Doctoral Degrees (Soil Science)

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The concentration of selected trace metals in South African soils
(Stellenbosch : University of Stellenbosch, 2007-12) Herselman, Jacoba Elizabeth; Fey, M. V.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.
Trace elements occur naturally in soils, usually at low concentrations (<0.1% or <1000 mg kg-1 of the earth’s crust), as a result of weathering and pedogenic processes acting on the rock fragments from which soil develops (parent material). Since about 98% of human food is produced on land, soil is the primary source supplying these elements to the food chain. Although cases of trace element deficiency and toxicity have been documented in many parts of South Africa, no comprehensive description of trace element concentration has yet been attempted for South Africa as a whole. The Natural Resources Land Type mapping project, initiated in the mid-1970s, has provided a collection of samples (approximately 4500) from soil profiles selected to represent the main soil forms in each land type and therefore to provide representative coverage of most of the soils of South Africa. These archived samples have now been analysed for a spectrum of trace elements, in terms of both available and total concentrations as well as other soil properties. Although detailed information is available on a wide range of trace metals, the seven trace metals considered to be of most interest in a South African context due to natural geological occurrences were selected for this study, including Cd, Co, Cr, Cu, Pb, Ni and Zn. This data was used to: • determine baseline concentrations in SA soils; • determining threshold values for South African agricultural soils receiving sewage sludge at agronomic rates; • determining the influence of certain soil properties on the baseline concentrations of these trace elements in SA soils; and • development of a bioavailable trace element distribution map for SA. The range, the mean and standard deviation (both arithmetic and geometric), and the median were used to summarize the data statistically. The baseline concentration range was calculated using the quotient and product of the geometric mean and the square of the geometric standard deviation, including data below the instrument detection limit. The upper limit of the baseline concentration range was set at the 0.975 percentile value of the population in order to minimize the influence of contamination and the lower limit at the 0.025 percentile value to minimize problems that might be associated with analytical uncertainty near the lower limit of detection. The quantile regression statistical approach was followed to illustrate the relationship between soil properties and trace element concentrations in soils. The soil properties that showed the strongest relation were CEC, clay content, pH (H2O) and S value (base status). The soils were then divided into different classes according to these soil properties and baseline concentrations were derived for the different classes. Soils with low clay contents have lower trace element concentrations than soils with higher clay contents, soils with low or high pH levels have lower trace element contents than soils with intermediate pH values and mesotrophic soils have higher trace element concentrations than dystrophic soils. This information is useful for the compilation of trace element distribution maps for South Africa where different soil forms and series/families could be classified into different classes to determine areas of potential deficiencies as well as toxicities. South Africa, with its diverse geology, has areas of both trace element toxicities and deficiencies and for decision-making purposes it is necessary to identify these areas. Mapping of trace element levels based on soil samples would provide valuable information, which cannot be obtained from geological or geographical maps. Statistical analyses of the data (clay %, base status, pH (H2O) and NH4EDTA extractable trace element concentrations) indicated that soils could be divided into five trace element classes based on their clay content, pH and base status (dystrophic, mesotrophic and eutrophic). The soil series according to the binomial soil classification system for South Africa were then divided into these different classes. The geometric means for each clay class were determined and the baseline concentration range for each class was calculated. The land type maps were used as basis for the distribution maps. A general trace element distribution map for South Africa was derived from this data as well as Cu and Zn distribution maps. A random selection of 500 soil samples across the country was used to verify the accuracy of the distribution map. The general trace element distribution map indicate, with a confidence level between 89 and 96%, where the potentially available trace element content of South African soils are low (deficient) too moderately high, excluding rocky areas and areas with limited soil. The Cu and Zn maps indicate the distribution and expected baseline concentrations of these specific elements in South African soils. The same methodology could be applied to derive risk maps for all the individual trace elements to indicate the distribution and expected baseline concentrations of the elements in South Africa. This presentation of baseline concentrations, reflecting likely natural ranges in South African soils, is the first quantitative report on the spatial extent and intensity of Zn, Cu and Co deficiency in South African soils. The proposal of new threshold values for trace elements in agricultural soils will be valuable in setting more realistic norms for environmental contamination that accommodate the geochemical peculiarities of the region, one example being rather high Cr and Ni concentrations with low bio-availability. This information should be of value not only in environmental pollution studies but also in health, agriculture, forestry and wildlife management. The following recommendations are made: • The baseline concentrations could be used to determine site specific threshold values based on soil properties and soil type. Soils with lower pH, clay content and CEC would require more protection than soils with high pH, clay content and CEC and therefore the threshold levels for these soils should be lower. • Although the distribution maps can be used to indicate broad areas of trace element deficiencies and toxicities, more detailed investigations are recommended for areas where problems are experienced. The same methodology could be applied on smaller scale to increase the value of the map and to add more value on a regional scale. The maps could be used for regional soil quality assessment especially in areas where trace element deficiencies or toxicities could result in negative effects on plants and animals.
The contribution of indigenous vegetables to food security and nutrition within selected sites in South Africa
(Stellenbosch : Stellenbosch University, 2013-12) Mavengahama, Sydney; De Clercq, W. P.; McLachlan, Milla; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: South Africa is rich in biodiversity among which are semi-domesticated vegetable species which are known as wild or indigenous vegetables. These wild indigenous vegetables have been reported to be good in nutritional qualities such as macro and micronutrients. However, there is still a high prevalence of malnutrition; especially micronutrient deficiencies among low or marginal income bracket of the population. The use of indigenous vegetables has been proposed as part of the solutions to the problems of micronutrient malnutrition among these populations. Indigenous vegetables are an important source of food in the maize based subsistence farming sector of rural South Africa. Their main role is as relish as they are used as an accompaniment for staple cereal based diets. They are also generally reported to be rich in micronutrients. Although they may be consumed in small quantities, they influence the intake of cereal staples, manage hunger and play a central role in household food security for the poorer rural groups. Mixing several indigenous vegetables species in one meal contributes to dietary diversity in terms of more vegetable types as well as in terms of choice of relish. For some very poor families indigenous vegetables are substitutes for some food crops. The seasonal occurrence of these vegetables leaves many families without a food source during the off-season. Indigenous vegetables increase agro-biodiversity at the household level. This agro-biodiversity helps in buffering against the accumulation and multiplication of pests and diseases and provides important cover for the soil. Further research on agronomic, social and economic dimensions is required to understand the roles of IV in subsistence farming systems in South Africa. The survey study revealed that indigenous vegetables were important in the diets of most rural people in the