Table 1:

The interpretation of steel rod corrosion patterns after Carnell & Anderson (1986).

Red/brown rust:indicates a well aerated soil.
Raised black:occurs where rusting has started but has been interrupted, or where rust has been knocked off during removal of the rod from the ground.
Shiny metal:can indicate the presence of substances (usually polyphenols or oil products) which have protected the rods from rusting. These can arise from organic residues that are under anaerobic conditions, and the soil can be classed as inhospitable for root growth.
Smooth black:occurs where anaerobic bacteria utilize soil sulphates producing hydrogen sulphide, which reacts with the surface of the metal.
Matt grey:indicates totally anaerobic conditions.