I have thought about the smell of exudates a bit – I think what you probably smell more than the exudate as such are the microbial metabolites produced of it. There are some metabolites in exudates that have a smell on their own, but many of them are rather neutral in the concentrations that they are exuded – sugars, citrate, malate.

Like sweat with or without bactericidal antritranspirant. Some bacterial metabolites have an intense smell at very low concentrations – esp. short chain fatty acids.

Which roots would you recommend for a beginner to smell an obvious difference?