The Influence Of Soil Organic Matter (SOM) In Soils From Long-Term Experiments: Measurements Of Compression Properties, Water Release Curves, Thermogravimetry, Acoustical Impedance And Neutron Tomography

Suravi, Kamrun Nahar (2022). The Influence Of Soil Organic Matter (SOM) In Soils From Long-Term Experiments: Measurements Of Compression Properties, Water Release Curves, Thermogravimetry, Acoustical Impedance And Neutron Tomography. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00014a7b

Abstract

Soil Organic matter (SOM) is a very important factor in soil productivity and health. As SOM is the basis of nutrient cycling in soil, it has important consequences for both plant growth and the development of the root system. Also, organic matter influences the physical and mechanical properties of soils by improving soil structure and changing the soil pore system. Soil physical properties determine soil health and regulate plant growth by controlling root development and maintaining the availability of water and nutrients to plants. The effect of soil organic carbon (SOC) on the consolidation behaviour of soil from two long-term field experiments at Rothamsted (the Broadbalk winter wheat experiment and Hoosfield spring barley) has been investigated. These experiments are located on soil with similar particle size distributions, but with SOC contents ranging from approximately 1 to 3.5 g/100g. Soils taken from plots with contrasting SOC contents were compressed and deformed in a triaxial cell and the normal consolidation and critical state lines were determined. It has been found that the compression index was independent of SOC, but the void ratio at any given effective stress was highly correlated with organic carbon content. By comparison with uniaxial compression data, the apparent influence of SOC on the compression index is likely to be due to its effect on soil hydraulic properties rather than any intrinsic effects on strength. The plastic limit test appears to be a useful, simple and direct way of comparing soil physical behaviour and expected soil density. Thermogravimetric analysis (TG) has been used to deduce the organic matter content and its composition. Water release characteristics, and soil strength have been measured with the suction plate method and indirect tension method, respectively. As well the traditional measurements, the application of two relatively novel methods including neutron scattering by thin soil slices and non-invasive acoustical reflection from samples in an impedance tube is reported. The neutron scattering data has not proved useful as a result of the difficulties in obtaining and using soil samples in the required form. However, impedance tube measurements show that (a) the sound absorption of the surfaces of sand and soil samples decreases with increasing water content and (b) changes in absorption coefficient spectra deduced from impedance tube measurements on soil samples extracted from the long-term agriculture experiments, involving different organic matter content but similar water content, are consistent with the predicted effects of changes in porosity and permeability in rigid-porous air-filled media. The increase in bulk porosity with organic matter increase suggested by the acoustic measurements agrees with the results of the water retention and compression tests.

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