organic compounds formed during pyrolysis. Therefore, it is critical to test biochars on intended soils and crops to select the most suitable biochars. The aim of this study was to investigate the effect of fresh and aged biochars produced from IAPs from different feedstocks and pyrolysis temperatures on soil quality and crop production. Five biochars were produced at a range of temperatures (500, 600, 700, and 800 °C) from eucalypt and a single temperature (600 °C) from black wattle biomass using low-vacuum pyrolysis. In June 2022, the biochars were applied at 1.5% (w/w) on acid sandy soil and cauliflower was cultivated in an optimally fertilized greenhouse experiment. Some of the soil-biochar mixtures were left outside for 11 months so that the biochar could age naturally in the soil. After 11 months of ageing, the cauliflower greenhouse experiment was repeated using the same cauliflower cultivar and experimental conditions. In both trials (fresh and aged biochar trial), soil chemical, physical and microbiological properties were determined at planting and harvest. At harvest, the cauliflower fresh and oven-dried curd, and above-ground biomass were determined. The oven-dried curd and leaves were analysed for total elemental macro and micronutrient analysis (N, P, S, Ca, Mg, K, Na, Fe, Mn, Cu, Zn and B). In the fresh biochar trial, the 600 °C eucalypt biochar had the most positive effect on cauliflower yields (53% increase), attributed to higher soil basic cation exchange capacity and N retention. Whereas the higher temperature (700 and 800 °C) eucalypt and 600 °C black wattle biochars suppressed crop yields by 26-79% attributed to phytotoxicity, alkalinity, and reduction of B and soil water availability. All fresh biochars were found to reduce total available water due to high microporosity. Whereas in the aged biochar trial, only low-temperature eucalypt biochar (500 °C) suppressed the total available water. Biochar ageing reduced soil total N, but slightly increased effective cation exchange capacity and easily available water content however not compared to the aged sand control. The ageing of eucalypt and black wattle biochar also seemed to reduce the phytotoxicity that was present in certain fresh biochars. However, none of the aged biochars had a significant effect on cauliflower yields compared to the control sand. The results of this study highlight the importance of biochar properties, as influenced by pyrolysis conditions and feedstock, on crop growth response.
- ItemEvaluating the effect of low flow drip irrigation on selected growth parameters of citrus trees(Stellenbosch : Stellenbosch University, 2023-03) Pretorius, Johannes Jacob; Hoffman, Josias Eduard; Raath, Pieter Johannes; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.ENGLISH ABSTRACT: The use of low flow drip irrigation is preferred nowadays in South Africa due to easier irrigation management, increased ability of precision irrigation and claims of increased water-use efficiency. The aim of the study was to investigate the validity of such claims since there is mounting pressure for all industries to find new methods to save water. Furthermore, it is imperative to investigate the effect that any new irrigation method has on both the soil environment and plant productivity. This study investigated the performance of three different drip delivery rate treatments (T1-1.6 L.h⁻¹, T2-0.7 L.h⁻¹ and T3-0.4 L.h⁻¹) with regards to soil water content (SWC) distribution, soil electrical conductivity and pH distribution, water-use efficiency, tree stress, fruit yield and quality. The soil water content distribution showed that the use of pulsed irrigation with a higher emission rate such as T1-1.6 L.h⁻¹ was most successful in preventing the movement of water out of the active rootzone, followed by T3-0.4 L.h⁻¹ and T2-0.7 L.h⁻¹. For T3-0.4 L.h⁻¹ it was concluded that the application rate was very similar to the overall uptake rate of the trees since there was also a reduced movement of water beyond the active root zone and very small changes in the water content of the main wetted core even as irrigation was applied. The higher application rate of T2-0.7 L.h⁻¹, compared to T3-0.4 L.h⁻¹ led to a greater degree of water movement beyond the active root zone. The calculated water-use efficiencies showed very good agreement with the measured SWC distributions since T1- 1.6 L.h⁻¹ had the highest water-use efficiency, followed by T3-0.4 L.h⁻¹ and T2-0.7 L.h⁻¹ respectively. The electrical conductivity showed good agreement with the soil water distributions since there was an increase in salts at the periphery of the wetted volumes; this became more prominent as the drip delivery rate decreased. All treatments showed severe acidification directly beneath the drippers which was ascribed to the form and quantity of nitrogen that was applied to the trees. The predawn leaf and midday stem water potential measurements taken during the main phenological phases of fruit growth during the 2022 production season concluded that no single treatment was constantly subject to more stress compared to the other. The results did however indicate that all treatments displayed symptoms similar to trees that only have a fraction of the root zone exposed to sufficient water content ranges. Considering the results already discussed, it could be concluded that the availability of soil water to the plants was not only influenced by the physical distribution thereof, but it was also less available due to chemical constraints. The fruit yield of the 2021 production season was mainly influenced by differences in the applied irrigation volumes with T1-1.6 L.h⁻¹ having the highest yield, followed by T2-0.7 L.h⁻¹ and T3-0.4 L.h⁻¹ respectively. Alternate bearing also seemed to have played a role in the yield differences observed for the 2021 and 2022 production seasons. The yield of the 2022 production season was mainly affected by tree physiology and T1-1.6 L.h⁻¹ again had the highest yield, followed by T3-0.4 L.h⁻¹ and T2-0.7 L.h⁻¹ respectively. Overall, the fruit quality was always at acceptable export levels for all treatments.
- ItemThe effect of plant water potential-based deficit irrigation on physiological reproductive responses of grapevines (cv. Shiraz) in different climatic regions.(Stellenbosch : Stellenbosch University, 2023-03) Visser, John-Murray; Lategan, Eugene Lourens; Hoffman, Eduard Josias; Van Zyl, Jan Louis; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.ENGLISH ABSTRACT: This thesis explores the effects of three midday stem water potential (midday ψS) thresholds on the physiological and reproductive responses of grapevines cv. Shiraz. Threshold treatments were applied, between pea berry stage and harvesting during three growing seasons i.e. from 2019/20 until 2021/22. The experiment was carried out in three main wine grape growing regions of South Africa namely, Olifants River Valley (Klawer), Breedekloof Valley (Slanghoek) and Coastal region (Stellenbosch). The following midday ψS thresholds were applied: -1.1 MPa (T1); -1.5 MPa (T2) and - 1.8 MPa (T3). Over the three seasons and in all regions, T3 produced bunches with lowest mass, compared to that of T1. Improvements in overall grape quality also occurred due to significant reductions in water applications in all three regions. The Klawer region had a saving in irrigation water of 38% for T2 and 55% for T3 compared to T1 while these savings were 64% and 81% in Slanghoek a similar trend occurred in the Stellenbosch region with a reduction of 39% for T2 and 89% for T3 compared to T1. The study concluded that there is a correlation of midday ψS with yield. The research can greatly help the wine industry with water saving and improved wine quality through the use of midday ψS as an aid in irrigation scheduling.