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How to relate X-ray Computed Tomography derived porosities to physical soil porosity in a randomised 3x3 factorial traffic and tillage field experiment

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2018 ASABE Annual International Meeting  1801651.(doi:10.13031/aim.201801651)
Authors:   William A Millington, Paula A Misiewicz, David R White, Edward T Dickin, Sacha J Mooney, Richard J Godwin
Keywords:   compaction, computed tomography, no till, porosity, soils, tillage, tires, traffic, x ray.

Abstract. Compaction of soils from agricultural machinery alters soil aggregate and pore structure whilst increasing bulk density. This leads to decreased soil aeration and water and nutrient uptake and increases root penetration resistance that can result in reduced crop yields. A randomised 3x3 factorial traffic (Random Traffic Farming, Controlled Traffic Farming and Low Ground Pressure system) and tillage (Deep, 250mm; Shallow, 100mm and No-till) field experiment at Harper Adams University, UK, was set up in 2011. An investigation was conducted in 2016 using X-ray Computed Tomography (CT) to assess the effects of tillage and traffic on the soil pore size and distribution.

The heterogeneous nature of soil makes assessment of structure difficult. Porosity in the soil consists of a variety of pore shapes and sizes that have different effects on the movement and storage of water, aeration and resistance to root growth. Determination of dry bulk density is a widely accepted means of identifying changes in soil compaction and total soil porosity in response to vehicular traffic and mechanical breaking from tillage operations but does not allow the quantification of pore sizes and distribution within the soil.

X-ray computed tomography (CT) is a non-destructive 3D imaging technique that can be used to measure soil pore size and distribution allowing visualisation of changes in pore system structure through the soil profile. The percentage porosity derived from the CT is lower than porosity measured by physical methods even at high resolutions.

A comparison of physical soil porosities calculated from bulk density measurements and X-ray CT derived porosities found that for a sandy loam soil, the X-ray CT derived porosities corresponded to the air filled pore space within the soil. In addition when a constant equal to the water filled pore space identified by previous researchers, of 31% porosity was added to the CT derived porosity for each traffic system it gave the same value of the total soil porosity as calculated from the bulk density data.

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