Quastel remained in Hopkins’ department as a demonstrator and lecturer from 1923 to 1929, during which he pioneered the research of microbial enzymology.
By analyzing soil as a dynamic system, rather than an inert substance, he was able to apply techniques such as perfusion, with which he had become familiar in studies of animal organs.
[7] On this approach, Quastel later wrote (1946) that "soil as a whole can be considered an organ comparable in some respects to a liver or a gland to which may be added various nutrients, pure or complex degraded plant materials, rain, air, and in which enzymatic reactions can occur.
Best known is the compound commonly labeled as 2,4-D, one of the first systemic or hormone herbicides, a class of chemicals responsible for triggering a worldwide revolution in agricultural output and still the most widely used weed-killer in the world.
During his nineteen years at McGill, Quastel supervised seventy PhD candidates and his Institute published over three hundred scientific papers on topics including metabolism of micro-organisms, soil biochemistry, neurobiochemistry, neurotropic drugs, anaesthesia, cancer biochemistry, enzyme inhibition, and transport of nutrients and ions across membranes.
Once he reached McGill's retirement age in 1966, Quastel accepted a professorship of neurochemistry at the University of British Columbia in the Department of Psychiatry, the first such position for that institution.