Masters Degrees (Soil Science)

Permanent URI for this collection


Recent Submissions

Now showing 1 - 5 of 83
  • Item
    The 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.
  • Item
    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.
  • Item
    Quantifying water movement in soils irrigated by continuous drip irrigation with citrus in the Western Cape
    (Stellenbosch : Stellenbosch University, 2022-04) van der Merwe, Johan Herbst; Hoffman, Eduard; Raath, Pieter; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The development of low discharge rate emitters by Netafim opened new possibilities to approach irrigation and fertilization and introduced continuous low flow drip irrigation, allowing for intensive management of fruit production on difficult to manage soils. This entails decreasing the application rate of the irrigation system to balance with the maximum daily water use over the total consumptive period of the crop addressing many challenges in managing irrigation in an orchard. A key for efficient drip irrigation is to adjust the shape of the wetted soil volume created under an emitter to the main root distribution, which is well known for conventional drippers in different soil types, but not for low flow continuous drip irrigation. In this study the daily water requirement of young ‘Nadorcott’ mandarin citrus trees were applied in three continuous drip irrigation treatments with different drip discharge rates (T1-0.4L.hˉ¹, T2-0.7L.hˉ¹ and T3-1.6L.hˉ¹ allowing different irrigation cycle lengths, to study the effect on soil water content distribution, root distribution and crop response . T1-0.4L.hˉ¹ maintained an average higher SWC in the active root zone along the season followed by T2-0.7L.hˉ¹ and T3-1.6L.hˉ¹ respectively. Significantly higher SWC was observed close to the soil surface with T1-0.4L.hˉ¹ followed by T2-0.7L.hˉ¹ and T3-1.6L.hˉ¹ respectively. Relative average lower SWC with daily conventional irrigation was expected due to narrower spacing of drippers and lower volume of water applied per emitter. 2-Dimentional water distribution patterns parallel and across the ridge showed that T1- 0.4L.h resulted in a smaller wetted soil volume compared to T2-0.7L.h with less horizonal and vertical water distribution from an emitter, which may have encouraged fibrous roots to grow closer to the soil surface and to the tree. More overlap was observed in T2-0.7L.h parallel with the ridge resulting in greater vertical distribution of water between two adjacent drippers where wetted soil volumes merge which favoured roots to grow and proliferate deeper down the soil profile and further away from the tree. A wetted strip was formed under T3-1.6L.h due to narrower spacing of drippers, compared to the ‘pots’ formed with T1-0.4l.h and T2-0.7L.h, resulting in a dense root strip across the profile in the 20 to 40cm depth. Highest fine root density was recorded in T3-1.6L.h in the 20 -30 cm layer, followed by T2-0.7L.h in the 30 to 40 cm layer and T1-0.4L.h in the 20-30 cm layer respectively. A bimodal distribution of water was observed in T1-0.4L.h and T2-0.7L.h early in the morning when a shorter irrigation event took place characterized by a main wetted soil volume in the upper 50 cm and concentrated areas of higher soil water content in the lower 50 cm. Approaching midday as irrigation continued SWC in the main core of the wetted soil volume increased and lower SWC was observed in the rest of the soil profile indicating a ‘shrinking’ of the wetted bulb midday after which a bimodal water distribution resumed. T3- 1.6 showed very little fluctuation during the day. It seems as if the antecedent soil water content may contribute largely to soil water content fluctuation and distribution patterns. Overall little fluctuation of the wetted soil volume was observed daily, maintaining very constant SWC levels in the root zone.
  • Item
    Structure and chemistry of two heuweltjies in areas with contrasting aridity in the Olifants/Doorn catchment: evidence for downward salt movement
    (Stellenbosch : Stellenbosch University, 2022-04) Hattingh, Magdaleen; Clarke, Catherine E.; Francis, Michele Louise; Miller, Jodie A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The Olifants/Doorn catchment in the West Coast region of South Africa is variably affected by saline groundwater. Other areas along the coast of southern Africa experience similar mean annual precipitation rates but do not display the same variability in saline groundwater. This suggests additional contributions to groundwater salinisation in the West Coast. Regularly spaced earthen mounds, termed heuweltjies (up to 2 m high and about 32 m wide), occur in abundance in the Olifants/Doorn catchment area. These mounds are characterised by significantly higher salinity levels compared to surrounding soils. Variable saline groundwater seems to spatially correspond with salt affected heuweltjies, suggesting that mound salts might be contributing to groundwater salinisation in the region. Two heuweltjies, one in a semi-arid climate (Klawer) and the another in a more Mediterranean climate (Piketberg), were excavated to determine the morphological properties and distribution of soluble salts and ions within the mounds. The study was conducted to determine and compare if salts in heuweltjies with different mean annual precipitation rates could be contributing to groundwater salinisation of the Olifants/Doorn catchment. The mineralogy, soil texture, electrical conductivity, pH, anion and cation profiles