PotassiumExtractableElements units

[rapw3k]

check if units can really be in centiMol per KG. In LUCAS they use MilliGM-PER-KiloGM. If it can have both units, we can either

• remove unit constraint in PotassiumExtractableElementsValue
• create two observations, and two values
• create two subclasses of the current value, e.g., PotassiumExtractableElementsValueInMoles and PotassiumExtractableElementsValueInMG.

simpler is first one.

[ldesousa]

These are the combinations of Potassium properties and units of measure in WoSIS:

             desc_attribute_standard_id             | desc_unit_id
----------------------------------------------------+--------------
 Potassium (K) - Bray I extraction                  | mg/kg
 Potassium (K) - Mehlich 3 extraction               | mg/kg
 Potassium (K) - total                              | mg/kg
 Potassium (K+) - soluble                           | cmol(c)/l
 Potassium (K) - extractable solution not specified | mg/kg
 Potassium (K) - exchangeable                       | cmol(c)/kg

Extractable Potassium is measured as mg/kg and Exchangeable Potassium as cmol(c)/kg. But in the ontology both are expressed with cmol(c)/kg. This is probably an error, I will check this and soon submit a correction.