study area. They were consumed as relish although they were not being cultivated. Their method of acquisition was gathering from homesteads and the wild. These vegetables were also believed to be medicinal. The local naming of wild vegetables varied among villages in the same district such that a vegetable in one village was assigned to a different species of vegetable in another village. They were reportedly abundant during summer and there was a decrease in availability off-season leaving vulnerable people who rely on them with a food shortage. The utilisation of wild vegetables among South Africans is reported to be declining due to over reliance on introduced temperate species. Efforts to domesticate and cultivate wild vegetables could be hampered by several factors including seed dormancy and premature flowering. In this present study dormancy was observed in C. olitorius. The response of wild genotypes of C. olitorius with different seed sizes to various dry heat and hot water treatments was evaluated. Steeping seeds in boiling water (95oC) for ten seconds and soaking seeds in a hot water bath at 80oC for ten minutes resulted in the highest response to germination in this species. The study also recorded significant interactions between heat treatment and seed sizes. We concluded that C. olitorius seeds of different sizes require diverse durations of exposure to heat treatment methods to break dormancy caused by an impermeable seed coat. Cleome gynandra is another species that is consumed as a vegetable in various parts of the world including Africa. The plant is also used as a medicinal herb for the treatment of various human diseases. Among the wild vegetables, C. gynandra has been reported to flower prematurely, a phenomenon known as bolting and common in many vegetable crops. Premature flowering (bolting) can be as a response to temperature extremes and photoperiod and affects many other leafy vegetables such as lettuce (Lactuca sativa), spinach (Spinacea oleracea) and mustard rape (Brassica juncea). Bolting leads to production losses in leaf vegetable crops as they flower before they have produced an economic yield. The removal of flowers and nitrogen application resulted in significant increases in the fresh and dry weight of cleome leaves. Removal of flowers resulted in a 46% increase in fresh weight of leaves. The observed positive response of leaf yield to removal of flowers offers a possible way to deal with the problem of bolting. The continuous removal of the flowers leads to increased utilisable leaf yield. The application of incremental amounts of nitrogen top dressing results in increased leaf yield in C. gynandra. The response of selected indigenous vegetables (Corchorus olitorius and Amaranthus cruentus) to micronutrients added to the soil was compared with the response of a reference crop; Swiss chard (Beta vulgaris var. cicla). For all the levels of micronutrients applied, Swiss chard accumulated Cu, Zn and Mn in the leaves at significantly (p<0.01) higher concentrations than the wild vegetables. Variations between the vegetables in the micronutrients were greater for Zn (72–363 ppm) and Mn (97.9–285.9 ppm) for Cu (8.8–14 ppm). C. olitorius had the least capacity to concentrate Mn and Zn in the leaf, which suggested that this vegetable is a less attractive candidate for agronomic bio-fortification of these elements. However, C. olitorius accumulated Fe at a significantly higher concentration (327 ppm) in the leaves than did Amaranthus (222 ppm) or Swiss chard (295 ppm). Sulphur as a macronutrient varied little in the plant species tested. The mean S concentration in the leaves ranged from 0.26% in C. olitorius to 0.34% in Amaranthus cruentus and Swiss chard. We concluded that the different vegetables have different abilities to take up Cu and Zn in the order Swiss chard > Amaranthus > Corchorus, and that they responded to micronutrients added to the soil but only up to certain limits of supplementation. The results from this current study seem to contradict the belief that wild vegetables have the inherent ability to concentrate mineral micronutrients in their tissue. Factors such as environment, anti-nutrients, dietary diversity, plant parts, plant age, and varieties result in differences in reported nutritional composition of indigenous vegetables. Post-harvest handling, storage, cooking and preservation also alter the composition. The need to optimise protocols for each vegetable type and for different laboratories makes analysis expensive. Equipment and methods of analysis are varied and may not be comparable, making it difficult to generalise on the composition of these vegetables. The Agricultural Research Council of South Africa and other stake holders are conducting studies on some aspects of these vegetables. There are still many information gaps regarding many aspects of these vegetables which require research attention. These include; the selection and improvement of genotypes, seed biology and germination studies, agronomic (population, fertiliser, crop mixtures) studies and phyto-chemical evaluation of these important species in order to encourage the overall use of these important indigenous resources. Finally, there is need to promote their increased utilisation.
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.
Determination and modelling of evapotranspiration of bearing and non-bearing apple trees at Grabouw in the Western Cape
(Stellenbosch : Stellenbosch University, 2024-03) Meyer, Aline; Van Zyl, J. L.; Hoffman, J. E.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: The irrigation of apple trees is important to ensure sustainable production and good quality fruit, especially in regions where rainfall does not contribute adequately to the water demand. Effective scheduling is the key to efficient water use to ensure profitability and sustainability on farms. Quantitative knowledge of the water use and the effect of irrigation application on young apple trees will improve on-farm decision making regarding scheduling. A study was conducted on Malus domestica “Bigbucks‟ (a mutation of “Corder Gala‟) trees grown in a gravelly soil at Grabouw in the Western Cape to determine the effect of three irrigation cycles on the water use, root growth characteristic and tree performance over four growing seasons (October to May) from planting to the first year of bearing. Treatment one (T1) was a short irrigation cycle receiving ca. 15 mm of water per irrigation with an average of 42 irrigations through the growing season, treatment two (T2) was a medium irrigation cycle receiving ca. 27 mm of water with an average of 21 irrigations through the growing season and treatment three (T3) was a long irrigation cycle receiving ca. 37 mm of water with an average of 13 irrigations through the growing season. Crop evapotranspiration (ETC) was determined for all three treatments based on the soil water balance. The ETC of all three treatments increased from the first to the fourth growing season as the leaf area index (LAI) of apple trees increased. T1 had a higher consumptive water use than T2 and T3. Studies done using micro-lysimeters to determine the orchard floor evaporation revealed that T1 lost more water through evaporation compared to T2 and T3, but water loss from the soil mainly occurred through transpiration, irrespective of the treatment. In situ and destructive root studies revealed that both root length density (RLD) and the number of fine roots within the soil profile is strongly related to soil water extraction (SWE). SWE increased with an increase in RLD and the number of fine roots. These results revealed that growing roots can continuously grow into moist regions of the soil. Significantly more fine roots penetrated deeper soil layers and at a greater distance from the tree for the two drier irrigation cycles (T2 and T3). The root index (RI) of T2 and T3 was also higher in deeper soil layers compared to T1 suggesting that soil moisture conditions of T2 and T3 were more favourable in deeper soil layers. It was concluded that short irrigation cycles will favour shallow root growth while longer irrigation cycles promote roots into deeper soil layers. There were no significant differences among treatments in terms of diurnal plant water status, vegetative growth, yield and fruit quality. These results suggest that longer irrigation cycles can be used to save water while simultaneously increasing root growth to deeper soil layers without compromising tree performance. Statistical analysis performed on ETC and RLD data revealed that there is a strong, positive correlation (R² = 0.741) between ETC and RLD. The data was used to develop a statistically significant model in which ETC can accurately be predicted using RLD data or vice versa. The model can be used as a reference for apple producers in South Africa to encourage more precise and effective irrigation scheduling while simultaneously increasing RLD for better water and nutrient uptake resulting in optimal crop production and quality.