and dissolved silica was analysed to determine if these heuweltjies are potentially contributing to the groundwater chemistry. Dominant salts exclusively present in heuweltjie soils were identified in both the mineralogy and modelling of ion concentrations. Calcite (in both heuweltjies) and gypsum (in the mound in Klawer) were identified to be enriched in mound soils. The less soluble calcite was saturated at closer to the surface compared to more soluble gypsum at greater depths in both mounds. This sequence of precipitation of increasing soluble salts suggested that the net direction of water movement occurs downward in mounds of both high rainfall and lower rainfall areas. The clay mineralogy did not support the parent material as a provenance. Additionally, increased coarse soil texture with depth and large macropores of heuweltjies indicated that groundwater is an unlikely cause of mound salinity. The chemical signature of salts indicated a marine origin. Concentrated hotspots of ions and minerals within biogenic features of mound soils suggested that burrowing fauna are responsible for accumulating marine-derived salts in heuweltjie soils. Preferential flow paths that aid solute movement were observed in gravelly dorbank, fractured platy dorbank and as termite channels and rodent burrows in the mound centres of both Piketberg and Klawer. This study reinforced the hypothesis that heuweltjie salts are generated within mounds, possibly through biological activity, and that salts are potentially translocated to the groundwater through preferential flow paths in mounds of the Olifants/Doorn catchment.
  • Item
    The effects of long-term tillage and crop rotation practices on nutrient stratification in the Western Cape
    (Stellenbosch : Stellenbosch University, 2022-04) Van der Merwe, Annemarie; Hardie-Pieters, Ailsa G.; Labuschagne, J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The Western Cape is one of the most successful provinces in South Africa in converting to Conservation Agriculture (CA), with an adoption rate of around 80%. CA is a production system that promotes minimal soil disturbance, maintaining crop residues on the soil surface combined, with crop rotation with different species, including legumes. The absence of soil mixing in CA systems can lead to the stratification of immobile nutrients at the surface of the soil profile. Rapid drying of top soil layers may prevent roots from absorbing nutrients from these layers. Up until now, little was known regarding the extent of nutrient stratification in CA systems in the Western Cape. The first objective of this study was to determine the vertical distribution of plant-available nutrients under different tillage practices and rotation sequences at Tygerhoek (34 ̊29’32” S, 19 ̊54’30”E) and Langgewens (33 ̊16’34” S, 18 ̊45’51” E) Research Farms. Soil samples were collected at 0-5, 5-10, 10-15, 15-20 and 20-30 cm depth intervals in zero-till (ZT), no- tillage (NT), minimum tillage (MT) and conventional tillage (CT) treatments combined with 4 crop rotation sequences - wheat monoculture (WWWW), wheat and medics rotation (WMWM and MWMW) and canola/wheat/lupine/wheat (CWLW). Crop rotation and its interaction with tillage and soil depth did not influence (p < 0.05) the distribution of nutrients in the soil at Tygerhoek, but the distribution of K and S in the soil at Langgewens was influenced. Tillage significantly influenced nutrient stratification at both sites. The amount of extractable Ca, Mg, P, K and C were significantly higher in the surface 0-5 cm of the soil under ZT and NT compared to CT. The higher soil organic carbon (SOC) in the topsoil under CA (ZT, NT and MT), may be due to reduced soil disturbance and retention of crop residues. The organic C in the 0-5 cm layer decreased as degree of soil disturbance was increased (ZT2.6%>NT 2.23%> MT 2.15% > CT 1.96%). The percentage difference in soil exchangeable K at Langgewens between 0-5 cm (308 mg kg-1) and 5- 10 cm (172 mg kg-1) layers were the most at 79% for ZT. The second objective was to evaluate the extent of soil nutrient stratification in a wide range of CA systems on commercial farms. Stratification was observed in both the natural veld and the cultivated soil. The cultivation and addition of ameliorates (fertiliser and lime) in the cultivated soil have accentuated the stratification compared to the natural veld. The use of incremental soil sampling identified layers with levels of deficiency for K, S, Zn as well as the occurrence of subsoil acidity. On farms where CA was practiced for more than 25 years, C was built up and the highest values of Ca, K, P and S were present. The third objective was to determine the adoption rate of CA principles and awareness of soil nutrient stratification through a questionnaire. Most of the respondents (63%) indicated that they used CA as farming system. Minimum soil disturbance (47%) was indicated as the most important CA principle, followed by crop rotation (37%) and stubble retention (10%). More than half of the respondents (54%) indicated that the carbon content in the soil were higher than 1.5 %. This could be influenced by the fact that 82% of the respondents were from the southern Cape. More attention to sampling depth is required in conservation agriculture when sampling for lime and fertiliser recommendations to reflect the nutrient status of the soil. In the shallow soil of the Western Cape, soil increments of 0-10 cm and 10-30 cm are recommended for a better reflection of the nutrient status of the soil.