Masters Degrees (Soil Science)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/570

Browse

Now showing 1 - 20 of 27

Comparison of nutrient use efficiencies in citrus under different fertilization and irrigation management practices
(Stellenbosch : Stellenbosch University, 2024-03) Parker, Imaan; Hardie-Pieters, Ailsa G.; Raath, Pieter J.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Excessive applications of nitrogen and phosphorus can lead to soil acidification, runoff, leaching and groundwater contamination. Additionally, the production of synthetic nitrogen fertilisers is an energy- intensive process, while phosphorus fertilisers are derived from non-renewable sources. This underscores the need for the citrus industry to adopt more sustainable management practices, with focus on improving nutrient use efficiency (NUE). Despite extensive research on the NUE of annual crops such as cereal grains and maize in South Africa, little research has been conducted on perennial crops such as citrus. Additionally, there is a need for NUE benchmarks that producers can use to compare their own NUE values with to. The main aim of this study was therefore to gain an understanding of the factors affecting nutrient use efficiency (NUE) of citrus, and to quantitively compare the NUE of different production systems currently used within the citrus industry. Various NUE indices were evaluated over two production seasons, in four of the main citrus production regions of South Africa. Within each region, three to four different irrigation-fertiliser production systems were selected, namely, conventional drip [1.6 L h-1] with fertigation (Drip), low-flow continuous drip [< 1 L h-1] with fertigation (LowFlow), micro-sprinkler irrigation with fertigation (MicroFert) and micro-sprinkler irrigation with granular fertiliser (MicroGran). There were significant differences between production systems in terms of partial factor productivity (PFP), partial nutrient balance (PNB) and internal utilization efficiency (IE). The MicroGran treatment consistently had higher PFP in both seasons of the study. Furthermore, the results showed that production systems (treatments) that applied excessive fertiliser rates or had low yields, exhibited significantly lower PFP and PNB values, this was seen in the LowFlow production systems (treatments) during both seasons. In the 2021/22 season the MicroGran treatment exhibited the highest average yield (62 ton ha-1), while in the 2022/23 season the MicroFert treatment obtained the highest average yield (81 ton ha-1). No significant differences were found between PFP and PNB values among the Drip, MicroFert and MicroGran production systems (treatments), when looking at the main effects of production systems (treatments). When yields between 50-80 ton ha-1 are obtained, producers can expect to achieve PFPN of 406- 424 kg kg-1, and PNBN values 0.68-0.76 kg kg-1, when using Drip, MicroFert and MicroGran production systems. Whereas for LowFlow production systems (treatments) can expect PFPN and PNBN values of 178 and 0.30 kg kg-1. Additionally, the study calculated indices that make use of data from a control plot which received no fertiliser. These indices include agronomic efficiency (AE), crop recovery efficiency (RE) and physiological efficiency (PE). These indices showed the crop response specifically to the fertiliser applied. Results highlighted the significance of interpreting leaf analysis and NUE along with soil analysis, as there were some significant correlations between soil properties and these factors. Soil analysis is especially important for interpreting PNB values. Furthermore, the study showed that NUE is not only dependent on the fertiliser applied but also influenced by mineralizable nutrients in the soil and the remobilization of nutrients stored within the plant biomass. To achieve optimal NUE, it is important to consider all these factors.
Conversion of soil pH values and exchangeable base cation quantities determined by different methods
(Stellenbosch : Stellenbosch University, 2021-12) Nel, Teneille; Hardie-Pieters, Ailsa G.; Clarke, Catherine E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Soil management recommendations based on chemical test results rely on comparisons with norm values developed using specific analytical methods and soil taxonomic classes. Conversions of soil pH and exchangeable base cation concentration between different methods are required for the standardization of data from scientific literature, soil test reports and soil databases. The combined effects of soil physicochemical properties on the accuracy and parsimony of soil pH and cation concentration conversions are unknown. Rapid soil analyses can be obtained by infrared (IR) spectroscopy, but few studies have evaluated the accuracy of IR spectral-based models to determine soil pH and exchangeable base cations in Western Cape soils. The aim of this study was to develop equations for conversion of soil pH values and exchangeable base cation concentrations between different methods of measurement. An additional aim was to develop IR spectral-based models for determination of soil pH and exchangeable base cation content of Western Cape soils. Soil pH was measured in different matrices (water, 0.1M KCl and 0.01M CaCl2) of varying soil: solution ratios (1:1, 1:2.5, 1:4 and 1:5). The exchangeable base cation content was measured according the ammonium acetate (1M, pH = 7), compulsive exchange, ammonium acetate (0.2M, pH = 7), Ambic- I, Mehlich-III, Bray-II and Citric acid (1%) methods. Models converting soil pH and cation content between values obtained by different methods were developed using simple linear regression (SLR). In addition, multivariate linear regression (MLR) equations incorporating soil physicochemical properties as parameters were developed. Spectral measurements of the soil samples were taken with three spectroscopic instruments in the near- to mid-IR range. IR spectral-based models predicting exchangeable base cation content and soil pH were calibrated using partial least squares regression. SLR equations for predicting soil pH (RMSE = 0.12-0.32) were more accurate than the conversion factors. MLR equations constructed by stepwise regression to convert exchangeable base cation content (RMSE = 1.4-1.5 cmolc kg-1, 0.4 cmolc kg-1, 16-20 mg kg-1 and 27-39 mg kg-1 for Ca, Mg, Na and K, respectively) were more accurate than SLR models. Exchangeable K content was accurately converted between the most extraction methods. Models converting base cation content may be chosen based on the purpose of application and relative size of the prediction error. IR spectroscopy is recommended to determine exchangeable Ca and Mg content in Western Cape soils (RMSE = 1.2-1.4 cmolc kg-1 and 0.37 cmolc kg-1 for Ca and Mg, respectively). Subsetting soil sample sets by similarities in texture and organic carbon content led to more accurate predictions of soil pH and exchangeable Ca content (RMSE decreased by up to 0.19 pH units and 0.79 cmolc kg- 1, respectively). In summary, use of the developed SLR as well as MLR models are recommended to convert soil pH and exchangeable base cation concentrations of Western Cape soils. Models should only be as complex as practical context requires. Moreover, IR spectroscopy may be used for determination of exchangeable Ca and Mg content in Western Cape soils.
Distribution and stability of soil carbon in spekboom thicket, Eastern Cape, South Africa
(Stellenbosch : Stellenbosch University, 2012-03) Mchunu, Sinethemba Euginia; Hardie-Pieters, Ailsa G.; Mills, A. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Soils of semi-arid regions generally have low organic matter contents, primarily attributed to prevailing climatic conditions. Yet, the spekboom thicket, located in the semi-arid region of the Eastern Cape (South Africa) has been distinguished to accumulate large amounts of soil carbon. To further understand this remarkable accumulation, a detailed investigation was carried out to evaluate the distribution, speciation and stability of the soil carbon from directly beneath spekboom (Portulacaria afra) and other “nonspekboom” vegetation within the intact thicket, and from adjacent, degraded (overgrazed), open thicket sites. In the first experimental chapter, the distribution of organic and inorganic C, as well as general soil properties (pH, EC, exchangeable and water soluble cations, hydrophobicity, water stable aggregates) were investigated in the intact (spekboom and nonspekboom) and degraded (open) thicket soils. There were no significant differences in the C content (organic or inorganic) between the spekboom and nonspekboom sites in the intact thicket. Soil organic carbon (SOC) content however was greatly influenced by thicket degradation, whereas, inorganic C remained unaffected. Mean organic C (0-50 cm) ranged from 19-34, 22-62, and 19-58 g C kg-1 for open, spekboom, and nonspekboom sites, respectively. Whereas, mean inorganic C ranged from 2-9 g C kg-1 irrespective of sites, representing 4-22 % of total soil C. However, no significant differences were detected between degraded and intact sites for all (total, organic and inorganic) C stocks, attributed to variation in bulk density. Spekboom soils contained significantly higher concentrations of exchangeable and water soluble Mg, Na, and K, and consequently had higher pH and EC compared to the other sites. Soils tended to be hydrophobic especially at shallower depths in the intact thicket irrespective of vegetation type, whereas soils from the adjacent open sites were not significantly hydrophobic. Macro-aggregates were considerably more stable under intact sites compared to open sites. The second experimental chapter investigated the effects of degradation and vegetation type on the stability and structural chemistry of SOC. This involved partitioning the soil organic matter into particulate and mineral-bound (stable) fractions, and then examining the relationships between the stable organic C fraction and various soil properties as an attempt to elucidate the stabilization mechanism(s). Particulate organic C ranged from 7.0, 9.3, and 14.4 g C kg-1 for open, spekboom, and nonspekboom respectively; representing 22-34 % of total SOC. Stable (mineral-bound) organic C accounted for the largest fraction ca. 60-66 % of total SOC; 28.0 and 26.2 g C kg-1 for spekboom and non-spekboom respectively, versus 17.2 g C kg-1 for open sites. It was concluded that SOC stability was influenced by the inherent residue quality (recalcitrance) rather than soil properties, attributed to the aliphatic (lipids and waxes) and highly aromatic nature of the spekboom litter substrate and particulate OM. These research findings are of fundamental significance in understanding soil organic matter stabilization in semi-arid environments.