Determination of optimal soil conditions and foliar nutrient levels in commercial rooibos tea production
(2023-03) Smith, Jacobus Francois Naude; Hardie-Pieters, Ailsa G.; Hoffman, J. E.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
Digital soil mapping techniques across multiple landscape scales in South Africa
(Stellenbosch : Stellenbosch University, 2019-12) Trevan, Flynn; Clarke, Catherine E.; Rozanov, Andrei Borisovich; De Clercq, W. P.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Digital soil mapping has seen increasing interest due to environmental concerns and increasing food security issues. Digital soil mapping offers a quantitative approach which is cost effective as less soil observations are needed to produce large area soil maps. However, digital soil mapping has only recently been addressed in South Africa. This research aimed to produce two digital soil mapping (DSM) frameworks with the available resources in South Africa. The methodologies incorporate advanced geostatistics and/or machine learning techniques to be able to produce quantitative soil maps from the farm to catchment scale. First, a framework that optimises both feature selection and predictive models was developed to produce farm-scale soil property maps. Four feature selection techniques and eight predictive models were evaluated on their ability to predict particle size distribution and SOC. A boosted linear feature selection produced the highest accuracy for all but one soil property. The top performing predictive models were robust linear models for gravel (ridge regression, RMSE 9.01%, R2 0.75), sand (support vector machine, RMSE 4.69%, R2 0.67), clay (quantile regression, RMSE 2.38%, R2 0.52), and SOC (ridge regression, RMSE 0.19%, R2 0.41). Random forest was the best predictive model for silt content with a recursive feature selection (RMSE 4.12%, R2 0.53). This approach appears to be robust for farm-scale soil mapping where the number of observations is often small but high-resolution soil data is required. Second, 24 geomorphons (landform classification) were evaluated on their association with soil classes. The geomorphon with the highest association was aggregated into a 5-unit system which was evaluated on how well the system stratified soil lightness, soil EC, SOC, effective rooting depth, depth to lithology, gravel, sand, silt, and clay. It was found that an aggregated geomorphon stratified all soil attributes except EC. Additionally, the aggregated geomorphon predicted 6 out of 9 soil properties with the greatest accuracy (RMSE) when compared to the original geomorphon (10-unit system) and a manually delineated system (5-unit system). This study shows that aggregating geomorphons can stratify the soil landscape even at the farm-scale and can be used as an initial indication of the soil spatial variability. Third, a framework to disaggregate the Land Type Survey (LTS) through machine learning was developed. Geomorphons, together with the original LTS were overlaid to produce terrain morphological units. The polygons were disaggregated further to produce a raster map of soil depth classes through a disaggregation algorithm known as DSMART. The first most probable class raster achieved an accuracy of 68% and for the two most probable class rasters, an accuracy of 91% was achieved. The two-step approach proved necessary for producing a farm-scale soil map. Forth, a study aimed to compare 10 algorithms, implemented through a modified DSMART model, in their ability to disaggregate two polygons into soil associations in two environmentally contrasting locations (Cathedral Peak, KwaZulu-Natal Province and Ntabelanga, Eastern Cape Province). At Cathedral Peak (high relief with clear toposequences), nearest shrunken centroid was the top performing algorithm with a kappa of 0.42 and an average uncertainty of 0.22. At Ntabelanga (low relief with strong geological control), the results were unsatisfactory. However, a regularised multinomial regression was the top performing algorithm, achieving a kappa of 0.17 and an average uncertainty of 0.84. The results of this study highlight the versatility of a technique to disaggregate South Africa’s national resource inventory. Disaggregation was then used to simultaneously disaggregate 20 land types in the Mvoti catchment covering 317 km2 in KwaZulu Natal province. First, the optimal geomorphon was chosen through a spatially resampled Cramer’s V test to determine the association between the soil legacy polygons and the geomorphon units. Second, feature selection algorithms were embedded into DSMART. Third, the feature selection techniques were compared using 25, 50, 100, and 200 resamples per polygon. The results indicate that the Cramer’s V test is a rapid method to determine the optimal input map. Feature selection algorithms achieved the same accuracy as using all covariates but had greater computational efficiency. It is recommended that 10 to 20 times the amount of soil classes be used for the number of resamples per polygon.
Effect of different biochars on inorganic nitrogen availability
(Stellenbosch : Stellenbosch University, 2018-03) Aghoghovwia, Makhosazana Princess; Hardie-Pieters, Ailsa G.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Biochar (a fine pyrolysed organic material) is an amendment used to increase and sustain productivity, reduce environmental pollution and sequester carbon (C) in soils. Successes were reported in improving acidic, sandy and highly weathered soil. However, the effects are strongly influenced by biochar physico-chemical characteristics, which vary widely depending on feedstock and pyrolysis conditions. The main objective of this study was to determine the effects of six biochars (commercially-produced in South Africa under various pyrolysis conditions from maize stover, grape pip, grape skin, pine wood, rubber tyre and sugarcane pith) on nitrogen (N) interactions in a sandy soil. The physico-chemical properties of the above biochars were characterised, three main experiments were conducted to study the effects of biochar addition to soil on (1) inorganic (ammonium and nitrate) N adsorption and desorption of added ammonium and nitrate in aqueous solution; (2) soil C and N mineralisation; and (3) leaching of inorganic N fertiliser. Maize stover and grape skin chars were suggested to be imperfect biochars due to low total C contents. Characterisation suggested that the sugarcane pith char was either not a suitable raw material for biochar or it was not actually a biochar due to its low stability and high chemical reactivity. However, its high ash content (66%) suggests good nutrient delivery as a soil amendment. Pine wood biochar was the most recommendable because of its low ash (3.5%), high total C (80%) and high surface area (344 m2 g-1), which all aid nutrient and water holding. However, the grape pip biochar had a low surface area (9.8 m2 g-1) and the highest fixed-C content (87%) which can be good for soil C storage. This work shows that despite many positive effects of biochar application to soil reported in literature, the negative effects of such applications on N availability are clear. All six biochars had a stronger nitrate removal affinity (82-89%) compared to ammonium (33-39%). It was also shown that adsorbed nitrate was not desorbable (0.01-0.23%) compared to adsorbed ammonium removal which was around 50% desorbable with KCl. Based on the shape of the adsorption isotherms, physisorption was the suggested mechanism for this behaviour. Competing reactions such as redox reactions in nitrate adsorption and volatilisation of ammonium were also suggested to have influenced the adsorption study results. Laboratory incubation studies showed that biochars enhanced N immobilisation along with increase in absolute and suppression of relative soil respiration. Pine wood and sugarcane pith biochars were found to reduce inorganic N availability the most due to net N immobilisation. The following biochar property may be linked to N immobilisation: inherent inorganic N in the soil-biochar system. Suppression of relative soil respiration may be due to biochar fixed-C content. Sugarcane pith char had the least effect on relative respiration because of its low fixed-C content (15.6%). However, the remaining biochars were substantially limiting the relative CO2 emissions. Rubber tyre char was the best performer in this regard with 75% lower cumulative relative CO2 emissions compared to the control. Among the plant-derived biochars, grape pip had the lowest CO2 released with 59% lower cumulative relative CO2 release. The leaching column experiment showed that application of biochars at 2.5% (w/w) to sandy soil reduced cumulative leaching of NH4+ and NO3- by 15-26% and 11-54%, respectively, compared to unamended soil. Using 15N labelled ammonium nitrate, it was found that 0.77-10.81% of applied fertiliser N remained in soil-biochar treatments after leaching. Only the pine wood and sugarcane pith biochar treatments significantly increased N fertiliser retention by 136 and 157% compared to the control soil. Whereas, the rubber tyre biochar treatment significantly reduced N fertiliser retention by 81%. The study concludes that all six biochars make inorganic N less available by mechanisms such as nitrate capture which is related to aromaticity and metal content of the chars and by enhancing biological immobilisation.
The effect of saline irrigation on selected soil properties, plant physiology and vegetative reproductive growth of Palsteyn appricots (Prunus armeniaca L.)
(Stellenbosch : University of Stellenbosch, 2005-12) Volschenk, Theresa; Kotze, W. A. G.; Cramer, M. D.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science.
Approximately 45% of apricot tree plantings in South Africa are centered on Montagu, Koo and Barrydale in the Little Karoo. Below average production in this area could be ascribed to the deteriorating water quality of the Breede River and highly saline groundwater from boreholes which provide this area with irrigation water. Profit margins for farmers are such that decreased yields cannot be tolerated. Correct management of low quality water could improve production and net farm income and could decrease irrigation return flow into the river system. The objective of this work was to establish whether international water quality guidelines for apricot are applicable under a different set of climatic conditions for a locally important cultivar and to revise guidelines if necessary for the management of irrigation with saline water. A drainage lysimeter was used to evaluate the effect of saline irrigation on apricot (Prunus armeniaca cultivar Palsteyn) trees over a period of four years at Stellenbosch (S33° 55’; E18° 53’) in the Western Cape.
