I played the Yoke minor improvement, and there was some disagreement as to whether it referred to the number of fields currently on the board, the number of times the "Plow 1 Field" action has been taken, or the number of "plow" minor improvements that had been played. What is actually meant by this card?
Card Text:
When you play this card, you can immediately plow 1 field for each Plow that has been played (by any player) or Harrow if it has been played.