Tillage effects on soil quality and plant productivity in the Swartland region, South Africa

  • Flackson Tshuma

    Student thesis: Doctoral ThesisDoctor of Philosophy


    Many farmers have stopped the practice of conventional tillage and have adopted conservation agriculture. Nonetheless, the practice of no-tillage in conservation agriculture can lead to weed and pest infestations, therefore most farms are currently
    managed using synthetic agrochemical (herbicide, insecticide, fungicide, and fertiliser) applications. Excessive utilisation of synthetic agrochemicals can be harmful to the environment. If the current conservation agriculture systems are to limit the
    environmental damage caused by synthetic inputs, then some form of tillage is likely to be necessary. Infrequent tillage practices could be adopted as a means of preventing intensive tillage and minimising problems associated with long-term
    conservation agriculture.

    This research explored the effects of contrasting tillage regimes and agrochemical applications on soil quality, crop productivity, and weed dynamics in South Africa’s Mediterranean climate zone. Seven tillage treatments, within a long-term (44-years) tillage experiment, were investigated: continuous mouldboard (MB), tine-tillage (TT), shallow tine-tillage (ST), no-tillage (NT), and infrequent tillage treatments: ST conducted once in two years (ST-NT), ST conducted once in three years (ST-NT-NT), and ST conducted once in four years (ST-NT-NT-NT). Three rates of synthetic agrochemical applications were used (standard, reduced, and minimum); the reduced and minimum rates involved the application of synthetic agrochemicals in combination
    with bio-chemicals (chemicals derived from natural compounds). It was hypothesised that infrequent tillage and application of reduced synthetic agrochemicals would improve soil quality, crop productivity and increase weed seedbank diversity relative
    to the NT and MB treatments.

    Contrary to the research hypothesis, infrequent tillage practices failed to significantly reduce the stratification of soil chemical parameters and could not improve the soil microbial diversity and enzyme activity; wheat and canola yield and quality and weed
    seedbank diversity. The MB was able to prevent stratification and weed infestation but depleted the soil organic carbon and led to a reduced soil enzyme activity. Nonetheless, the combined results from the system with standard and reduced use of synthetic agrochemicals for 2018 and 2020 showed that there were no differences in yield and grain quality in four of the seven tillage treatments. And no differences were found in canola seed yields in 2019. Further reduction in the application of synthetic
    agrochemicals, as was done in the system with minimum synthetic agrochemicals, did not yield positive crop productivity results due to severe weed problems.

    Overall, results from this study highlight the importance of reducing both the intensity of tillage and the application of synthetic agrochemicals as doing so can improve soil quality and crop productivity. However, there are trade-offs. Some form of tillage is required to prevent nutrient stratification, but this should not be so intensive or frequent as to deplete the soil organic matter stocks. Also, the application of standard synthetic
    agrochemicals, as conducted in most conservation agriculture systems, can be reduced, but it is risky to completely avoid the synthetic agrochemicals as shown by crop failure in the system with minimum synthetic agrochemicals in 2020. Furthermore,
    results from the system with reduced synthetic agrochemicals shows that the Western Cape province has the potential to gradually introduce more agroecological farming practices in wheat and canola production by using bio-chemicals although further research is needed to optimise these approaches.
    Date of Award2022
    Original languageEnglish
    Awarding Institution
    • Coventry University
    • Stellenbosch University
    SponsorsWestern Cape Agricultural Research Trust
    SupervisorJames Bennett (Supervisor), Francis Rayns (Supervisor), Pieter Andreas Swanepoel (Supervisor) & Johan Labuschagne (Supervisor)

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