Evaluating soil and terrain variables in a production environment: implications for agricultural land assessment
(Stellenbosch : Stellenbosch University, 2022-12) Barichievy, Kurt Russell; Clarke, Catherine E.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Agricultural land in South Africa is under increasing pressure to produce more food from an ever-shrinking land base, as more land is being converted to non-productive uses. Additional to these pressures, is the concept of land reform and strategic land acquisition, aimed at agrarian transform within the rural landscape. It is estimated that less than 15% of South Africa is suitable for dryland cultivation. Consequently, the sustainable utilisation of these scarce resources and preservation of agricultural land is of paramount importance, to ultimately ensure some measure of national food security in the years to come. Agricultural land evaluation is a critical tool that can achieve this goal. Unfortunately, in recent decades the development of revised or novel land evaluation methodologies has stalled for South African farm-level assessments, the scale at which land release decisions are made. Further, the relationship between productivity and individual land assessment attributes has not been adequately quantified or incorporated into contemporary local assessment procedures. It is envisaged that this study would influence and help guide in-field methodologies, as well as draft legislation and best-practice strategies, with a view of both standardising and improving agricultural land assessment techniques. By emphasising the importance of agricultural land and the accurate assessment thereof, this research also aims to increase our understanding of production-based approaches at an operational scale, though the novel combination of traditional approaches and use of newer technologies. It is anticipated that this improved understanding will be employed to not only protect more agricultural land, which may have been undervalued by historical methods, but also as an intuitive assessment tool to highlight the yield gap between potential and actual production levels. A review of pertinent literature identified the need for local verification studies to evaluate the performance of land assessment methodologies currently used in industry. To address this, five methods were verified using land assessment polygons in a commercial production environment, in the Province of KwaZulu-Natal, South Africa. The resultant classifications, derived from 225 soil observations, were compared to actual land use and precision yields achieved by dryland maize and soybean, across five growing seasons (2016 - 2020). By comparing land use with broad arability, four of the five land assessment methods were found to adequately classify arable land. Additionally, land evaluation polygons, linked to dryland precision maize and soybean yields can provide a general overview of method performance. However, it was concluded that yield performance and variation, across land evaluation methods and classes, is only explicit on or near a soil observation point where measurements are taken. Accordingly, seasonal variograms for maize and soybean were developed, to establish a representative yield buffer around individual soil observation points. This, along with yield normalisation strategies were employed, to improve verification procedures across multiple growing seasons. To determine crop productivity drivers, significant land assessment attributes inter alia slope, effective rooting depth, soil texture, soil group and soil wetness limitations were analysed against maize and soybean yields. It was found that the two crops respond differently to individual land assessment attributes and these differences should be taken cognisance of in new, crop-specific land evaluation methodologies and weighted accordingly. In an attempt to improve productivity-based land classification 78 attributes; derived from land assessment methodologies, digital terrain analysis, the pedological survey and soil colour spectrophotometry were collated. From these attributes, three new approaches, one based on biophysical scoring criteria and two based on machine learning, were developed across two commercial farming operations, in northern KwaZulu-Natal. These new methodologies were then tested on three separate commercial operations, located in different regions of the province. The biophysical scoring classification generally outperformed machine learning models and was particularly accurate when classifying observations associated with either extremely poor or extremely advantageous soil and terrain attributes. The transferability of the models to other regions, with different resources produced mixed results, highlighting the need for wider calibration in some instances. The study also found that the new productivity-based approaches can have useful applications in commercial farm management, where crop specific classification can identify underperforming areas and yields gaps, which can be ringfenced for appropriate interventions. The newly developed biophysical scoring classification was used to demonstrate the utility of these approaches in broader agricultural land release applications. The study found the new approaches better reflect production potential and should be used to supplement existing methodologies in land release assessments. Ultimately, the application of these production- based approaches can assist the land assessor to better classify the production potential of the land, as well as the decision-making authority to justify preserving more land for agricultural purposes.
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.
Evaluation of selected legumes for sustainable weed ecology/soil fertility/livestock management interactions in crop-livestock systems of the moist savannah of Nigeria
(Stellenbosch : Stellenbosch University, 2004-03) Owoeye, Lawrence Gbadebo; Pieterse, P. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: This project aimed at enhancing the net benefit in production systems. It took a holistic approach to evaluate the potential interactions of herbaceous legumes in relation to weed dynamics, soil fertility and livestock management in the crop-livestock system in Nigeria. The project was carried out between 2000 and 2002 in two localities. These were the National Animal Production Research Institute (NAPRI) at Zaria in the northern Guinea savannah and the International Institute of Tropical Agriculture (IITA) at Ibadan in the derived savannah. The main experiment was carried out in the northern Guinea savannah, while the secondary experiments were simultaneously conducted in the derived savannah and the northern Guinea savannah. The experimental design for the three experiments reported in this thesis is a splitsplit plot, fitted into randomised complete block design (RCBD), with four replications. Main plot treatments were herbaceous legumes, namely Vigna unguiculata, Arachis hypogaea, Glycine max, Aeschynomene bistrix, Centrosema pascuorum, Stylosanthes guianensis and natural vegetation. Sub-plot treatments were management systems (1) M1, 'residues left in the field'; (2) M2, 'residues taken out of the field' and (3) M3, 'residues fed to livestock, manure/urine/refused feeds returned'. Sub-plot treatments were administered in a sequence following rotational fallows of herbaceous legumes and natural vegetation. However, plots in the secondary experiments were not subdivided before the cropping of maize in 2002, and for logistical reasons only two sub-plot treatments, M1and M2, are featured in this experiment. Herbaceous legumes were established at the start of the rainy season, approximately in June, in 2000, 2001 and 2002. All herbaceous legumes received single super phosphate (SSP) at 20kg ha" P20S at planting, while minimum hand weeding was done to maintain pure legume stands during the establishment phase. Forage biomass was higher in the derived savannah than in the northern Guinea savannah. Similarly, higher forage yields were observed after two consecutive years of legume fallow and natural vegetation, compared to the first year plots. Grain yield for Glycine max was consistently higher than for the other two grain legumes in 2001 and 2002. Chemical analysis of herbaceous legumes and natural vegetation showed that crude protein values ranged between 11.2% to 17.3% for legumes; that was significantly (p<0.05) higher than the 8.6% value found for natural vegetation. Moreover, all herbaceous legumes and natural vegetation, except Arachis hypogaea, had dry matter digestibility values of more than 30%. Maize grain and stover yields on herbaceous legumes fallowed plots were evaluated and compared with those for natural vegetation. Results in 2001, i.e. after a one-year fallow with legumes, indicated that the dry matter of maize grain and stover yields in the Stylosanthes guianensis plots were higher among the forage legumes. Arachis hypogaea gave the highest grain and stover yields among the grain legumes in the northern Guinea savannah. Results in 2002, i.e. after a two-year fallow, also showed that the productivity of maize planted on Arachis hypogaea and Glycine max fallowed plots were consistently higher across the three management systems tested in the Centrosema pascuorum, Glycine max, Aeschynomene histrix, Vigna unguiculata and least weight gain was recorded for the natural vegetation. Objective functions in linear optimisation, or linear combinations in algebra, used to link dynamic processes in livestock production (Iiveweight gain) with the dynamic processes in soils (soil nitrogen), weeds (weed biomass), herbaceous legumes (legume biomass) and crop production (maize grain and stover yields) under varying management systems took the form: Management system 1, Y, = f (XI, Xz, x4); Yr= 0 Management system 2, Y, = f (x" x2); Yz = 0 Management system 3, Y, = f (XI, Xz, xl); Yz= f (XI, Xz, X4. x5) Where (1) (2) (3) Y1 = Crop in kg; Y2 = Livestock weight gain in kg; X1= Weed in kg; X2 = Soil N g kg-1; X3 = Livestock compost in kg; N= Herbaceous legumes in kg; X5 = Maize stover in kg, for the three management systems considered in this experiment. Deductions from these equations showed that Aeschynomene histrix performed better under M1, i.e. when legumes residues were left on the field. Natural vegetation performed better than the herbaceous legumes under M2, i.e. when legumes residues were exported out of the field. However, the presence of manure in M3 enhanced soil fertility in the system and improved the overall productivity across all the legumes and natural vegetation. Overall rankings, conducted by pooling all components in the system, indicated that Glycine max performed best among the legumes, followed by Stylosanthes guianensis and Arachis hypogaea, which ranked second and viithird respectively. In relation to the specific legume groups, Stylosanthes guianensis performed better than the other two forage legumes, while Glycine max also performed better than the other two grain legumes tested. We can see from these on-station research results that there are indications of positive opportunities for improving overall productivity and resources. This can be done through integrating and complementing crop and livestock production, to provide a sustainable intensification of agriculture.