The effect of biochar application to sandy soil on nitrogen nutrition of maize (Zea mays)
(Stellenbosch : Stellenbosch University, 2023-03) Kemp, Bernitia; Rozanov, Andrei Borisivich; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Sustainable agricultural development requires a holistic systems approach to address environmental, economic, and social challenges associated with agricultural productivity. Sustainable management of sandy soil in agricultural systems requires fertilization strategies that maintain and enhance soil physio-chemical properties while sustaining optimum crop yield for the long run. Biochar production and utilization are linked directly to agriculture, the environment, and sustainable development. Biochar application is widely considered a sustainable management strategy to sequester carbon in the soil and improve soil properties. Reservations have been expressed in the literature regarding this soil amendment, particularly regarding the possible adverse effects of biochar application to soils on nitrogen nutrition of crops. A pot trial was conducted to understand the effects of biochar application on the nitrogen nutrition of maize, by investigating the impact of locally produced biochar application on the growth response (vigour) of maize and its foliar N content. This study compared the effect of biochar addition to soil with other soil amendments commonly used in agricultural systems. To clarify, this study compared biochar with compost and nitrogen fertilizer, applied independently and in combinations. Biochar was derived from black wattle pyrolyzed at 730 ° C. This study has shown that the compound application of biochar with the recommended dosage of nitrogen fertilizer had a clear positive synergistic effect on the performance of maize. This positive synergistic effect may be attributed to various improvements in soil quality typically observed in biochar-amended soils. Applying biochar had a liming effect on the acidic sandy soil when used on its own and in combination with organic/ inorganic fertilizers. The potentially mineralizable N in the soil did not differ for any treatments relative to the control. Biochar may serve as a long-term soil management strategy in agricultural systems to effectively manage nutrients, enhance the nutrient use efficiency of cropping systems, reduce N loss and subsequently limit environmental pollution, and reuse and recycle waste, while increasing agricultural production. Biochar should be designed according to the context of any given agricultural system and modified to optimally perform specific functions. Subsequently, the production and utilization of biochar will be not only efficient and profitable but also sustainable. In addition, this study emphasized that excessive N fertilization is both unnecessary and an unsustainable soil management strategy. Thus, balanced fertilization schemes may be highly beneficial for the productivity of maize cropping systems. Additional studies are required to enhance future biochar development. Especially since biochar production can significantly contribute to the sustainable development of economies, society, agricultural systems, and the environment.
Effect of biochar on chemistry, nutrient uptake and fertilizer mobility in sandy soil
(Stellenbosch : Stellenbosch University, 2012-03) Sika, Makhosazana Princess; Hardie-Pieters, Ailsa G.; Hoffman, J. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Biochar is a carbon-rich solid material produced during pyrolysis, which is the thermal degradation of biomass under oxygen limited conditions. Biochar can be used as a soil amendment to increase the agronomic productivity of low potential soils. The aim of this study was to investigate the effect of applying locally-produced biochar on the fertility of low-nutrient holding, sandy soil from the Western Cape, and to determine the optimum biochar application level. Furthermore, this study investigates the effect of biochar on the leaching of an inorganic nitrogen fertilizer and a multi-element fertilizer from the sandy soil. The biochar used in this study was produced from pinewood sawmill waste using slow pyrolysis (450 °C). The soil used was a leached, acidic, sandy soil from Brackenfell, Western Cape. In the first study, the sandy soil mixed with five different levels of biochar (0, 0.05, 0.5, 0.5 and 10.0 % w/w) was chemically characterised. Total carbon and nitrogen, pH, CEC and plant-available nutrients and toxins were determined. The application of biochar resulted in a significant increase in soil pH, exchangeable basic cations, phosphorus and water holding capacity. A wheat pot trial using the biochar-amended soil was carried out for 12 weeks and to maturity (reached at 22 weeks). The trial was conducted with and without the addition of a water-soluble broad spectrum fertilizer. Results showed that biochar improved wheat biomass production when added at low levels. The optimum biochar application level in the wheat pot trial was 0.5 % (approximately 10 t ha-1 to a depth of 15 cm) for the fertilized treatments (21 % biomass increase), and 2.5 % (approximately 50 t ha-1 to a depth of 15 cm) for unfertilized treatments (29 % biomass increase). Since most biochars are alkaline and have a high C:N ratio, caution should be taken when applying it on poorly buffered sandy soil or without the addition of sufficient nitrogen to prevent nutrient deficiencies. In the second study, leaching columns packed with sandy soil and biochar (0, 0.5, 2.5 and 10.0 % w/w) were set up to determine the effect of biochar on inorganic nitrogen fertilizer leaching over a period of 6 weeks. It was found that biochar (0.5, 2.5, and 10.0 % w/w) significantly reduced the leaching of ammonium (12, 50 and 86 % respectively) and nitrate (26, 42 and 95 % respectively) fertilizer from the sandy soil. Moreover, biochar (0.5 %) significantly reduced the leaching of basic cations, phosphorus and certain micronutrients. This study demonstrated the potential of biochar as an amendment of acidic, sandy soils. Our findings suggest that an application rate of 10 t ha-1 should not be exceeded when applying biochar on these soils. Furthermore, biochar application can significantly reduce nutrient leaching in sandy agricultural soils.
Effect of biochar on selected soil physical properties of sandy soil with low agricultural suitability
(Stellenbosch : Stellenbosch University, 2012-03) Zeelie, Angelique; Hoffman, J. E.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Biochar has been labelled to be a key factor in the global carbon mitigation act and has been described as the modern day equivalent (terra nova) to the terra preta dark earth soils of the Brazilian Amazon. Globally biochar has been evaluated as a means to improve soil fertility and to mitigate greenhouse gases (GHGs). Little research has however been published on the effects of biochar incorporation on soil physical properties. The objective of this study was to evaluate the effect of pine sawmill waste derived biochar (locally-produced via slow pyrolysis – 450°C) on selected soil physical properties, soil-water dynamics and crop production and- performance, when amended to a Kroonstad (Kd 1000 – Morgendal) soil form. This soil form is commonly found in the Western Cape area (South Africa) and can be classified as having low agricultural suitability for perennial- and annual crop species. Two pot trials were carried out in an atmospheric controlled greenhouse, where winter wheat and green beans respectively were planted, with five different application levels of biochar (0t/ha, 1t/ha, 10t/ha, 50t/ha and 200t/ha). Soil physical properties namely, water-stable aggregates, bulk density and water-retention capacity along with physiochemical characterisation of the sandy soil and biochar was determined. The water-use was monitored throughout the trials (evapotranspiration, volumetric water content and biomass water use efficiency, BWUE). The above- and below ground (specific leaf traits for the green bean and the root structural development for the winter wheat) biomass was collected and analysed at harvest. There was significantly higher volumetric water content measured for the 50t/ha and 200t/ha biochar treatments. This effect can be ascribed due to a change in the soil’s tortuosity and porosity where more meso- and micro-pores were present as the biochar rate increased. The same results were evident when a water-retention curve was established in vitro by means of the sandbox method. The bulk densities were only significantly lower for the 200t/ha biochar treatments. The wheat root systems differed greatly among the fertilised biochar treatments: the 50t/ha and 200t/ha treatments had a more complex fibrous root system (more extensive branching and thinner roots) than 0t/ha, 1t/ha and 10t/ha application levels. This is attributed to the increased water-holding capacity along with a reduction of N- and P availability with increasing addition of biochar. Several leaf traits were measured for the green bean crops; however the leaf nitrogen- and carbon content, chlorophyll content index (CCI) and carbon isotope fractionation yielded the most interesting findings. Concerning the fertilised biochar treatments, there was established that the 10t/ha treatments had the highest leaf nitrogen- and carbon content. The leaf chlorophyll content did not differ significantly between the fertilised biochar treatments; however a very interesting observation was evident regarding the measured leaf CCI for the unfertilised treatments. A decreasing trend and lower leaf CCI was measured as the biochar application levels increased. This effect was ascribed to be due to a decrease in N uptake by the plants as the biochar application increased, the C/N ratio also increased, and this leading to N immobilisation. The lowest leaf carbon isotope fractionation was measured for the 10t/ha fertilised treatments and is inversely correlated with BWUE and therefore endorses the conclusion that the 10t/ha biochar application had a positive effect on the long term water use efficiency for the green bean plants. Biochar promoted aggregation in the sand-rhizosphere interface for winter wheat, increased water-holding capacity and enhanced crop performance for green beans. The findings reported here provide new information on the effect of biochar on the structural development of sandy soil, combined with biochar- and root growth effects for winter wheat; along with detailed interpretations of specific leaf traits associated with crop production for commercial green beans. The addition of biochar at low application levels (approximately 1-10t/ha to 15 cm depth) increased the biomass yield and water use efficiency of the crop species. Besides long term carbon storage, biochar can have immediate positive effects on the physical properties of sand and plant growth.
