Database table structure

I'm working on a program to track plants for a nursery and am having an problem structuring the database.  For each plant there can be multiple crops each year. Each crop is divided into different descriptors when its entered in inventory. (graft, size1, size2, etc...) As the plant grows it can be shifted from one descriptor to the next.  
 I need to be able to track by crop number and only use a descriptor once for the plant.  For example: Crop1, Crop2, Crop3...  Descriptor: size1   I need to be able to get a total count from all the plants that have a specific descriptor regardless of what crop its from.  
Right now I have a table for Plants, Crop, Inventory. Each Plant has multiple Crops, each Crop has multiple inventory records.  Portions of the crop may be pre-sold so the same descriptor could be used for two different inventory records with one being marked as pre-sold.