Die gronde van die Karoo
(Stellenbosch : Stellenbosch University, 1988) Ellis, Freddie; Lambrechts, J. J. N.; Stellenbosch University. Faculty of AgriSciences. Department of Soil Science.
ENGLISH ABSTRACT: Soil information pertaining to one of the most extensive arid and semi-arid areas of South Africa is limited. This area, known as the Karoo, comprises 36 million hectares or 32% of the surface area of South Africa. The purpose of this study was to gather as much information as was possible regarding the distribution, morphology, classification, physical, chemical and fertility status of the soils of this area. The pedosystem concept was used to identify and demarcate the terrain and soils of this study area. This assumes that a fair degree of interdependence exists between the environmental characteristics, terrain morphology and the soil. From this a general map (scale 1: 1 000 000) of the Karoo was produced. This illustrates broad soil patterns, while topsoil texture classes and underlying materials can easily be read from the map. In addition, terrain characteristics (i.e. percentage level land and relief) are also given. For the interpretation of soil and terrain characteristics it was necessary to define a small number of natural regions of the Karoo. In these regions the natural resources of soil, terrain and climate are strongly interdependent. They have been called broad physiographical regions. Twenty such regions have been identified in the Karoo. For each broad physiographical region a description is given of the nature and extent of the terrain together with the distribution of the soils with their morphology, classification, physical and chemical characteristics and fertility status. The soils of the Karoo can conveniently be arranged into seventeen broad soil patterns. It was clear that shallow calcareous lithosols and red apedal soils with a high base saturation occupy the largest area of the Karoo. The red soils usually have sandy topsoils (A horizon) while the clay content of the calcareous lithosols is slightly higher varying from 6- 15%. The deep, weakly structured soils which are associated with unconsolidated deposits and which should have a high potential for irrigation, occupy ca. 1 million hectares and occur mostly in the central , northern and eastern parts of the Karoo. Approximately 22 million hectares (60%) of the soils of the Karoo are stony and a further 14% are covered by desert pavement.
In situ denitrification on nitrate rich groundwater in South Africa
(Stellenbosch : Stellenbosch University, 2015-12) Israel, Sumaya; Rozanov, Andrei Borisovich; Jovanovic, Nebojsa; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science
ENGLISH ABSTRACT: South Africa is a water scarce country and in certain regions the quantity of surface water is insufficient to provide communities with their domestic water needs. In many arid areas groundwater is often the sole source of water. This total dependence means that groundwater quality is of paramount importance. A high nitrate concentration in groundwater is a common cause of water being declared unfit for use and denitrification has been proposed as a potential remedy. In many areas of South Africa nitrate levels exceed the recommended maximum concentration of 40 mg/L NO3- as N. Concentrations of 100 mg/L NO3- as N or even greater than200 mg/L NO3- as N are found in various places. Water with nitrate concentrationsexceeding 40 mg/L NO3- as N, belongs to the category of “dangerous” drinking waterquality (“purple”, i.e. Class IV) according to DWA (1996, 1998) water quality guidelines. Concentrations in this range have been reported in case studies to cause conditions like methaemoglobinaemia (“blue baby syndrome”), spontaneous abortions, stomach cancers and livestock deaths. The purpose of the study includes laboratory experiments to compare the denitrification efficiency, reaction rates and reaction mechanisms between woodchips, biochar and a mixture of woodchips and biochar. Further work included modelling of denitrification using the PHREEQC-2 1D reactive transport model. Field implementation of a denitrification technique was tested at a site which previously experienced some NH4NO3 spills, to determine the lifespan of the woodchips used during the experiment based on available data. The underlying intended purpose of this research is to contribute to the wellbeing of rural South Africans in areas where groundwater is plentiful, but elevated nitrate levels prevent the use of this water. The purpose of the laboratory experiment was to establish the efficiency of carbon sources and compare their rates, sorption properties and processes by which they react. Laboratory experiment consisted of three leaching columns containing two layers of building sand on either side of a carbon containing layer. The carbon containing layers were made of about 600g of woodchips, biochar and woodchip and biochar mixture respectively. Parameters analysed from the effluent from the columns included NO3-, NO2-, SO42-, NH4+, Alkalinity, DOC (dissolved organic carbon content) and Phosphate. The purpose of the field experimental work was to install a barrier containing a cheaply available carbon source to treat groundwater and to monitor changes with time in order to determine the efficiency and life span of carbon source used for the experiment. Experimental work was done at a site in Somerset West (South Africa) that had experienced spills in the past from agrochemical storage factory premises. Somerset West normally receives about 568 mm of rain per year. It receives most of its rainfall during winter; it thus has a Mediterranean climate. It receives the lowest rainfall (10 mm) in February and the highest (96 mm) in June. The “reactor”/ tank with dimensions- 1,37m height, 2.15m diameter used for the experiment was slotted for its entire circumference by marking and grinding through the 5mm thick plastic material. The top section was left open to allow for filling and occasional checking of filled material during the experiment. The tank was packed with Eucalyptus globulus woodchips which was freely available at the site. Concentrations of groundwater nitrate at the site were well over what could be expected in any naturally occurring groundwater systems, and would result only by major anthropogenic activities in unconfined aquifer areas of South Africa. Nitrate levels in monitoring boreholes at the site ranged from about 20 mg NO3--N/L at background boreholes up to about 600 mg/L NO3--N. Woodchips used to denitrify groundwater in the field experiment were sampled after 27 months and 35 months of being active in the treatment zone. Various depths of samples were collected namely the top section, bottom of the tank and a full core sample of the tank. Main results from the laboratory studies showed that biochar on its own as a carbon source for nitrate removal would not be viable, however, the presence of biochar in the mixture of woodchips and biochar increased the rate of denitrification. Biochar on its own was able to remove some nitrate, but results showed incomplete denitrification and limited reactivity. The results also confirmed that different processes were in play, while the redox reaction of denitrification was taking place in woodchips and biochar and woodchip mixtures, the biochar treatment followed a physical process and had only a small percentage of incomplete denitrification. This was confirmed by sulphate reduction and increased alkalinity in the woodchips and biochar and woodchip mixture treatments. Rates deduced from the data also showed that the woodchip and biochar mixture would take a shorter period to affect total denitrification. Main results from the field work showed that nitrate was totally removed at the treatment zone and surrounding boreholes, and even sulphate and NH4+ were removed during the experiment. This shows that the woodchips were successful in affecting denitrification for 35 months. Data also shows that boreholes further downstream from the tank had reduced NO3-, SO42- and NH4+ levels. This would relate to higher permeability flow paths possibly present on the downstream side of the treatment zone. This became evident when pumping boreholes during sampling and noting that upstream boreholes had to be allowed for a recovery period, while downstream boreholes could be pumped continuously for 30 minutes without any reduced yields. This shows that not only did the treatment zone work at removing nitrate, but migration of excess available carbon from the tank may have further treated nitrate rich areas on the site. During monitoring on the site, woodchips were sampled and analysed for their components at time period 27 months and 35 months of the experiment respectively. Results showed that woodchips were considerably more degraded than a) woodchips of the same species of tree that had undergone natural degradation on the floor and b) un-degraded woodchips of the same tree species. Comparing data from the two time series samples, a rate of woodchip degradation could be calculated. Using the available biodegradable carbon for the woodchips based on its composition, a barrier lifespan could be determined. The results of calculations show that the barrier would be effective for at least another 6.9 years from the period of the last sampling date. A total lifespan of about 10 years can thus