The six turns to mature a seed into a plant takes on more significance in the campaigns. In the introductory scenario, it's more about learning the concepts that will come into play on future missions.
Also, don't think of the plant as a salad that you eat all at once and then it's gone. Think of it more like an avocado tree that will produce edible fruit for a long time.
That helps a little.
So the four black cubes(greenhouses) are producing one food each turn correct? And each astronaut consumes one food each turn?