Doctoral Degrees (Soil Science)

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Browsing Doctoral Degrees (Soil Science) by browse.metadata.advisor "Hoffman, Josias Eduard"

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    Deficit irrigation and canopy management practices to improve water use efficiency and profitability of wine grapes
    (Stellenbosch : Stellenbosch University, 2024-03) Lategan, Eugene Lourens; Hoffman, Josias Eduard; Myburgh, Philipus Albertus; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Grapevines irrigated at low plant available water (PAW) depletion levels required more than double the irrigation volumes compared to those irrigated at high depletion levels. The accelerated sugar accumulation of sprawling grapevine canopies resulted in earlier harvest dates, reducing pre-harvest irrigation requirements. Different canopy manipulations did not affect total leaf area (LA) per grapevine within an irrigation strategy, but negatively affected LA as less water was applied. Non-suckered grapevines produced more shoots and more vigorous shoot growth, while non-suckered vertical shoot positioned (VSP) grapevines tended to produce lower cane mass. The LA distribution provides a good indication of canopy orientation, and non-destructive measurements of primary and secondary shoots can estimate winter pruned cane mass. This would allow viticulturists, producers, or irrigation consultants to estimate the maximum cane mass and use the VINET© model to predict grapevine water requirements in real-time throughout the season, as the LA is estimated using cane mass. Grapevines with sprawling canopies had lower mid-day leaf water (ΨL) and stem water (Ψs) potentials compared to those with VSP canopies. Grapevines experiencing severe water constraints ripened more rapidly than those without or with medium water constraints. Low frequency irrigation increased water constraints compared to high frequency irrigation. Diurnal ΨL cycles showed that grapevines with sprawling canopies had lower ΨL after 18:00 and throughout the night, suggesting that their water status could not recover as fast as VSP grapevines. High irrigation frequencies led to higher grapevine row evapotranspiration (ETGR) losses, with losses from sprawling grapevines, particularly those irrigated at ca. 30% plant available water (PAW) depletion, being higher in January and February than those with VSP canopies. Seasonal full surface evapotranspiration was more sensitive to irrigation frequency than to canopy manipulations. Grapevines irrigated at ca. 30% PAW depletion had higher mean full surface crop coefficient (Kc) values compared to other strategies, with those irrigated at ca. 90% PAW depletion having the lowest Kc values. The mean peak Kc was generally obtained in February for grapevines irrigated at frequencies, while the lowest Kc was found during the same period for low frequency irrigation applications. The fraction of soil wetted during irrigation applications under grapevine row (Kc,GR) could be a more realistic coefficient than Kc for producers and consultants to use in irrigation scheduling requirements. Irrigation frequency had a more significant impact on yield than canopy manipulation. Higher rainfall in 2013/14 increased vegetative growth and yield, with low frequency irrigations resulting in higher production water use efficiency compared to medium and high frequency irrigations. The incidence of grey rot was higher during the wetter season, with grapevines with sprawling canopies experiencing higher yield losses due to sun burn and less frequent irrigation. The highest incidences and yield loss to grey rot were found in grapevines left un-suckered and irrigated at ca. 30 PAW depletion, while irrigation at around 90 PAW depletion resulted in the absence of grey rot. Grapes were harvested near the target total soluble solids level of 24ºB, with severe water constraints enhancing berry maturation. Non-suckered VSP grapevines produced poorer quality at lower levels (30% and 60% depletion levels), with the highest overall wine quality obtained when irrigated at ca. 90% PAW depletion. Less frequent irrigations reduced summer canopy management requirements, but grapevines with more shoots required higher labour inputs at harvest. Pruning labour input requirements were affected by the number of shoots produced per grapevine and the mass per individual shoot. Sprawling canopy grapevines generally required lower labour costs, and pump costs were affected by the frequency of irrigation applications. During low to normal rainfall seasons, grapevines with sprawling canopies irrigated at ca. 60% PAW depletion produced the highest gross margins incomes, followed by box pruned grapevines irrigated at ca. 90% PAW depletion. In high summer rainfall seasons, box pruned grapevines and non-suckered VSP canopies had the highest gross margins. Grapevines with sprawling canopies, particularly those irrigated at ca. 60% PAW depletion, produced the best balance between yield and quality, ensuring the best gross margin incomes. The gross margin water use efficiency (WUEGM) increased with an increase in PAW depletion level irrigation, with box pruned grapevines consistently having the highest WUEGM. The study found that grapevines with sprawling canopies experienced lower diurnal and cumulative evaporation losses compared to VSP grapevines, regardless of PAW depletion levels. The higher mean leaf area per grapevine resulted in denser canopies, and treatments irrigated at approximately 30% PAW depletion were always within stage 1 of evaporation. Grapevines irrigated at around 60% PAW depletion occasionally went into stage 2, particularly in sprawling canopies. The water content of soil under grapevines irrigated at around 90% PAW depletion spent most of the season in stage 2. The vegetation coefficient (Kv) of sprawling canopies was lower than VSP grapevines, irrespective of PAW depletion. The VINET© model generally underestimated transpiration rates in wet soil regimes and overestimated them during dry soil regimes. Adjusting the model by addition of Kv and adapted transpiration water predictions can be done using two multilinear regressions after a few grapevine canopy measurements inputs have been considered.