be estimated. These calculations are tree species composition specific and rate specific. PHREEQC-2 modelling was used to estimate the use of carbon in the experiment by adding incremental moles of carbon to the influent composition. Saturation indices from PHREEQC-2 showed that mineral phases of iron may precipitate from solution during the experiment. Experimental data were plotted against results of intermittent carbon reactions in PHREEQC-2 and it was found that initial rates in the experiment were higher and agreed with up to 100mg/L of carbon consumption when a 24 hour residence time was used while later stages agreed with about 37.5 mg/L carbon consumption, where a 72 hr residence time was used. It was concluded that biochar and woodchips combined are more effective than woodchips on their own at denitrifying groundwater. Also woodchips successfully denitrified groundwater at the Somerset West site for 35 months, with added removal of sulphate and NH4+. Barrier life span calculations show that the barrier could remain active for an additional 6.9 years which relates to a total period of about 10 years of denitrification should the rates remain constant. It was concluded that nitrate removal and barrier lifespan would be extended by testing variable lignin content in different tree species prior to use in a denitrification barrier as lignin is unlikely to degrade in an anaerobic environment. It was recommended that implementation or field test should be done using a biochar and woodchip mixture. Improved results may be achieved by analysing wood or plant material for comparative lignin content, cellulose content and hemicellulose contents. Wood types or plant species with higher lignin content would be more resistant to degradation in anaerobic conditions.
Interrelationships among soil water regime, irrigation and water stress in the grapevine (Vitis vinifera L.)
(Stellenbosch : Stellenbosch University, 1984-12) Van Zyl, Jan Louis; Visser, J. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: No abstract available
Lipid humification by soil clays
(Stellenbosch : Stellenbosch University, 2019-04) Adams, Adrian Richard; Clarke, Catherine E.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: We studied three aspects of the natural polymerisation (humification) of lipids by soil clays – namely, the products formed, reaction mechanism and kinetics – at environmental temperatures (c. 20–50°C). Various clays were reacted with oleic acid (our chosen model lipid). The Mn-oxide birnessite was the most reactive toward oleic acid, polymerising it into quasi-solid polyesters. A probing of the birnessite-oleic acid reaction mechanism revealed that the formation of a surface exchange complex between oleic acid carboxyl groups and birnessite surface sites (>Mn(III)/>Mn(IV)) is a crucial first step of the reaction. Subsequent chelation and one-electron reduction of Mn(III) to Mn(II) forms radical oleic acid species which couple and thereby polymerise. Kinetic studies revealed that the birnessite-oleic acid reaction was near-linearly dependent on birnessite mass-loading (rate order ~ 0.75) but virtually independent of birnessite surface pH (rate order ~ 0.2). A determined activation energy for the reaction of 12.8 ± 4.2 kJ/mol revealed that it is energetically more spontaneous than the usual autoxidation pathways. These findings broaden our understanding of the role soil clays play in lipid humification in soils.
Mapping soil organic carbon stocks by combining NIR spectroscopy and stochastic vertical distribution models : a case study in the Mvoti River Catchment, KZN, South Africa
(Stellenbosch : Stellenbosch University, 2019-03) Wiese, Liesl; Rozanov, Andrei Borisovich; De Clercq, W. P.; Seifert, Thomas; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: The agricultural and environmental importance of maintaining and increasing soil organic carbon (SOC) has been increasingly recognized globally. To a large extent, this recognition can be attributed to soil being the largest terrestrial carbon pool, as well as to soil’s responsiveness to land use and management. Land use and land use change are major factors affecting SOC levels with changes from natural vegetation (forests, grasslands and wetlands) to croplands, for example, causing significant SOC losses. The topsoil (0-30 cm depth) is especially sensitive to changes in land use and management and the highest variation in SOC levels is observed in this zone. In this study SOC stocks in the first meter of soil were quantified and mapped under different land uses and management systems using a vertical SOC distribution model, applying near-infrared (NIR) spectroscopy for SOC analysis and estimating the uncertainty of the maps created using different approaches. The study area was chosen as a quaternary catchment of 317 km-2 south and southeast of Greytown in the Midlands area of KwaZulu-Natal, South Africa. The catchment exhibits complex topography and predominantly shale and dolerite parent material. Soils in the area have high organic carbon content ranging from 0.08 to 22.85 % (mean = 3.48 %), with clay content ranging from 3 to 49 % (mean = 14.7 % clay) and pH(H20) between 3.3 and 6.7 (mean pH(H20) = 4.5). Vertical SOC distribution functions were developed for 69 soil profiles sampled from different land uses (mainly forestry plantations, grasslands and croplands) in and around the study catchment. Bulk density samples were taken at 2.5, 7.5, 12.5, 17.5, 30, 40, 50, 75 and 100 cm depths. The aim was to reduce the number of soil observations required for SOC accounting to one point close to the soil surface by applying negative exponential vertical depth functions of SOC distribution. To achieve this, the exponential functions were normalized using the volumetric SOC content observed close to the surface and grouped as a function of land use and soil types. Normalization reduced the number of model parameters and enabled the multiplication of the exponential decline curve characteristics with the SOC content value observed at the surface to present an adequately represented value of soil carbon distribution to 1 m at that observation point. The integral of the exponential function was used to calculate the soil carbon storage to 1 m. The vertical SOC distribution functions were refined for soils under maize production systems using reduced tillage and conventional tillage. In these soils, the vertical SOC distributions are described by piecewise, but still continuous functions where the distribution within the cultivated layer (0-30 cm) is a linear decline under reduced tillage or a constant value under conventional tillage, followed by an exponential decline to 1 m (30-100 cm). The value of predicting SOC concentrations in soil samples using wet oxidation (WalkleyBlack method) and dry near-infrared (NIR) spectrometry was assessed by comparing them to the dry combustion method. NIR spectrometry is considered to be an especially promising method, since it may be used in both proximal and remote sensing applications. In addition, the effect of using paired samples with single SOC determination versus paired samples with replicated (three times) analysis by all (reference and test) methods was tested. It was shown that the use of paired tests without replication dramatically decreases the precision of SOC predictions of all methods, possibly due to high variability of SOC content in reference values analysed by dry combustion. While reasonable figures of merit were obtained for all the methods, the analysis of non-replicated paired samples has shown that the relative RMSE for the SOC NIR method only falls below 10 % for values above ~8 % SOC. For the corrected SOC Walkley Black method the relative RMSE practically never falls below 10 %, rendering this method as semi-quantitative across the range. It was concluded that for method comparison of soil analysis, it is essential that reference sample analysis be replicated for all methods (reference and test methods) to determine the “true” value of analyte as the mean value analysed using the reference method. Finally, the above elements of vertical SOC distribution models as a function of land use and soil type, predicting SOC stocks to 1 m using only a surface (0-5 cm) sample, and the use of NIR spectroscopy as SOC analysis method were combined to assess the changes in SOC stock prediction errors through mapping. Results indicated a dramatic improvement in precision of SOC stock predictions with increasing detail in the input parameters using vertical SOC distribution functions differentiated by land use and soil grouping. Still, the relative error mostly exceeded 20 % which may be seen as unacceptably high for carbon accounting, trade and tax purposes, and the SOC stock accuracy decreased in terms of map R 2 and RMSE. The results were generally positive in terms of the progressive increase in complexity associated with SOC stock predictions and showed the need for a substantial increase in sampling density to maintain or increase map accuracy while increasing precision. This would include an increase both in surface samples for the prediction of SOC stocks using the vertical SOC distribution models, as well as an increase in the sampling of profiles to include more soil types and increase the profile density per land use to improve the vertical SOC prediction models.