The effect of composted biochar on compost properties and mineralisation
(Stellenbosch : Stellenbosch University, 2016-12) Botha, Ockert Guillaume; Hardie-Pieters, Ailsa G.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Pyrolized carbon, also known as biochar, is a widely used soil conditioner recognized for its adsorption, C sequestration and agricultural qualities. This led to the investigation into the possible use thereof by small-scale sustainable farmers as a filter for agricultural olive or wine effluent, where after the spent biochar can be incorporated into composts to sterilize it from toxins and pathogens before being used as soil amendment. However, before these used biochar filters can be applied to compost, research is required to assess the affect that biochar could have on the composting process. This research project was therefore initiated to investigate the feasibility of adding biochar to composts, specifically focusing on the effect of type and amount of biochar on the composting process and mineralisation of the composts in soils. The final aim was to construct a method for quantifying biochar content in compost and soil that can be used to assess the stability of biochar in soils. Furthermore, none of this research has previously been done in South Africa or on two locally produced biochars. The first experiment was constructed to evaluate the effect of two contrasting commercial biochars on composting; a relatively low-cost, crude, pine wood biochar produced using a low-tech slow pyrolysis technique at 450°C, and a significantly more expensive, refined eucalyptus biochar produced using a high-tech slow pyrolysis technique at 900°C. The biochars were applied at two application rates (10% and 20% dry weight) to a mixture of green and animal waste. The effect was measured through composting indices such as temperature, C/N ratio, pH and EC, and microbial activity. Results showed that the robust, low temperature pine biochar applied at 10% (d/w) is the most suitable for composting due to higher composting temperatures measured, lower C/N ratios in the final product and higher cumulative microbial activity relative to the other biochar treatments. However, all biochar and control composts were all classified as successfully matured and stabilized according to the indices used, indicating that both types of biochar and application rates can be used to produce compost. The second experiment was aimed at comparing the carbon (C), nitrogen (N) and phosphorus (P) mineralisation of the composted biochar in relation to compost with biochar and biochar only under ideal laboratory conditions. The incorporation of these treatments into the soil showed that the composting process increased the composted biochars degradability with 7.6 – 11.7% more carbon dioxide (CO2) being respired than compost with biochar of the same quantity. Biochar type and quantity influenced the mineralisation as eucalyptus char in general, and all treatments containing 20% biochar proved to be least degradable by microbes. Nitrogen mineralisation results showed that regardless of biochar type, quantity or composting, all biochar containing treatments caused net N immobilization and reduced nitrification. Phosphorus availability was found to be improved for both biochars through composting and the addition of compost, especially for eucalyptus biochar of which the amount of available P surpassed that of pine biochar although pine biochar only applications released more P. A 6-month field trial experiment was also constructed to further evaluate the five composts’ C mineralisation under natural conditions. In this experiment there was found that all biochar containing compost produced 7.6 – 20.1% less CO2 than the control compost, of which eucalyptus biochar showed the least amount of respiration. Loss on ignition results also revealed that composted eucalyptus biochar was the least degradable composts as only 7.4% and 7.8% of the total SOM was lost. Density fractionation further illustrated that composted biochar remains in the soil in particulate form longer than conventional compost and is slower to transform into the mineral fraction. No discernable difference in biochar content within the composts could be seen after field application at 50 t/ha. The final aim of developing a rapid and cost-effective quantification method with the use of near-infrared spectroscopy (NIRS), was completed by constructing a calibration range of soils and compost from both types of biochar. The spectra acquired was then used to create regression models that were used to predict biochar content in the final mature composts and field trial soils. The results showed that NIRS can be used to quantify biochar, to within the same order of magnitude, in both composts and soil mixtures, which is of great importance for C stock audits and assessing biochar decay over time. Selecting the type of biochar for water filtration, composting and soil conditioning, would be dependent on the purpose of the application. Both biochars show the ability to be successfully composted and used as soil amendment with good C sequestration capabilities. However, pine biochar is more suitable for the composting process and sterilization as it results in higher temperatures and increased microbial activity. Eucalyptus biochar however, would be the best option for phosphorus mineralisation and building soil carbon stocks.
The effect of domestic greywater on soil quality of urban soils from the Cape Town and Stellenbosch areas
(Stellenbosch : Stellenbosch University, 2020-03) Madubela, Ncumisa; Hardie-Pieters, Ailsa G.; Clarke, Catherine E.; Lategan, Eugene; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: During a recent drought and water scarcity in the Western Cape, the reuse of greywater for garden irrigation was encouraged. Greywater, although considered less polluted than same other wastewaters, can be environmental hazardous due to the pathogens, salts, alkalinity and micropollutants it contains. Some greywater streams are easier to capture and reuse than others, and types of detergent can have a significant effect on greywater quality. In previous research the role of soil properties in soil susceptibility to greywater degradation has received little or no attention. Therefore, this study investigated the effect of irrigation with different domestic greywater streams on soil quality of a variety of representative urban soils from the Greater Cape Town area. Six domestic greywater streams were characterised in terms of water quality parameters. Two of better (shower and liquid laundry detergent) and two of poorer quality greywater streams (dishwasher and powdered laundry detergent) were selected for use in subsequent soil application experiments. Twenty soil samples, representing the five major soil groups from the Cape Town and Stellenbosch areas, were collected and characterised. These groups consisted of aeolian coastal sands (avg. 5% clay), alluvial soils (avg. 10% clay), granite-derived soils (avg. 11% clay), shale-derived soils (avg. 20% clay) and Fe-rich chromic soils (avg. 23% clay). In the first experiment, a laboratory soil column infiltration experiment was used to investigate the vulnerability of the five soil groups to degradation (pore sealing and dissolved organic carbon removal) by liquid laundry detergent (LLD) and powdered laundry detergent (PLD) greywaters in comparison to tap water (TW). Application of 200 mm PLD greywater had significantly more detrimental effects on soil permeability, clay dispersion and dissolved organic carbon (DOC) removal compared to 200 mm LLD or TW. This was attributed to PLD’s high pH (ca. 9.95) and SAR (ca. 147). The saturated hydraulic conductivity (Ksat) of the LLD greywater was 1.3 - 2.3 times lower than that of TW, while PLD Ksat was 2.2 - 8.4 times lower. Granite and shale soils were more inclined to Ksat reduction (ca. -81% and -82%, respectively) while the chromic soils were the least susceptible (ca. -47%). PLD greywater resulted in greatest extent of DOC removal, with aeolian sands being most susceptible to DOC stripping (ca. 7.5% C lost) while the chromic soils were the least susceptible (ca. 1.5% C lost). In the second leaching column experiment, the effect of the shower (SH) and dishwasher (DW) greywaters on soil degradation was compared to that of the laundry greywaters and TW on a smaller selection of (11) soils. Application of 200 mm of SH and DW reduced soil infiltration by ca. 50% compared to TW, although it was not statistically significant. Shower and dishwasher greywaters did not significantly remove DOC from the soils as compared to TW. In the third experiment, a column experiment was conducted to simulate the effect of repeated summer greywater irrigation, followed by winter rainfall, on soil properties. The effect of repeated application (370 mm applied over 10 weeks) of the four greywater streams on soil quality of a representative dispersive (granite – SP1) and stable (chromic – BD1) soil types was determined. This was followed by repeated application of 370 mm of rainwater to see whether the soils could be rehabilitated. As expected, the PLD and DW had the most harmful effects on soil quality, resulting in the formation of alkaline and saline-sodic soils. Powdered laundry detergent greywater and DW also significantly increased plant available P. All the treatments lowered soil bacterial diversity, while no significant change was observed on the fungal community. Subsequent application of rainwater showed that no water was able to infiltrate into the dispersive granite soil after treatment with PLD or DW. This indicated that it would be very difficult to remediate this soil type after irrigation with these types of greywaters. Application of all four greywaters significantly decreased rainwater infiltration in the chromic (ca. -42% to -93%) and granitic (ca. -25% to -100%) soils. Application of rainwater was, however, able to decrease the exchangeable sodium percentage of the DW and PLD irrigated soils to around ca. 13%, but the pH values remained high. Total C content of the PLD treated chromic soil was significantly decreased (ca. -22% of total C) due to DOC stripping. The results of this study demonstrate that soils vary in their susceptibility to degradation due to greywater application, depending mainly on texture and clay mineralogy. It is concluded that PLD and DW greywater should not be used for soil irrigation, whereas LLD and SH greywater should be used cautiously, especially on dispersive granite and shale-derived soils. The results of this study should be incorporated into the establishment of greywater irrigation guidelines.
