unit g/Mg vs mg/kg

[HGarre]

Two "options" are used for essentially the same unit within SOIL_PROFILE_LAYERS and SOIL_LAYERS: g/Mg e.g. for nitrate_KCl but mg/kg e.g. for phosphorus_extractable. More similar units exist such as g/kg. Decide whether to keep for historic reasons or to unify.

[chporter]

Agree, we should probably standardize these.