It sounds like you only want to mix soil and/or fertilizer in large batches so as not to waste the capacity of the machine.

Perhaps this configuration will work:

1. Make Soil a Stockable Product with a Reordering Rule.  MIN is the smallest amount you will make, MAX is how much room you have to store the product and QUANTITY MULTIPLE is in this case the same as MIN:

This tells Odoo you don't want to ever procure (in your case make) a batch smaller than 12 yards.  With a QUANTITY MULTIPLE of 12, you will only ever get asked to make 12, 24, 36, 48, etc.

Bill of Materials:

MO after the first Sale of 12 yards (nothing in stock at all):

Note that Odoo wants to make 120 (to keep) and 12 (to sell).

With this setup, MO's are only created when you run the scheduler (usually daily, or perhaps before a shift change).  They will be for the projected demand - uncluding demand for all confirmed Sales since the scheduler was last run.

You can also set the MIN to 0 if your goal is to have NOTHING in stock, and still be told to make a new batch when needed.  In this case, if someone orders 5 yards and you have none in stock, you will still be asked to make 12.