The effect of gypsum form and source on soil amelioration
(Stellenbosch : Stellenbosch University, 2024-03) Du Plessis, M; Hardie-Pieters, Ailsa G.; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Studies evaluating the effectiveness of South African gypsum sources are scarce or outdated, as new products, such as granulated and liquid gypsums, have been introduced. The efficacy of calcium (Ca) nitrate and micro-fine (MF) calcitic limes in comparison to multiple gypsum sources has also never been evaluated. Consequently, the main aim of the study was to characterise and compare the effectiveness of various South African gypsum sources for use in crop production. A controlled laboratory study was performed to compare the effectiveness of mined and by-product gypsums and other Ca sources in terms of soil sodicity remediation and saturated hydraulic conductivity (Ksat) improvement. Twelve mined and by-product gypsums sources, MF calcitic limes and calcium nitrate were surface-applied to a sodic, sandy loam soil to achieve an exchangeable sodium percentage (ESP) of 5%. The soils received 300 mm simulated rainfall. Finer by-product gypsum sources (particularly liquid and flue gas desulphurisation (FGD) gypsums) exhibited faster dissolution rates and superior performance in lowering soil ESP compared to Class A mined gypsums. Only the liquid, phospho-, FGD gypsums and calcium nitrate significantly increased soil hydraulic conductivity compared to the control. The MF calcitic limes had no effect on the sodic soil properties. In a separate controlled laboratory study, 7 gypsum sources (reagent grade gypsum, phosphogypsum, granulated phosphogypsum, mine water, FGD, mined gypsums), two calcitic lime products (granulated MF and agricultural lime), hydrated agricultural lime, and a 1:1 combination of mined agricultural gypsum and calcitic lime were surface-applied to two Western Cape acid (sandy and sandy loam) soils and compared in terms of soil acidity remediation in 50 mm increments to a depth of 400 mm. The soils received 1000 mm simulated rainfall. Gypsum could not substitute lime in terms of acidity neutralisation but did show capability in displacing acidity away from the topsoil. The 1:1 combination of mined Class A gypsum and calcitic lime proved to be a viable substitute for lime alone. In a greenhouse pot trial, 6 gypsum products (FGD gypsum, mine water gypsum, phosphogypsum, granulated phosphogypsum, mined recrystallised gypsum and mined gypsum) were surfaced applied to an acidic, sandy soil (5.1% clay) to raise soil Ca levels to 400 mg kg⁻¹. The objectives of this study were to investigate the effect of different sources and forms of gypsum on the chemical properties of an acid, sandy soil and assess their effects on canola growth and nutrient uptake. Canola (Blazer TT cultivar) plants were grown in the pots and harvested after 7 weeks. Shoot and root biomass, foliar nutrient content soil chemical properties were then determined. In the short- term, granulated phosphogypsum and mined recrystallised gypsum were superior in terms of correcting acidity and improving root growth compared to the other gypsums, but all gypsum sources adequately addressed Canola Ca and S deficiency.
The effect of inorganic fertilizer application on compost and crop litter decomposition dynamics in sandy soil
(Stellenbosch : Stellenbosch University, 2015-03) Van der Ham, Ilana; Hardie-Pieters, Ailsa G.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of Agrisciences. Dept of Soil Science.
ENGLISH ABSTRACT: Inorganic fertilizer applications are common practice in commercial agriculture, yet not much is known regarding their interaction with organic matter and soil biota. Much research has been done on the effect of inorganic N on forest litter decomposition, yet very little research has focused on the effect of inorganic fertilizers on crop litters and, to our knowledge, none on composted organic matter. Furthermore none of the research has been done in South Africa. The main aim of this research project was to determine the effect of inorganic fertilizer applications on the decomposition of selected organic matter sources commonly used in South African agriculture and forestry. Two decomposition studies were conducted over a 3-month period, one on composts and the other on plant litters, using a local, sandy soil. In the first experiment a lower quality compost, compost A (C:N ratio, 17.67), and higher quality compost, compost B (C:N ratio, 4.92) was treated with three commercially used fertilizer treatments. Two were typical blends used for vegetable (tomato and cabbage) production: tomato fertilizer (10:2:15) (100 kg N, 20 kg P, 150 kg K per ha) and cabbage fertilizer (5:2:4) (250 kg N, 100 kg P, 200 kg K per ha). The third fertilizer blend, an equivalent mass application of N and P applied at 150 kg of each element per ha, is more commonly used in pastures. In the second experiment, five commonly encountered crop and forestry litters, namely kikuyu grass, lucerne residues, pine needles, sugar cane trash and wheat straw, were selected to represent the labile organic matter sources. The litters were treated with the tomato and cabbage fertilizer applications rates. Both decomposition experiments were conducted under ambient laboratory conditions at field water capacity. Decomposition rates were monitored by determining CO2 emissions, DOC production, β-glucosidase and polyphenol oxidase activity (PPO). At the start and end of decomposition study, loss on ignition was performed to assess the total loss of OM. Based on the results obtained from these two experiments, it was concluded that the addition of high N containing inorganic fertilizers enhanced the decomposition of both composted and labile organic matter. For both compost and plant litters, DOC production was greatly enhanced with the addition of inorganic fertilizers regardless of the organic matter quality. The conclusion can be made that inherent N in organic matter played a role in the response of decomposition to inorganic fertilizer application with organic matter low in inherent N showing greater responses in decomposition changes. For labile organic matter polyphenol and cellulose content also played a role in the responses observed from inorganic fertilizer applications.
The effect of invasive alien plants (IAPs) biochar properties on soil quality and crop production
(Stellenbosch : Stellenbosch University, 2024-03) Mncedi, Sipho; Hardie-Pieters, Ailsa G.; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Woody invasive alien plants (IAPs), such as black wattle (Acacia mearnsii) and eucalypt (Eucalyptus camaldulensis), are a major problem in South Africa. IAPs can decrease surface water runoff and groundwater recharge and are actively being cleared on a large scale. The woody biomass generated from the clearing of IAPs can serve as feedstock for biochar production. By sequestering carbon and improving soil functions, biochar has shown promise for enhancing agricultural practices. The efficacy of biochar in enhancing soil quality and crop production varies widely depending on its properties which are influenced by the type of feedstock and pyrolysis conditions used. According to the literature, biochar can contain certain toxic compounds such as heavy metals and volatile organic compounds formed during pyrolysis. Therefore, it is critical to test biochars on intended soils and crops to select the most suitable biochars. The aim of this study was to investigate the effect of fresh and aged biochars produced from IAPs from different feedstocks and pyrolysis temperatures on soil quality and crop production. Five biochars were produced at a range of temperatures (500, 600, 700, and 800 °C) from eucalypt and a single temperature (600 °C) from black wattle biomass using low-vacuum pyrolysis. In June 2022, the biochars were applied at 1.5% (w/w) on acid sandy soil and cauliflower was cultivated in an optimally fertilized greenhouse experiment. Some of the soil-biochar mixtures were left outside for 11 months so that the biochar could age naturally in the soil. After 11 months of ageing, the cauliflower greenhouse experiment was repeated using the same cauliflower cultivar and experimental conditions. In both trials (fresh and aged biochar trial), soil chemical, physical and microbiological properties were determined at planting and harvest. At harvest, the cauliflower fresh and oven-dried curd, and above-ground biomass were determined. The oven-dried curd and leaves were analysed for total elemental macro and micronutrient analysis (N, P, S, Ca, Mg, K, Na, Fe, Mn, Cu, Zn and B). In the fresh biochar trial, the 600 °C eucalypt biochar had the most positive effect on cauliflower yields (53% increase), attributed to higher soil basic cation exchange capacity and N retention. Whereas the higher temperature (700 and 800 °C) eucalypt and 600 °C black wattle biochars suppressed crop yields by 26-79% attributed to phytotoxicity, alkalinity, and reduction of B and soil water availability. All fresh biochars were found to reduce total available water due to high microporosity. Whereas in the aged biochar trial, only low-temperature eucalypt biochar (500 °C) suppressed the total available water. Biochar ageing reduced soil total N, but slightly increased effective cation exchange capacity and easily available water content however not compared to the aged sand control. The ageing of eucalypt and black wattle biochar also seemed to reduce the phytotoxicity that was present in certain fresh biochars. However, none of the aged biochars had a significant effect on cauliflower yields compared to the control sand. The results of this study highlight the importance of biochar properties, as influenced by pyrolysis conditions and feedstock, on crop growth response.
