Update doc/calculating_npk_current_model.rst

doc/calculating_npk_current_model.rst

@@ -175,6 +175,12 @@ zero. The reason for this is twofold:
 * The fertilizer recommendations for crops included in the pasture sub-categories are
   close to zero.
 
+The above is true for future scenario work, however for the Current scenario, in order to
+match Metson et al. 2021 assumptions, a slightly different total cell nutrient demand method
+is used. We assume that the proportion of animal excreta not collected (so the reverse of the below section),
+is used to decrease crop demand (including an estimated crop pasture demand). In a few instances this can
+cause a cell today demand to be below 0 and these were put to 0 manually in the Sweden.xlsx sheet.
+
 .. _animals_npk_contrib:
 
 Animals