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    Evaluation of old store-and-release covers on discard dumps and backfilled pits to improve and predict their performance for rehabilitated mines at Mpumalanga Highveld, South Africa
    (Stellenbosch : Stellenbosch University, 2021-12) van Schalkwyk, Roeline; Hoffman, Josias Eduard; Van Zyl, Johan Hendrik Combrink ; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Store-and-release covers (SRCs) are an important mitigation method to protect the environment at rehabilitated mines in the Mpumalanga Highveld, South Africa. The long-term performance of SRCs can be influenced by soil cover-, soil hydraulic-, and vegetation properties. Currently, a Technical Guideline for Soil Covers Development is not in place in South Africa. In addition, data sets of well- and poorly constructed covers, and the availability of data on appropriate input parameters for predicting long-term performance of such covers are limited. This need includes data for saturated hydraulic conductivity (Ksat), soil water retention curves (SWRCs), and photosynthetic active leaf- area index (LAI). Moreover, the measurement of Ksat and SWRCs is time-consuming, labour intensive and costly. Consequently, a multidisciplinary study to investigate the impact of soil cover-, soil hydraulic- and vegetation properties on long-term performance of SRCs was initiated. Most importantly, pedotransfer functions (PTFs) to predict Ksat and SWRCs were developed from particle- size distribution, soil organic matter (SOM) and bulk density. Leaf area index values for good and poor vegetation covers were determined for rehabilitated mines in Mpumalanga Highveld. Soil cover properties viz. cover configurations, soil texture, Atterberg limits, bulk density and soil nutrient availability were determined. Saturated hydraulic conductivity were measured using two types of double-ring infiltrometer, a single-ring infiltrometer, and a constant-head permeameter. Soil water retention curves were established using the pressure plate apparatus. The SRCs data-set was split into training and testing sets to validate the SWRC model. After the SRCs data-set was split into moderately- and very dense SRCs data sets, and an additional site was used to validate the moderately dense Ksat model. The data-set of very dense SRCs was also split into training and testing sets to validate the very dense Ksat model. Monthly LAI from September 2018 to August 2019 was destructively measured using a LI-3100C Area Meter. The dual-layered SRCs were constructed with sandier growth medium (top layer) underlaid by a loamy to clayey water retention layer (sub-layer). Monolithic SRCs were constructed of sandy loam or sandy clay loam soil covers. After the SRCs were split into moderately- and very dense soil cover conditions, the moderately dense SRCs performed significantly better and had acceptable bulk densities, good vegetation covers with good root distribution in the growth medium, steep slope in the desaturation function of the growth medium and high water-holding capacity (WHC) in the water retention layers. The Ksat and WHC of the moderate SRCs over 20 years had values similar to that of the soils, but the values of sandier soil cover layers were lower than critical threshold values due to low resistance to compaction. The statistical analysis of best-fit moderately- and very dense Ksat. models yielded an adjusted R2 of 0.749 and 0.999, respectively from sand-, silt- and clay content, SOM and bulk density. The statistical analysis of the best-fit SWRC model of 14 matric potentials had an adjusted R2 = 0.827 from three fractions of sand-, two fractions of silt-, clay content, SOM, and bulk density. The photosynthetic active LAI for good and poor vegetation cover of rehabilitated mines at Mpumalanga Highveld was ~1.2 and 0.8 m2.m-2, respectively. Poorly constructed soil covers result in high bulk density, low to very low Ksat and WHC values and poor vegetation properties and should be avoided at any cost. The critical threshold values for bulk density, Ksat and WHC of soils can be used to evaluate long-term soil cover performance. The developed PTFs can be used to predict soil covers’ hydraulic properties having soil physical properties similar to the old SRCs. These results can be considered as a possible amendment to the Technical Guidelines on Soil Cover Development in South Africa.