Effect of subsoil constraints on the chemical potential of soil potassium in a catena in NSW southern tablelands : how potassium potential, or isotherms, can be used to predict the soil response to nutrient depletion or addition in dryland agriculture

  • Seija Tuomi

    Student thesis: Master's Thesis


    Soil potassium (K) is of great agronomic importance to crop yield quality and quantity in Australia. A soil catena (toposequence) relates soil profiles at a functional scale, enabling interpretation of the landscape position effect on soil K, subsoil characteristics, soil horizon configurations and related patterns in soil chemical properties. The Fairview Farm soils have developed over a regionally extensive Siluro-Devonian felsic volcanic substrate. Linking soil parameters to hillslope processes at Fairview Farm likely mirrors processes operating at sites in a similar landscape setting and with similar agricultural land use.
    Naturally occurring K in soils comes from the weathering of rocks and the breakdown products of minerals in the long term (Sparks et.al, 1987), and in the short term it comes from the turnover of organic materials and further breakdown of soil particles. The nutrient capacity of K in the soil is determined by the total amount of exchangeable K present in the soil colloid and the energy at which it is delivered. This thermodynamic term is expressed as the relative activity ratio and exchange of K+ with Ca2+ and Mg2+. The K potential of a soil is a free energy measure of the amount of work plants need to do to acquire K from the soil and this is unique to each soil. Soil K potential in dryland agriculture can be measured by the isotherm technique. At Fairview Farm, situated between Binalong and Boorowa in New South Wales (NSW), the isotherms describe soil K behaviour on a catena with greater precision than measuring plant-available K.
    For this area, publicly available soil data mostly describe the upper 10 cm of analysed profiles, but typical crop rooting depth is much deeper (40–150cm). Agricultural soil testing is vital for farmers but has traditionally been expensive with a lag in data provision, meaning soil information is not readily available for ‘just-in-time’ monitoring. Use of the isotherm technique to evaluate K behaviour is a relatively inexpensive and timely approach that may address this issue and provide a level of detail otherwise not available.
    This research improves our understanding of K behaviour in an agricultural setting, using proximal sensing (Diffuse Reflectance Infrared Fourier Transformed (DRIFT) Vis-NIR spectroscopy) to measures secondary K-bearing clay minerals to illustrate the effect that secondary minerals have on soil K potential and Fe oxides to reflect on the soil structure. The K-bearing minerals are a source of K and they are associated with storage/release of K. Also, spectroscopy is used to map total K distribution in the soil. This new method is compared with the traditional method for mineral analysis, x-ray diffraction (XRF).
    Isotherms reveal that Fairview Farm soil preferentially adsorbs K from the soil solution, which is possible in heavy clay soils when K bearing secondary clay minerals are present. The exchange sites on K-bearing minerals must be filled before K accumulates in the soil solution, thus becoming plant available. All the landscape positions show preferential adsorption to some extent, but it decreases from the top of the hill to the bottom of the hill.
    Although this site has a small amount of plant-available K and only clusters of poorly crystalline K bearing secondary minerals, plants find it easy to extract K from the soil. The Crest plant available K is always at luxury levels, whereas the Mid-slope and Lower slope have decreasing amounts until the energy required to extract the plant available K becomes too much. i.e., K potential decreases too low. In this way, plant available K becomes a subsoil constraint, which is reflected in the uneven crop growth in the paddock.
    The soil overall is illite/kaolinite dominated, but geochemistry tells us that the formation of the secondary minerals is not dependent on the geology but rather it depends on what is in the soil solution. The top 80 cm of the soil has not evolved from the same geological origin as the soil below. Interstratification of illite with smectite makes it difficult to identify these two minerals as they are poorly crystallised. Farmers and their advisors should consider the mineralogy before deciding on how and when to apply K rich fertilizer.
    Date of Award2023
    Original languageEnglish
    SupervisorDuanne White (Supervisor), Leah MOORE (Supervisor) & Fiona Dyer (Supervisor)

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