Effect of lime source, form and placement on soil pH neutralisation and permeation
(Stellenbosch : Stellenbosch University, 2021-03) Du Toit, Dawid Jacob Johannes; Hardie-Pieters, Ailsa G.; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Currently, agricultural producers in South Africa are uncertain about the relative effectiveness of locally available class A and granulated micro-fine lime sources, especially in terms of soil permeation and correction of subsoil acidity. There is also limited information regarding the effect of lime placement (surface, band-placement and incorporation) on subsoil acidity alleviation and crop growth. Therefore, the main aim of the study was to compare the effectiveness of various local lime sources (calcitic and dolomitic) and forms (class A, micro-fine and granulated micro-fine) at increasing soil pH and permeating through different textured soils. A laboratory soil-lime incubation study was conducted to compare the acid neutralisation capacity of various local dolomitic and calcitic lime sources (i.e., different quarries) and forms (class A, micro-fine and granulated micro-fine lime) at increasing soil pH over time under controlled conditions in three acid soils (sand, sandy loam, and loam). All liming materials were applied (adjusted for CCE) to reach a target pHKCl of 5.5 on the three soils, incubated at around 85% field capacity and room temperature, and then pH measured at intervals of 1, 4, 7, 14, 21, 30, 47, 60, 90, 180, 275 and 365 days. Ungranulated micro-fine limes were found to have a higher rate of dissolution than class A lime, but the difference in pH obtained attenuated over time. The granulated micro-fine lime was found to be the least effective in correcting soil acidity due to largely remaining intact. The difference in performance between commercially available class A limes was of little importance, if applied based on CCE. Chemical purity (Ca%, CCE) was found to be the main factor effecting the reaction of a liming material in soil and particle fineness only determines the short-term reaction (4-7 days) of the limes. A winter rainfall laboratory column study was carried out on two soils (sand and loam) to compare the soil permeation of different calcitic lime forms (class A, micro-fine and granulated micro-fine). Lime was surface applied to soil columns (40 cm) and irrigated in six events with a total of 350 mm water over 12 weeks. The soil chemical effects of all the surface applied limes were limited to the top 5 cm only. Lastly, a greenhouse barley (Hordeum vulgare) pot trial was conducted to assess short-term crop response to lime form and placement. Lime (class A, micro-fine and granulated micro-fine) was applied (surface, band-placed or incorporated) to an acid loamy sand soil before barley planting. The effects of liming materials on soil chemical attributes were confined to its point of placement. Thorough incorporation of lime more effectively neutralised soil acidity below 5 cm, especially Mn toxicity, resulting in larger barley shoot biomass after six weeks. The granulated lime was once again the least effective due to partial dissolution. The results of this study highlight that, if class A limes are corrected for CCE, the difference between the respective class A lime sources and micro-fine limes attenuates over time and the effects of surface applied lime is confined to its point of placement.
The effect of long-term no-till crop rotation practices on the soil organic matter functional pools
(Stellenbosch : Stellenbosch University, 2014-04) Smith, Jacques De Villiers; Hardie-Pieters, Ailsa G.; Strauss, J. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Total soil organic matter (SOM) and its different functional pools (fractions) are important attributes of the physical, chemical and biological quality of the soil and are seen as key factors in the evaluation of the sustainability of management practices. Until now, limited information was available regarding soil C accumulation and stabilization under conservation tillage managed soils in the Western Cape grain production regions of South Africa. Long-term field experiments investigating different crop and crop/pasture rotation systems under no-tillage were initiated in 2002 at the Tygerhoek Research Farm of the Western Cape Department of Agriculture, near Riviersonderend, Overberg, Western Cape, South Africa. The study site enabled us to compare the following five dryland cropping systems; permanent Lucerne (100% pasture), Medic-Medic-Wheat (MMW) (67% pasture; 33% crop), Medic-Medic-Wheat-Wheat (MMWW) (50% pasture, 50% crop) and two 100% cropping systems (continuous cropping) in different phase [Wheat-Barley-Canola-Wheat-Barley-Lupin (WBCWBL4 & WBCWBL1)]. The numbers “1” and “4” in rotation code refers to the first and fourth crop planted in the cropping system, respectively. The underlined crop in rotation code represents the crop that was on the field at time of sampling. Natural vegetated soil (non-cultivated area) acted as a reference for this study. In 2012, soil samples were taken at four depth increments; 0-5, 5-10, 10-20, 20-30 cm. The objectives of the study were to investigate the effect of long-term crop/pasture rotation systems on: i) the total soil organic carbon (SOC) storage under different cropping systems, ii) the SOC and N content in different functional pools (fractions); free particulate organic matter (fPOM) fraction (labile fraction), occluded particulate organic matter (oPOM) fraction (moderately stabile intra-aggregate C) and mineral-associated fraction (stabile fraction), (iii) the main C stabilizing mechanisms operative in these soils and (iv) the relationship between the extent of C sequestration and crop yields. After 11 years, the medic-wheat rotations had the highest total SOC contents (15.2-18.6 g kgˉ¹ in 0-30 cm depth, P ≤ 0.05), compared to the continuous cropping (13.3-14.1 g kgˉ¹ in 0-30 cm depth), permanent lucerne pasture (15 g kgˉ¹) or natural vegetated soil (13.2 g kgˉ¹). Higher belowground C inputs through roots and the lower extent of disturbance in the 0-10 cm depth are the main reasons for higher total C content in the wheat-medic systems compared to the other systems. The contribution of the fPOM fraction (labile C) to total C content in the cultivated treatments (6-9%) was lower than the natural vegetated soil (13%) in the 5-10 cm depth. The fPOM fraction is the most sensitive soil organic C and N pool to detect changes due to management practices, which include quantity and quality of OM inputs, extent of physical disturbance, and fertilization. The medic-wheat rotations had the highest C (1.37-1.74 g kgˉ¹ in 5-10 cm depth) and N (0.107-0.110 g kgˉ¹ in 5-10 cm depth) contents in the fPOM fraction of the cultivated treatments. Compared to the natural vegetated soil, the cultivated treatments had a lower C content in the oPOM fraction (moderately stabile fraction) and concomitantly a lower aggregate stability. On average, the oPOM fraction only contributed 0.4-2.4% to total C content at all sites. A significant positive correlation (R²= 0.77) was found between C occluded in aggregates (oPOM fraction) and aggregate stability with the highest aggregate stability found in the medic-wheat rotations of the cultivated treatments. The major part (85-93%) of the SOC was associated with the mineral fraction (stabile fraction) in the natural vegetated and agricultural soils. The MMWW treatment contained the highest C content (18.7 g kgˉ¹, 5-10 cm depth) in the mineral-associated fraction and the two continuous cropping systems the lowest (14.2-14.7 g kgˉ¹, 5-10 cm depth) of the cultivated treatments. A significant positive correlation was found between mineral-associated SOC fraction and clay (R² = 0.74) and Fe-oxide (R² = 0.57) content. This helps explain the large mineral SOC fraction found in these soils and is the dominant SOM stabilization mechanism operative in these shale-derived soils. The mineral-associated organic matter is probably predominantly sorbed to the clay minerals (illite, kaolinite and sesquioxides) via ligand exchange resulting in very strong organo-mineral associations. Physical protection via occlusion in aggregates is not a dominant C stabilizing mechanism in these soils. The C:N ratios of the fractions decreased in the order fPOM > oPOM > mineral with a C:N ratio below 10 in the mineral fraction indicative of humified organic matter. The MMW and MMWW treatments produced higher wheat yields in 2012 with a significant positive correlation found between total soil C and N, and yields obtained. In a higher quality soil, higher agronomic production is expected. Findings in this study enabled us to conclude that due to effect of cropping system and soil properties, the MMWW treatment had the highest total SOC content, which included highest labile C and N content and highest.