Mitigation of soil and ground water pollution caused by on-land disposal of olive mill wastewater
(Stellenbosch : Stellenbosch University, 2016-03) Umeugochukwu, Obiageli; Rozanov, Andrei Borisovich; Sigge, G. O.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of Agrisciences. Dept of Soil Science.
ENGLISH ABSTRACT: Olive mill wastewater (OMW) is generated in large quantities, particularly in the regions with a Mediterranean climate where olive oil is produced on a commercial scale. Some producers collect the effluent and dispose of it as hazardous waste at significant expense, while others dispose of it directly on land, claiming the potential benefits to productivity from the plant nutrients present in the OMW. It was shown that the OMW also contains some phytotoxic phenols, which may have both immediate and cumulative negative effects on plant growth. The long-term effects on the soil and crop growth have been shown to be detrimental. Sandy soils are of particular concern due to the possibility of phenol penetration into deeper soil layers and potential ground water contamination. The study explores in-situ (soil amendment with biochar prior to the OMW disposal) and ex-situ (OMW filtration through a biochar bed) options to mitigate the negative effects of the OMW on-land disposal. A laboratory batch sorption experiment was set up using 0.2 g pinewood biochar to explore the possibilities of removing the phenols from 50 mL of the OMW or gallic acid (GA) solutions at different concentrations. The results showed that the sorption process was rapid and stabilized within one hour. The kinetic process followed a pseudo-second-order model and was described by the Freundlich multi-layer isotherm. The pinewood biochar had a sorption capacity of 30 mg·g-1 and 100 % removal was obtained with 300 g·l-1 of the OMW load. It was found that pinewood biochar could be used to remove the phenols contained in the effluent. A column experiment was set up to determine the effectiveness of biochar and biochar-soil mixtures in removal of phenol and Chemical Oxygen Demand (COD) from the OMW compared to sand filtration. The breakthrough curves for phenol and COD were determined, while the pH and EC of the filtrates were monitored. Ten PVC columns of 30 cm height and 5 cm diameter were filled with five different materials: sand, sand + biochar, Hutton clay loam soil, Hutton clay loam soil + biochar and biochar alone. Two different treatments were given to the columns; five of the columns were prewashed with 2 liters of deionized water and the other five were not washed before the OMW filtration. The performance of the columns was determined in respect of the phenol and COD removal capacities, hydraulic conductivities and porosity changes. The results showed that washing enhanced the phenol sorption but not the COD sorption. The addition of the biochar at 2%wt load significantly improved the effectiveness of the filtration. The best performance was achieved in terms of COD removal in pure biochar columns, but in terms of the phenol, the best performance was on a pre-washed Hutton clay loam soil with 2%wt biochar addition. Both the washing and biochar addition affected the porosity and reduced the hydraulic conductivity of the columns. The greenhouse experiments were conducted to confirm the above statement using pot trials laid out in a 4 x 4 factorial Randomized Complete Design (CRD) to determine the effect of effluent and biochar on wheat and green beans on alkaline sand. Results showed that the increasing effluent rate up to 200 m3·ha-1 gave significantly negative results on wheat growth, even with fertilizer application. But the effect was different for beans where low effluent loads gave positive results though not significant while with fertilizer (N and P) 50 m3·ha-1 performed better. With the addition of biochar there was no significant effect on wheat, but it significantly affected beans at the application rate of 2.5 and 5%wt. The interaction of biochar and effluent showed that the best performance was at 5% biochar application and effluent loads of 50 and 100 m3·ha-1, but increased effluent rate decreased production even with a 5% wt biochar application rate. It was suggested that a leguminous crop should tolerate OMW application better compared to wheat even in the adverse conditions of the alkaline sand. A second greenhouse experiment was conducted with another legume, an indigenous African crop, the bambara groundnut, on an acidic Hutton clay-loam soil (Oxisol) sourced locally. The experiment was laid out in a 2 x 6 CRD factorial design to determine the effect of the biochar and effluent combination on the yield and growth parameters of bambara as well as the effect on soil conditions and nutrient availability. The result showed that biochar addition improved seed germination, which was retarded by effluent loading. The effluent rate of 200 m3·ha-1 and biochar 2% gave the best yield performance. The biochar addition increased the pH and hence affected the release of P and N whereas Na and K availability were reduced. We conclude that biochar may be used for both ex-situ filtration to treat the OMW, and as a soil amendment to allow safe on-land disposal of the OMW. The estimations of safe disposal loads and the required application rates of the biochar should be made individually for a specific soil type. Pinewood biochar was proven to be a cheaper source of activated carbon for the treatment of olive mill wastewater organic contaminants in South Africa.
