diff --git a/premise/iam_variables_mapping/crops_variables.yaml b/premise/iam_variables_mapping/crops_variables.yaml
index 25f410ef..e2a6e478 100644
--- a/premise/iam_variables_mapping/crops_variables.yaml
+++ b/premise/iam_variables_mapping/crops_variables.yaml
@@ -1,5 +1,9 @@
 # mapping between biofuel types, crop types, climate types
 # and aliases for land use and land use change
+# land_use in hectares per GJ primary energy
+# land_use_change in ton CO2 per GJ primary energy
+# production in MT/year
+# fertilizer use
 
 ---
 sugar:
@@ -96,4 +100,4 @@ grain:
   production:
     image: Agricultural Production|Crops|Energy|Maize
   fertilizer use:
-    image: Fertilizer Use|Nitrogen|Energy-Crops|Non-woody crops
\ No newline at end of file
+    image: Fertilizer Use|Nitrogen|Energy-Crops|Non-woody crops