Effect of NPK application on rooibos (Aspalathus linearis) under Clanwilliam field conditions
(Stellenbosch : Stellenbosch University, 2018-03) Lourenco, Marcello Rufino; Hardie-Pieters, Ailsa G.; Valentine, A. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Currently no macronutrient fertiliser recommendations have been established for rooibos plants under field conditions. The aim of this study was to examine the interactive effect of NPK (nitrogen, phosphorus, potassium) on young rooibos plants’ growth and survival, and soil chemistry and fertiliser leaching under Clanwilliam field conditions with an aim to establishing soil and foliar nutrient norms for optimum rooibos production. A field trial was established at Vaalkrans Farm, Clanwilliam district in June 2016. Rooibos seedlings were fertilised at planting as a completely randomised design in combinations of various levels of N (0, 20, 40, 60 mg/kg) as NBPT-coated urea, P (0, 15, 30, 45, 60 mg/kg) as triple superphosphate and K (0, 20, 40, 60, 80 mg/kg) as potassium chloride (KCl). The N and K applications were split, 50% at planting and the remainder top-dressed after 2 months. The fertilisers and application rates were selected based on previous seedling greenhouse trials. Parameters measured during the trial included: soil pH, electrical conductivity (EC), total carbon (C) and N, mineral N, Bray II P, exchangeable cations, micronutrients, soil enzyme activity, plant height, survival, biomass, and foliar nutrient content. The movement of the applied fertilizer was also determined on selected treatments, and a pot trial to determine the effect of lime application of rooibos seedling growth was performed. Initially, during the wet winter months, the application of P at 15 and 30 mg/kg stimulated biomass production. However, after the dry summer it was observed that all P applications suppressed plant growth and decreased plant survival, and this effect was more pronounced as P application rate increased. Foliar P and shoot biomass were negatively correlated (R2=0.5929). No interactive effect between N and P on biomass response was found, and N application could not help rooibos to overcome P-toxicity, contrary to previous studies. The highest above-ground biomass yields were recorded at K application rates of 20 – 40 mg/kg. When yield was adjusted according to mortality, the 20 mg/kg K treatment had the largest yield (597 kg/ha), nearly double that of the unfertilised control. Due to the low intensity rainfall experienced in Clanwilliam during the field trial, the fertiliser had not leached significantly in the soil profile, and the majority remained where it was initially placed at planting (20 – 30 cm) and on surface (0 – 20 cm). Rooibos seedling biomass responded positively to lime application at all rates up to an equivalent of 1.29 t/ha in a greenhouse pot trial. Application rates of 1 – 1.3 t/ha nearly doubled the mass of rooibos seedlings after two months. The ideal pH for rooibos seedling growth in this study was found to be around pH (KCl) 7.4. This study highlights the importance of field trials, as opposed to short-term greenhouse trials, as the effect of nutrients combined with climate can have deleterious effects. It is recommended that young rooibos plants do not receive any P fertilisers at planting, but receive up to 20 mg/kg of N and between 20 – 60 mg/kg of K (applied as split application).
The effect of residue management on the nutrient cycle in the production of rooibos (Aspalathus linearis) at Nieuwoudtville, Northern Cape
(Stellenbosh : Stellenbosch University, 2017-03) Nieuwoudt, Stephanus Francois; De Clercq, W. P.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of AgriScience. Dept. of Soil Science.
ENGLISH ABSTRACT: Rooibos (Aspalathus linearis) is a sensitive fynbos species with a large genetic variation, adapted to acid, nutrient poor soils, and can only be grown in certain parts of the Western- and Northern-Cape. Rooibos yields are getting poorer with increasing age of the field and the lifespan of rooibos are also just a maximum of 5 years commercially produced. A lot of medicinal/health research has been done and published on rooibos, but not much on the agricultural production aspects of rooibos. In order to increase the production and lifespan of rooibos tea further research needs to be done to better understand the rooibos plant in its cultivated environment. The role of leaf litter in fynbos, particularly in the cultivated rooibos nutrient cycle is still a grey area that could open up key management principles regarding plant water availability and plant nutrition style. The hypothesis is that the method of harvesting the rooibos seed by removing the litter layer without returning it under the plant can have a negative impact on the nutrient pools and cycle and thus lead to a shorter lifespan. The main aim was thus to look at the effect of different residue treatments on the rooibos plant nutrient cycle (uptake and nutrient pools). Four sites across the Nieuwoudtville Bokkeveld region were selected with all the rooibos plants being ± 2 years old. Four different mulch treatments; a bare soil (leaf residue removed) treatment imitating seed harvesting (A), an added rooibos mulch (B), a natural leaf mulch (C) and an enriched rooibos mulch (D) were prepared at 4 sites. The chemical properties of soil and plant tissue from rooibos plants were tested. 5TE soil probes were used to measure the volumetric water content, EC and temperature at two soil depths of each treatment. All measurements were also duplicated using near-infrared spectroscopy (NIRS), to generate a database for future reference and to build calibrations that will be able to predict the nutrient content in the soils and plants. It was found that soil chemical properties including P (mg kg-1), Na (cmolc kg-1), K (cmolc kg-1), Ca (cmolc kg-1), Mg (cmolc kg-1), Zn (mg kg-1), Mn (mg kg-1), C (%) and % Na (at pH 7); and plant chemical properties including Na (%) and plant N (%), P (%), K (%), Al (mg kg-1) and Fe (mg kg-1) all had a significant effect of the regrowth models using multiple regression analysis. Soil P, Mg and K had the biggest positive influences on the regrowth models. During this process it was found that the N:P ratio in soil plays an important role in the uptake of N and growth. Only at treatment D, with the lowest soil N:P ratio, plant N (%) had a positive influence on the regrowth multiple regression model. Plant N and P had a moderate positive correlation (R2=0.56). Nutrient uptake by the rooibos plant was very high from July 2015 to September 2015. These nutrients included N, P, K, Ca, Mg, Zn, Mn, Fe and Al. From September 2015 to January 2016 however the uptake was lower for all the nutrients, but for K and Mg the uptake was higher compared to the other nutrients. The decrease in plant nutrient concentration from September 2015 to January 2016 is a result of less nutrient uptake and nutrient dilution following rapid growth of plant. There was an increase in soil exchangeable Mg and Ca from July 2015 to September 2015 due to increase in soil pH during this time. Soil exchangeable Ca (R2=0.49) and Mg (R2=0.61) correlated positively with pH, thus the increase in soil exchangeable Ca and Mg can be due to the increase in pH. For all the treatments there was a total decrease in soil N (significant for A and B) and plant N over the one-year period. The plant Al and Zn for all the treatments also increased over the one-year period. The increase in plant Al was not significant and the increase in plant Zn was significant for all treatments. From July 2015 to January 2016 there were differences in growth between the treatments. Treatment A resulted in the lowest estimated dry matter increase during this period and for treatment B it was the highest. The difference in estimated dry matter increase between these two treatments was also statistically significant. The estimated dry matter increase for treatment C and D was higher than treatment A but it was not significant. The mulch treatments, especially treatment B, resulted in higher P, K and Mg uptake. For all the treatments, except treatment D, the soil P decreased over the one-year period. For treatment B and D the plant P increased significantly compared to treatment A and C where the increase was not significant. The mulch treatments showed an increase in plant K, but it was only significant for treatment B over the one-year period. For all the treatments there was an increase in plant Mg, but only for treatment A the increase was not significant. All the mulch treatments also conserved more water for longer compared to treatment A. The combination of nutrient leaching from the mulch (Mg and K) and the conservation of more soil water may be the reason for the higher nutrient uptake by treatment B and thus the better growth. From the NIRS study it was found that for these sandy soils it was only exchangeable Mg that could be predicted with good accuracy (RPD>2). The soil chemical properties pH, H (cmolc kg-1), K (mg kg-1), Ca (cmolc kg-1), Fe (mg kg-1) and C (%) models showed satisfactory predictability. For plant samples NIRS predicted P (%) and Mg (%) with good accuracy. The prediction models for N (%), K (%), Ca (%) and Na (mg kg-1) were only satisfactory and for the rest it was unreliable. From these results it was thus not possible to quantitatively predict all the chemical properties in the soil and plant samples but there is potential for better calibrations in the future. Differences in growth and vigour can also be attributed to location. The micro conditions in which a single plant grows, related to the impact of normal agricultural practices, was found to also determine the success of rooibos production. The hypothesis was supported by treatment C (the plant where natural mulch was not removed) having a higher estimated dry matter increase compared to treatment A (bare soil), but this difference was not significant. Treatment B (added mulch) however showed to have a significant impact. Rooibos production systems are far from being optimized and the amounts of unknown impacts were narrowed down by this work. This research indicates that the rooibos plant is genetically unrefined and that agricultural practices should focus more towards the natural state of fynbos nutrient availability and growth.