Modeling and regulating hydrosalinity dynamics in the Sandspruit river catchment (Western Cape)
(Stellenbosch : Stellenbosch University, 2014-04) Bugan, Richard D. H.; De Clercq, W. P.; Jovanovic, N.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Bugan, R.D.H. Modelling and regulating hydrosalinity dynamics in the Sandspruit River catchment (Western Cape). PhD dissertation, Stellenbosch University. The presence and impacts of dryland salinity are increasingly become evident in the semi-arid Western Cape. This may have serious consequences for a region which has already been classified as water scarce. This dissertation is a first attempt at providing a methodology for regulating the hydrosalinity dynamics in a catchment affected by dryland salinity, i.e. the Sandspruit catchment, through the use of a distributed hydrological model. It documents the entire hydrological modelling process, i.e. the progression from data collection to model application. A review of previous work has revealed that salinisation is a result of land use change from perennial indigenous deep rooted vegetation to annual shallow rooted cropping systems. This has altered the water and salinity dynamics in the catchment resulting in the mobilisation of stored salts and subsequently the salinisation of land and water resources. The identification of dryland salinity mitigation measures requires thorough knowledge of the water and salinity dynamics of the study area. A detailed water balance and conceptual flow model was calculated and developed for the Sandspruit catchment. The annual streamflow and precipitation ranged between 0.026 mm a-1 - 75.401 mm a-1 and 351 and 655 mm a-1 (averaging at 473 mm a- 1), respectively. Evapotranspiration was found to be the dominant component of the water balance, as it comprises, on average, 94% of precipitation. Streamflow is interpreted to be driven by quickflow, i.e. overland flow and interflow, with minimal contribution from groundwater. Quantification of the catchment scale salinity fluxes indicated the Sandspruit catchment is in a state of salt depletion, i.e. salt output exceeds salt input. The total salt input to and output from the Sandspruit catchment ranged between 2 261 - 3 684 t Catchment-1 and 12 671 t a-1 - 21 409 t a-1, respectively. Knowledge of the spatial distribution of salt storage is essential for identifying target areas to implement mitigation measures. A correlation between the salinity of sediment samples collected during borehole drilling and the groundwater EC (r2 = 0.75) allowed for the point data of salt storage to be interpolated. Interpolated salt storage ranged between 3 t ha-1 and 674 t ha-1, exhibiting generally increasing storage with decreasing ground elevation. The quantified water and salinity fluxes formed the basis for the application of the JAMS/J2000-NaCl hydrological model in the Sandspruit catchment. The model was able to adequately simulate the hydrology of the catchment, exhibiting a daily Nash-Sutcliffe Efficiency of 0.61. The simulated and observed salt outputs exhibited discrepancies at daily scale but were comparable at an annual scale. Recharge control, through the introduction of deep rooted perennial species, has been identified as the dominant measure to mitigate the impacts of dryland salinity. The effect of various land use change scenarios on the catchment hydrosalinity balance was evaluated with the JAMS/J2000-NaCl model. The simulated hydrosalinity balance exhibited sensitivity to land use change, with rooting depth being the main factor, and the spatial distribution of vegetation. Revegetation with Mixed forests, Evergreen forests and Range Brush were most effective in reducing salt leaching, when the “salinity hotspots” were targeted for re-vegetation (Scenario 3). This re-vegetation strategy resulted in an almost 50% reduction in catchment salt output. Overall, the results of the scenario simulations provided evidence for the consideration of re-vegetation strategies as a dryland salinity mitigation measure in the Sandspruit catchment. The importance of a targeted approach was also highlighted, i.e. mitigation measures should be implemented in areas which exhibit a high salt storage.
Reciprocal relationships between vegetation structure and soil properties in selected biomes of South Africa
(Stellenbosch : Stellenbosch University, 2003-12) Mills, A. J.; Fey, M. V.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
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Remote sensing of salt-affected soils
(Stellenbosch : Stellenbosch University, 2013-03) Mashimbye, Zama Eric; De Clercq, W. P.; Van Niekerk, Adriaan; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Concrete evidence of dryland salinity was observed in the Berg River catchment in the Western Cape Province of South Africa. Soil salinization is a global land degradation hazard that negatively affects the productivity of soils. Timely and accurate detection of soil salinity is crucial for soil salinity monitoring and mitigation. It would be restrictive in terms of costs to use traditional wet chemistry methods to detect and monitor soil salinity in the entire Berg River catchment. The goal of this study was to investigate less tedious, accurate and cost effective techniques for better monitoring. Firstly, hyperspectral remote sensing (HRS) techniques that can best predict electrical conductivity (EC) in the soil using individual bands, a unique normalized difference soil salinity index (NDSI), partial least squares regression (PLSR) and bagging PLSR were investigated. Spectral reflectance of dry soil samples was measured using an analytical spectral device FieldSpec spectrometer in a darkroom. Soil salinity predictive models were computed using a training dataset (n = 63). An independent validation dataset (n = 32) was used to validate the models. Also, field-based regression predictive models for EC, pH, soluble Ca, Mg, Na, Cl and SO4 were developed using soil samples (n = 23) collected in the Sandspruit catchment. These soil samples were not ground or sieved and the spectra were measured using the sun as a source of energy to emulate field conditions. Secondly, the value of NIR spectroscopy for the prediction of EC, pH, soluble Ca, Mg, Na, Cl, and SO4 was evaluated using 49 soil samples. Spectral reflectance of dry soil samples was measured using the Bruker multipurpose analyser spectrometer. “Leave one out” cross validation (LOOCV) was used to calibrate PLSR predictive models for EC, pH, soluble Ca, Mg, Na, Cl, and SO4. The models were validated using R2, root mean square error of cross validation (RMSECV), ratio of prediction to deviation (RPD) and the ratio of prediction to interquartile distance (RPIQ). Thirdly, owing to the suitability of land components to map soil properties, the value of digital elevation models (DEMs) to delineate accurate land components was investigated. Land components extracted from the second version of the 30-m advanced spaceborne thermal emission and reflection radiometer global DEM (ASTER GDEM2), the 90-m shuttle radar topography mission DEM (SRTM DEM), two versions of the 5-m Stellenbosch University DEMs (SUDEM L1 and L2) and a 5-m DEM (GEOEYE DEM) derived from GeoEye stereo-images were compared. Land components were delineated using the slope gradient and aspect derivatives of each DEM. The land components were visually inspected and quantitatively analysed using the slope gradient standard deviation measure and the mean slope gradient local variance ratio for accuracy. Fourthly, the spatial accuracy of hydrological parameters (streamlines and catchment boundaries) delineated from the 5-m resolution SUDEM (L1 and L2), the 30-m ASTER GDEM2 and the 90-m SRTM was evaluated. Reference catchment boundary and streamlines were generated from the 1.5-m GEOEYE DEM. Catchment boundaries and streamlines were extracted from the DEMs using the Arc Hydro module for ArcGIS. Visual inspection, correctness index, a new Euclidean distance index and figure of merit index were used to validate the results. Finally, the value of terrain attributes to model soil salinity based on the EC of the soil and groundwater was investigated. Soil salinity regression predictive models were developed using CurveExpert software. In addition, stepwise multiple linear regression soil salinity predictive models based on annual evapotranspiration, the aridity index and terrain attributes were developed using Statgraphics software. The models were validated using R2, standard error and correlation coefficients. The models were also independently validated using groundwater hydro-census data covering the Sandspruit catchment. This study found that good predictions of soil salinity based on bagging PLSR using first derivative reflectance (R2 = 0.85), PLSR using untransformed reflectance (R2 = 0.70), a unique NDSI (R2 = 0.65) and the untransformed individual band at 2257 nm (R2 = 0.60) predictive models were achieved. Furthermore, it was established that reliable predictions of EC, pH, soluble Ca, Mg, Na, Cl and SO4 in the field are possible using first derivative reflectance. The R2 for EC, pH, soluble Ca, Mg, Na, Cl and SO4 predictive models are 0.85, 0.50, 0.65, 0.84, 0.79, 0.81 and 0.58 respectively. Regarding NIR spectroscopy, validation R2 for all the PLSR predictive models ranged from 0.62 to 0.87. RPD values were greater than 1.5 for all the models and RMSECV ranged from 0.22 to 0.51. This study affirmed that NIR spectroscopy has the potential to be used as a quick, reliable and less expensive method for evaluating salt-affected soils. As regards hydrological parameters, the study concluded that valuable hydrological parameters can be derived from DEMs. A new Euclidean distance ratio was proved to be a reliable tool to compare raster data sets. Regarding land components, it was concluded that higher resolution DEMs are required for delineating meaningful land components. It seems probable that land components may improve salinity modelling using hydrological modelling and that they can be integrated with other data sets to map soil salinity more accurately at catchment level. In the case of terrain attributes, the study established that promising soil salinity predictions could be made based on slope, elevation, evapotranspiration and terrain wetness index (TWI). Stepwise multiple linear regressions soil salinity predictive model based on elevation, evapotranspiration and TWI yielded slightly more accurate prediction of soil salinity. Overall, the study showed that it is possible to enhance soil salinity monitoring using HRS, NIR spectroscopy, land components, hydrological parameters and terrain attributes.

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