A Time Leap! Massive Harvests!

Black soil was formed under colder conditions when surface vegetation died, and the decayed matter eroded over a long period of time, evolving into humus. 

This sort of soil was extremely rich and high in organic matter. It was also fertile, light and airy, and world-famous as the best soil for cultivation—so much so it had been dubbed the "grain storehouse". 

As for the patch of black soil near the Underground Shelter, it was at its prime the minute it emerged. In addition, the large amount of snow it was buried under meant that even fertilizers were not needed to satisfy the nutritional needs of the seeds growing across all five acres of the land. 

Of course, the crops in the fields needed water to grow, so replenishing their water supply in a timely manner was extremely necessary.