The effects of long-term tillage and crop rotation practices on nutrient stratification in the Western Cape
(Stellenbosch : Stellenbosch University, 2022-04) Van der Merwe, Annemarie; Hardie-Pieters, Ailsa G.; Labuschagne, J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: The Western Cape is one of the most successful provinces in South Africa in converting to Conservation Agriculture (CA), with an adoption rate of around 80%. CA is a production system that promotes minimal soil disturbance, maintaining crop residues on the soil surface combined, with crop rotation with different species, including legumes. The absence of soil mixing in CA systems can lead to the stratification of immobile nutrients at the surface of the soil profile. Rapid drying of top soil layers may prevent roots from absorbing nutrients from these layers. Up until now, little was known regarding the extent of nutrient stratification in CA systems in the Western Cape. The first objective of this study was to determine the vertical distribution of plant-available nutrients under different tillage practices and rotation sequences at Tygerhoek (34 ̊29’32” S, 19 ̊54’30”E) and Langgewens (33 ̊16’34” S, 18 ̊45’51” E) Research Farms. Soil samples were collected at 0-5, 5-10, 10-15, 15-20 and 20-30 cm depth intervals in zero-till (ZT), no- tillage (NT), minimum tillage (MT) and conventional tillage (CT) treatments combined with 4 crop rotation sequences - wheat monoculture (WWWW), wheat and medics rotation (WMWM and MWMW) and canola/wheat/lupine/wheat (CWLW). Crop rotation and its interaction with tillage and soil depth did not influence (p < 0.05) the distribution of nutrients in the soil at Tygerhoek, but the distribution of K and S in the soil at Langgewens was influenced. Tillage significantly influenced nutrient stratification at both sites. The amount of extractable Ca, Mg, P, K and C were significantly higher in the surface 0-5 cm of the soil under ZT and NT compared to CT. The higher soil organic carbon (SOC) in the topsoil under CA (ZT, NT and MT), may be due to reduced soil disturbance and retention of crop residues. The organic C in the 0-5 cm layer decreased as degree of soil disturbance was increased (ZT2.6%>NT 2.23%> MT 2.15% > CT 1.96%). The percentage difference in soil exchangeable K at Langgewens between 0-5 cm (308 mg kg-1) and 5- 10 cm (172 mg kg-1) layers were the most at 79% for ZT. The second objective was to evaluate the extent of soil nutrient stratification in a wide range of CA systems on commercial farms. Stratification was observed in both the natural veld and the cultivated soil. The cultivation and addition of ameliorates (fertiliser and lime) in the cultivated soil have accentuated the stratification compared to the natural veld. The use of incremental soil sampling identified layers with levels of deficiency for K, S, Zn as well as the occurrence of subsoil acidity. On farms where CA was practiced for more than 25 years, C was built up and the highest values of Ca, K, P and S were present. The third objective was to determine the adoption rate of CA principles and awareness of soil nutrient stratification through a questionnaire. Most of the respondents (63%) indicated that they used CA as farming system. Minimum soil disturbance (47%) was indicated as the most important CA principle, followed by crop rotation (37%) and stubble retention (10%). More than half of the respondents (54%) indicated that the carbon content in the soil were higher than 1.5 %. This could be influenced by the fact that 82% of the respondents were from the southern Cape. More attention to sampling depth is required in conservation agriculture when sampling for lime and fertiliser recommendations to reflect the nutrient status of the soil. In the shallow soil of the Western Cape, soil increments of 0-10 cm and 10-30 cm are recommended for a better reflection of the nutrient status of the soil.
The effects of selected soil physical properties on the soil water balance, soil water redistribution and pyrrolizidine alkaloids loads in two diferent textured soils in the nieuwoudtville rooibos tea production area
(Stellenbosch : Stellenbosch University, 2021-03) Olifant, Kagisho Ignitious; Hoffman, Josias Eduard; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Rooibos (Aspalathus linearis) is an endemic crop of South Africa produced only in the south western parts of the country. This includes the Northern Cape where the production was reported to have declined over past five years. The first aim of this study was to look at the soil water dynamics in relation to the Normalized Difference Vegetative Index (NDVI) on Rooibos plantation around Nieuwoudtville area in the Northern Cape. Pyrrolizidine alkaloids (PA’s) contamination on Rooibos has been reported from several markets around the globe and pose a health risk towards consumers. Hence the second aim of the study was to investigate the PA’s in Rooibos plantation. Field trials were conducted at Rogland, Meulsteenvlei, Oorlogskloof and Klein Blomfontein farms near Nieuwoudtville while a pot trial was conducted at Vaalharts Research Station near Jan Kempdorp in the Northern Cape Province. Soil water content (SWC) was monitored at an hourly basis throughout the growing season (October 2017 to February 2019) using ECH2O sensors in the field. At the end of 2018/19 growing season, soil water balance and NDVI of selected sites around Nieuwoudtville were determined. The total rainfall received at Rogland, Meulsteenvlei, Oorlogskloof and Klein Blomfontein was 373, 495, 413 and 391 mm, respectively. The cumulative evapotranspiration ( surrounded the Rooibos plants and known to contain PA’s. Further investigation was conducted in pots to evaluate PA’s uptake by Rooibos plant. Soil collected from Rogland was treated with three different weeds (Chrysocoma oblingifolia [0.225 ppm total PA’s], Othonna coronopifolia [0.377 ppm total PA’s] and Raphanus rhaphnistrum [46.008 ppm total PA’s]). Weeds were applied at a rate of 1% of soil volume and replicated 6 times. The pots were planted with Rooibos seeds obtained from Oorlogskloof on the 25th of July 2018 and the experiment continued up until the 1st of February 2019. The total weed population density between the study sites was not significantly different. However, the significant difference between the study sites was only found on Arctotheca calendula, Chrysocoma oblongifolia, Cleretum papulosum, Cynodon dactylon, Ehrharta longiflora, Juncus capensis, Senecio arenarius, and Ursinia weeds species. The dominant weeds species found to contain PA’s at the study sites were Arctotheca calendula, Chrysocoma oblongifolia, Othonna coronopifolia and Raphanus rhaphnistrum with an average total PA’s of 5, 75, 2 817 and 15 330 μg.kg-1, respectively. The mean total PA’s concentration in Rooibos plantation at the study sites was 7.2 ppm (Klein Blomfontein). 15.5 ppm (Meulsteenvlei), 16 ppm (Rogland) and 43.2 ppm (Oorlogskloof). The higher total mean PA’s concentration at Oorlogskloof can be due to the high density of Chrysocoma oblongifolia as compared to the other study sites. The Chrysocoma oblongifolia could have released PA’s in soil during decomposition which could have let to the uptake of PA’s by Rooibos plants. Furthermore, this study confirm that Rooibos itself cannot produce PA’s but rather, it absorbs PA’s from the soil. The lateral transfer of PA’s from weeds to Rooibos was also found to be inconclusive.
Evaluation of a peri-urban smallholder farmers’ soil amendment practices on soil quality and crop growth, yield and quality
(Stellenbosch : Stellenbosch University, 2018-03) Gobozi, Thamsanqa Khanya Sikho; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Peri-urban, smallholder farmers surrounding Cape Town, which are the main producers of fresh vegetables in the region, are generally not producing at their optimum level due to lack of agronomic support, marginal sandy soils and socio-economic constraints. The aim of this study was to evaluate the soil amendment practices of an organic smallholder farmer from Raithby, near Stellenbosch, in comparison with potential alternative organic and chemical amendments on soil fertility and vegetable crop growth, yield and quality and economic profitability. During the first winter field trial, the farmer’s routine soil amendment practice of adding 10 t/ha of commercially bought compost was compared with three alternative organic amendment practices and a commercial chemical fertilizer programme on broccoli production. Two on-farm produced composts, composted plant and animal waste (CW) and composted waste containing 20% biochar (CB), and the commercial compost (CC) were applied at typical smallholder application rate of 10 t/ha. The CW was also applied at broccoli N requirement equivalent to 22 t/ha (CWCR). These organic treatments were compared with a control soil (C) and a chemical fertilizer (CF) programme designed specifically for broccoli. There were no significant differences in soil quality at planting or at harvest (pH, EC, ECEC, plant available macro or micro-nutrients) or broccoli head nutrient content between treatments. However, the CF significantly (p<0.05) increased soil mineral N compared to all other treatments, whereas, CB significantly (p<0.05) enhanced soil C. Application of CF significantly (p< 0.05) increased broccoli yields (88% increase compared to CC) which was correlated with the higher soil mineral N, followed by CW (28% increase compared to CC). Application of CC, CB and CWCR resulted in non-significant changes in yield compared to the control, which was attributed to too much C being added to soil compared to N. Compared to the farmer’s routine amendment practice (CC), the CF resulted in the greatest income increase (455%) followed by CW at 10 t/ha (151%). During the second summer field trial, the effect of two composts, i.e., university compost (UC) and farmer’s compost (FC), two commercial organic fertilizers, i.e., OF1 (blood and bone meal based) and OF2 (chicken manure based), and commercial chemical fertilizer (CF) programme was evaluated on green bean production. The commercial organic and mineral fertilizers were applied at green bean N requirement rate of 158 kg N/ha. Whereas, the two composts were inadvertently applied at different N application rates relative to the commercial fertilizers (UC was added at 8.9 ton/ha (~49 kg N/ha), while FC was added at 17.8 ton/ha (~181 kg N/ha) due to a commercial laboratory providing incorrect elemental analysis of the composts prior to the field trial. All compost and fertilizer treatments significantly (p< 0.05) increased soil Bray II P contents above the critical value 25 mg/kg at planting except FC. The commercial organic fertilizers increased soil EC by a factor of 2-3, which resulted in lower bean plant survival. There were no significant differences in bean nutrient content between treatments, except for OF1 which contained significantly lower Mg content. Application of CF significantly (p< 0.05) increased (56% compared to control) green bean yields which was associated with a significantly (p< 0.05) higher (168% compared to control) cumulative soil mineral N, while the FC applied at 17.8 t/ha produced the second highest increase (37% compared to control) which was associated with higher (5%) number of plants that survived to harvest and the order was consistent in terms of economic feasibility. The availability of mineral N was the main driver of crop yields and size of economic yield per plant in this study. Composts, especially commercial composts with low inherent N content, are not reliable sources of mineral N for intensive crop production. The commercial organic fertilizers, although better sources of mineral N, were prohibitively expensive and decreased plant survival. The organic smallholder farmer is likely to generate more income when he produces his own compost using animal and plant waste and applily the on-farm produced compost at N requirement of the crop in production rather than buying composts or organic fertilizers. The study also indicated that the farmer would generate much higher income, especially in winter when organic N mineralisation is slowest, if he would use a chemical fertilizer programme for both model crops.
