Similar issues are encountered in biomedical X-CT imaging, where there is a need to distinguish different fluids and soft tissues from one another, despite their similar electron densities. In this case, a common solution is to use contrast agents to label areas of interest, where contrast agents are solutions or particles that contain elements with high attenuation coefficients such as iodine, barium or lanthanides or other heavy metals. This concept has equally been applied to label liquid phases in investigations of flow through porous rock for applications in hydrocarbon recovery and carbon dioxide storage, typically using iodinated contrast agents. A number of studies have applied these contrast agents to soil imaging, visualizing both flows and plant root systems . However, it is notable that their phytotoxicity can cause dehydration, distortion and damage to plant tissues. Other contrast agents that have been applied in soil include osmium tetroxide, which exhibits limited penetration ability, along with very high toxicity, as well as bismuth tartrate, which requires a prolonged incubation period that makes it unsuitable for many applications. These pioneering investigations have shown the value that contrast agents can bring in soil science, underlining the potential benefits of an agent that is non-toxic and can be used at large scale in imaging biologically active soil.