One of the early problems that many players have is resource management - particularly that of crop production. This is often noticed when going to the field screen and seeing the production list: Crop: 2 per hour
Many buildings, fields, and troops place a constant drain on crop production as designated by the icon in production.
For example, a level 1 woodcutter requires:
40 100 50 60 2

This means it needs 40 lumber, 100 clay, 50 iron from the warehouse and 60 crop from the granary. In addition, it will constantly consume 2 crop per hour. This crop consumption is cumulative and the level 2 woodcutter consumes an additional unit of crop every hour (3 total). Across the top of the screen you can see your current stocks, crop production and crop consumption:
145/800 200/800 100/800 90/800 12/14

This means that in the stocks you currently have 145 lumber, 200 clay, 100 iron, 90 crop and consume 12 of the 14 crop produced per hour. At this point, if you were to try to upgrade a woodcutter to level 1 you would be unable to do that because the game tries to keep you from reducing the crop production below 2/hour.

To increase crop production, go to the fields screen and click on one of the croplands (yellow) This will tell you how many resources are necessary to upgrade this. Assuming you have the necessary requirements you will be able to do so. Please note that the level 1 cropland requires 20 crop itself to get constructed.

Negative crop production and its effects
When you have to feed more troops and population than the crop production itself can support, the crop production becomes a negative value. This indicates that the troops in the village will empty the granary. If the owner of the village does not fill it before the stocked crop reaches 0 the troops will slowly start to die of starvation. Once the crop production reaches 0 the starvation will stop.