- 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.
- ItemThe 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.
- ItemCarbon 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.
- ItemRole of soil carbon in crop yields and quality in crop rotational system(2023-03) Daiber, Rocco; Hardie-Pieters, Ailsa G.; Strauss, J. A.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.ENGLISH ABSTRACT: Conservation agriculture (CA), which includes no-till (NT) and crop rotation, can be implemented for a positive effect on soil organic matter (SOM) accumulation and carbon sequestration. This can lead to greater soil health and resilience, with subsequently increased yields. Previous research has shown soil organic carbon (SOC) accumulation is site-specific, with the climate, crops, and the soil properties to be considered. Studies about the role that SOC plays in wheat yield are lacking in the Western Cape. In this study, soil (0-15 cm) and yield data from long- term field trials under CA conducted at Langgewens Research Farm (24ᵗʰ year) and Tygerhoek Experimental Farm (20ᵗʰ year) in the Western Cape were analyzed. The rotational systems implemented during the trials included wheat monoculture, 100% crop, 50% crop/50% pasture, and 33% crop/67% pasture. The objectives of this research project were i) to look at the long-term effect of the no-till crop rotations systems on SOC and wheat yield, ii) explore the relationship between the SOC content and the yield and protein content of wheat, and iii) obtaining optimum values and sufficiency ranges for the factors affecting wheat yield, protein content, and SOC content using boundary Line Analysis (BLA). Over the entire trial period, a significant increase in SOC content was only found at Langgewens (0.4%) attributed to it its substantially lower starting SOC (0.91%). The average SOC content being higher at Tygerhoek (1.68%) compared to Langgewens (1.17%) attributed to differences in soil properties and climate. It was observed that the SOC had likely reached a ‘saturation point’ at both sites. At both sites, the wheat monoculture had a significantly lower yield, and the incorporation of natural vegetation (pastures and saltbush) had a benefit on the SOC content, wheat yield, and protein content. The rainfall showed a significant linear correlation with wheat yields. While soil pH, lime, gypsum, N fertilizer, soil nutrients, and the clay:C ratio did not show a significant correlation. No linear correlation was found between wheat yield and the SOC content, while some seasons showed a significant partial correlation. A panel regression showed significant correlations of some variables to wheat yield. Wheat monoculture and 100% crops had a significantly lower protein content at Langgewens and Tygerhoek respectively. The BLA for wheat yield showed that above and below a certain SOC point wheat yield will decrease at each site. Protein content decreased above 20 mg kg⁻¹ soil Sulphur. The BLA showed a decreased protein content above and below a certain SOC point. A significant negative correlation between average minimum temperatures and SOC content was found. The focus of this study was the long-term dynamics of SOC and yields, and their relationship. There was SOC accumulation with the implementation of CA but is dependent on the soil and climate, along with a ‘saturation point’. The SOC did not directly correlate with yield as it is likely not directly addressing the main yield limiting factors in this production area, which are mainly climate and soil related.