## Nutrients

Because the CHEBI nutrient roles don't work for plants, we need a
separate role hierarchy for plants. This is in plant-chemical-roles.obo

The purpose of this is primarily for normalization of ontologies and
reasoning.

Hierarchy:

 / PLANTCHEBIROLE:01 ! plant nutrient structure *** 
  is_a PLANTCHEBIROLE:02 ! plant micronutrient structure
   is_a CHEBI:18248 ! iron atom
   is_a CHEBI:18291 ! manganese atom
   is_a CHEBI:23116 ! chlorine atom
   is_a CHEBI:24866 ! salt
   is_a CHEBI:27363 ! zinc atom
   is_a CHEBI:27560 ! boron atom
   is_a CHEBI:28685 ! molybdenum atom
   is_a CHEBI:28694 ! copper atom
  is_a PLANTCHEBIROLE:03 ! plant macronutrient structure
   is_a PLANTCHEBIROLE:04 ! plant primary macronutrient structure
    is_a CHEBI:25555 ! nitrogen atom
    is_a CHEBI:26216 ! potassium atom
    is_a CHEBI:28659 ! phosphorus atom
   is_a PLANTCHEBIROLE:05 ! plant secondary macronutrient structure
    is_a CHEBI:22984 ! calcium atom
    is_a CHEBI:25107 ! magnesium atom
    is_a CHEBI:26833 ! sulfur atom

Note that this is not fully normalized as we create classes
representing structures-with-roles rather than roles.

This is somewhat redundent with EO, but EO deals with regimens. The
regimens should themselves ultimately be classified using CHEBI and
this role hierarchy.

It may be the case that future versions of CHEBI will include a more
relational concept of roles.

This is being used in trait_xp. E.g.:

  TO:0000248 ! primary macronutrient sensitivity

is defined using

  PLANTCHEBIROLE:04 ! plant primary macronutrient structure