An evaluation of lime Requirement methods for selected South African soils
(Stellenbosch : Stellenbosch University, 2017-03) Van der Berg, Vincent; Hardie-Pieters, Ailsa G.; Raath, P. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: A laboratory study was conducted to evaluate seven widely used methods to predict soil lime requirement (LR) using 20 acidic South African top and sub-soils with a wide range of properties. The LR methods which were evaluated against a standard CaCO3 incubation LR procedure, included: the original Eksteen method with organic matter correction factor (OMCF) , commonly used in the Western Cape; two modifications of the Eksteen method, namely: (i) Eksteen-KCl method, involving the use of 1 M KCl exchangeable acidity instead of titratable acidity at pH 7, and (ii) Modified-Eksteen method, where a correction factor was applied to titratable acidity that was derived from soil data obtained in this study; the Cedara method, most commonly used in KwaZulu-Natal; the ARC-SGI method, developed primarily for Free State soils by the ARC- Small Grain Institute in Bethlehem; the Shoemaker-McLean-Pratt single buffer (SMP-SB) method most commonly used in the North East and North Central regions of the USA; the Adams and Evans single buffer (AE-SB) method most commonly used in the South East and Mid-Atlantic regions of the USA. The original Eksteen method, although highly correlated with incubation LR, was found to be a relatively inaccurate predictor of LR. The Eksteen-KCl and Cedara methods were found to be highly correlated with incubation LR, yet consistently underestimated LR. The modified-Eksteen method was found to be highly correlated with incubation LR, and was a good predictor of LR. The ARC-SGI method was a considerably poorer predictor of LR, and tended to grossly overestimate LR. The SMPSB method was found to be highly correlated with incubation LR, and was shown to be reasonably accurate to achieve a target pHKCl of 5.5. Recalibration of the SMP-SB soil-buffer pH with incubation LR resulted in considerable increases in accuracy. The AE-SB method was found to be highly correlated with incubation LR, yet tended to overestimate LR. Recalibration of the AE-SB soil-buffer pH with incubation LR resulted in a sufficient increase in accuracy. A correlation study was conducted to investigate the relationship between soil properties and both incubation LR and LR methods. It was revealed that soil properties other than soil pH, which are useful indicators of LR are: soil C > variable charge > CECpH 7 > clay + silt. Soil C was found to be a significant contributor to LR due to its association with exchangeable Al and due to its high pH dependent acidity. Titratable acidity was found to be the soil property that most strongly related to soil LR. Variable charge was also shown to exhibit significant relationships with soil parameters that most strongly influence LR. For these reasons, a multiple regression equation was developed that utilised only titratable acidity and variable charge. The multiple regression equation was able to predict 96.76% of the variation observed for incubation LR, and was 97.86% accurate in predicting the LR for each specific soil to obtain a target pHKCl of 5.5. Regarding the relationship between soil properties and LR methods, it was revealed that the local methods, except the ARC-SGI method, were most strongly influenced by exchangeable acidity and Al, and had significant relationships with soil C. The American direct buffer methods were strongly correlated with essentially all of the soil properties studied. This is indicative of the ability of these methods to directly determine the amount of acidity that may originate from various sources in the soil, in order to make a sufficiently accurate LR. It is therefore recommended that the application of direct buffer methods be further developed for use on South African soils in order to further improve the accuracy of LR determination in South Africa. The existing method that was found to most accurately predict LR on a wide range of soils was the modified-Eksteen method.
Improving phosphate fertilizer recommendations using soil phosphorus buffer capacity and evaluation of various P extraction tests on a variety of South-African soils
(Stellenbosch : Stellenbosch University, 2019-04) White, Vivian George; Hardie-Pieters, Ailsa G.; Raath, Pieter; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Phosphorus (P) fertiliser recommendations are based on extractable P levels as measured by a variety of extraction tests and should incorporate factors such as depth, bulk density and the soils phosphorus buffering capacity. Internationally and in South-Africa a variety of extraction test are used with contrasting norms and crop norms are not available for all of these extraction tests. Therefore the first aim of this study was to examine the relative aggressiveness and relationships between the various P extraction tests using 49 soils with widely varying physicochemical properties. The relative efficiency as compared to Total P (acid digestion) of the extraction tests were as follows; 1:2 H2O < Olsen < Colwell < Bray I < Ambic 1 < Bray II < Mehlich III ≈ Citric acid. Bray II and Mehlich III were the only extractions test that correlated significantly (R2 = 0.78 & 0.76) with total P. Strong linear relationships were observed between Bray II, Olsen and Mehlich III extractions tests and based on the weighted standard error of measurement, direct conversions between these tests were most reliable. It is thus possible to convert with confidence from Olsen to Bray II and Bray I by using a factor of 5.20 and 3.88 respectively. For the conversion from Bray II to Mehlich III a factor of 1.10 can be used. Phosphorous buffering capacity (PBC) can be determined by a multiple-point sorption isotherm and through fitting the Freundlich equation or through a single-point isotherm method by making various assumptions. A subsample set of 10 soils varying in P sorbing capabilities was used, a large variance in parameter b was observed and ultimately two distinct groups with both a different value for b were determined. By equilibrating the samples with 1000 mg P kg-1 and by using the untransformed variation of the Freundlich equation and using the two distinct manually selected values for b (one for the low PBC soils and one for the high PBC soils) the best results were obtained. The single point estimate of PBC correlated significantly with PBC (Ozanne and Shaw, 1968) (Eq. 1 & 2). However, a large RMSE was observed and this predicted estimate of sorption is not reliable. These estimations utilise unrealistically high P levels and a 1:10 soil: solution, completely saturating the soil and allowing for most of the added P to react with the soil. This would be highly unlikely when incorporating P fertilizer into the soil thus a simple laboratory incubation method was investigated. A strong linear relationship existed between applied P and percentage extractable P by the three extraction tests used (Bray II, Mehlich III and Olsen) using the incubation method. It was found that at an application rate exceeding 150 mg P kg-1 the regression lines plateaued and that and the percentage extractable P at rates (100 – 150 mg kg-1) correlated highly significantly (R2 = 0.92 and 0.99 respectively) with the percentage extractable P derived from the slope of the regression line of applied P against extractable P. The influence of time also needed to be investigated and it was found that Bray II extractable P only significantly decreased after 1 month and that after 24 hours of equilibration the percentage extractable P remains fairly constant up to one month. Therefore it is possible to get an estimate of the amount of applied P that will be plant available with a single-application incubation method allowing for 24 – 72 h of equilibration. Lastly the Bray II and Olsen extractable P was correlated to plant response in high pH soils of the Northern cape planted to grapevine and citrus, no significant response was observed to the applied P fertilizer due to soil P already being at significant levels. Bray II was most sensitive in detecting applied P and excessive soil P levels, where Olsen was the least effective in detecting applied P. Therefore these soils do run the risk of reaching excessively high P levels which can in turn lead to P loss, micronutrient deficiencies and groundwater contamination.