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 and always consumes 2 crop per hour. This crop consumption is cumulative.

Across the top of the screen, you can see your current resource stores and crop consumption: 

To increase crop production, you can upgrade a crop field, mill or bakery. Assuming you have the requirements, you will be able to do so, noting that level 1 cropland requires 20 crop itself for construction.

Negative crop production and its effects 

When you have to feed more troops and population than your production, the crop production value becomes negative, this indicates that the troops in the village will empty the granary.  If you do not refill it before the stored crop reaches 0, the troops will slowly start to die of starvation. Once the crop production reaches 0, the starvation will stop.

 What is free crop?

 This is the crop production available to build or upgrade a building or resource field, clicking the crop balance icon (furthest right in the above image) will show you the totals for that village

•  Free crop is the crop production minus the population.
  • Includes Oasis, Mill and Bakery
  • Does not include hero bonus or plus crop bonus

You can view the details for each village from the crop balance icon as shown below

What is "croplocking"

 It’s the difference between crop production and free crop for buildings.  If you do not have enough crop production you may be prevented from building within your village until you have increased your production.

Limitations and Exclusions

• You can always upgrade a crop field.
• Grain mill and bakery may be built/upgraded if they would increase free crop.
• Main Building level 1 may always be built.
• If base crop production in the village is 276 per hour (which is the production of 6 crop fields at lvl 6) or more, and the amount of free crop is less than 0, then Main Building, Warehouse and Granary may be built/upgraded to level 10.
• World Wonders are excluded and may always be upgraded
• You cannot build or upgrade anything if it would reduce free crop below 1#
• You cannot destroy grain mill nor bakery if it would drop free crop below 1. (Any running destruction of these is already counted).
• The hero has a basic crop production of 6 crop per hour, no matter how the skill points are set. This basic production follows the same rules as the usual hero resource production. So it's added to the production of the village that the hero belongs to. But it does not increase the free crop.
  • A dead hero still produces 6 crop per hour.
  • When a hero is revived, then his resource production starts at the same time as his crop consumption.

• For all of the other buildings, you can only commence their construction if you have a positive total production (without troops, hero's production, Gold bonus) again.