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Type: Theses
Title: Development of the diffusive gradients in thin films (DGT) technique for plant available potassium measurement in Australian soils
Author: Zhang, Yulin
Issue Date: 2015
School/Discipline: School of Agriculture, Food and Wine
Abstract: Potassium (K) is an essential plant macronutrient and the most abundant cation found in plants. Except for nitrogen (N), K is taken up by plants more than any other nutrient from the soil (Havlin et al. 2005). Due to the complexity of the plant K uptake process, which is affected by multiple factors, the traditional soil K testing methods (e.g. CaCl₂ K, Colwell K, NH₄OAc K, etc.) have generally failed to provide an accurate indication of the amount of K fertilizer should be applied before planting. Beyond measurements of bioavailable fractions of trace elements, the relatively new diffusive gradients in thin films (DGT) method has successfully predicted plant-available phosphorus (P) in agricultural soils. As the main mechanism of K uptake by plants is by diffusion, which is the same mechanism of P uptake by plants, it is likely that the DGT could provide an accurate prediction of plant K requirements. The DGT K method has been improved to enable measurement of both plant-available P and K in soils by using a new mixed Amberlite and ferrihydrite (AMF) gel. Compared to the resin gel used by Tandy et al. (2012), the MAF gel has improved properties in terms of Amberlite distribution resulting in a flat shape, which avoids the difference in length of K⁺ diffusion pathways caused by unevenly-distributed Amberlite particles in the gel and aids in the process of preparing the DGT devices. With the new resin gel, it was revealed that the DGT method can be used at longer deployment times (>2 h) and was capable of measuring solution concentrations of K larger than 16 mg L⁻¹ - limitations which were reported by Tandy et al. (2012). It was also the first time that the diffusion coefficient of K through the diffusive gel was fully investigated in the presence of competing cations (e.g. Ca²⁺, Mg²⁺ and NH₄⁺). Since the MAF gel incorporates ferrihydrite, which is traditionally used for P measurement using the DGT method, the MAF gel was shown to have the ability to measure both P and K in solution and in soils, compared with the traditional gel containing ferrihydrite alone. Besides having the ability to take up K, the MAF gel also has the ability to bind calcium (Ca) and magnesium (Mg) from solution. With the measurement of elution and uptake efficiencies of the MAF gel for Ca and Mg and the diffusion coefficients of Ca and Mg through the diffusive gel, the DGT method can also be used to measure the available of Ca and Mg in solution and soil environments. Due to higher affinity of the MAF gel for Ca and Mg compared to K, measurement of available K in soil using the DGT method is mainly restricted by Ca, the main competing cation present in agricultural soil solutions. In some scenarios, high Ca concentrations in soils mean that shorter deployment times must be used or else measurements of K are affected. Larger effects of deployment time on the CDGT of K were observed at shorter deployment times. The effects of thickness of the diffusive gel on the CDGT of K were found to be inconsistent across soils. Finally the accuracy of the DGT K method and the traditional extraction methods for K were compared in terms of predicting wheat growth to K application in soils at two different root densities in a glasshouse trial. For predicting wheat relative yield, the Colwell K and NH₄OAc K methods were more accurate compared than the DGT and CaCl₂ K methods at low root densities, which is the situation most relevant to field conditions. The ability of the DGT K method to predict plant response to K varied with root density, and was poor at low root densities. However, the DGT K method was a good predictor of wheat tissue K concentrations irrespective of root density (R²≥0.84). Further investigation showed that the increases in concentrations of K measured by the DGT method as a function of rate of K fertilizer application were highly (inversely) correlated to the potassium buffering abilities (KBA) of the soils (R²=0.96). KBA may be a good predictor of soils that are potentially prone to depletion of available K and susceptibility to deficiency, as soils with low KBA have a reduced ability to resupply soil solution K pools in response to K removal by plant roots. The DGT K method is not recommended for adoption as a soil test K method for wheat before further evaluation of the performance using crop responses to K in field conditions. There is room for further improvement of the method to measure more strongly bound K in soil which appears to contribute to crop K nutrition, by changes to the binding gel and the diffusive gel in order to obtain more selective uptake of K by the binding gel and potentially change the transport of K through the diffusive gel. As K uptake varies between plant species, the ability of the DGT K method to predict K requirements by other crop types also requires evaluation.
Advisor: Mason, Sean
McNeill, Ann Marie
McLaughlin, Michael John
Dissertation Note: Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Agriculture, Food and Wine, 2015.
Keywords: DGT
potassium
soil testing
cations
diffusion
nutrition
Research by Publication
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals
DOI: 10.4225/55/5902ab924a93c
Appears in Collections:Research Theses

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