X-ray CT investigations of intact soil cores with and without living crop roots

Abstract

X-ray computed tomography (CT) was used to non-destructively visualise intact cores of soil (15 cm diameter and 50 cm depth) collected from a farming region in South Australia. The texture contrast soil comprised 20-30 cm of nutrient poor sand over dense sodic clay. Cores were taken from a non-ripped paddock and from an adjacent paddock that had been ‘deep ripped’ to 50 cm. Image-reconstruction software was used to visualise in three dimensions (3D) macro-morphological features of the soil cores. Canola was grown in some of the cores and several times during plant development the cores were scanned. Roots visualised were of a diameter equal or larger than 1 mm and the volume, surface area, length and position of these ‘exploratory’ roots was quantified. At the end of the experiment destructive root sampling and two-dimensional (2D) scanning were used to measure the total length and volume of all roots. The work revealed the architecture and morphology of root systems in situ and tracked response to soil macro-structures such as layers of organic matter in the sand, the clay domes at the interface with the sand, old root channels, calcium deposits, stones, and areas of soil that were naturally more loose. Root penetration was much slower in cores from the non-ripped paddock than in cores from the ripped paddock, indicating a restriction to growth even in the sand. Superimposition of root and non-ripped soil images showed that the roots grew preferentially in looser soil matrix, organic deposits and old root channels. However, roots did not necessarily seek sand-filled cracks between the clay domes as was expected from anecdotal evidence. Root growth in the ripped soil was not influenced by the redistributed macro-morphological features with roots growing in a straight fashion through both sand and clay clumps spread throughout the soil profile by the ripping procedure.