There is a fairly rare proverb that says "we must eat a peck of dirt before we die". A peck is about 9 liters. So, every human will eat about 9 liters of dirt in their life. The proverb is usually used to remind people that there is always a small amount of unpleasantness mixed with the good things in life.
If you eat meals off the ground, you would expect that you would quickly exceed your 9 liters--your peck of dirt would overflow. However, here, the ground was so clean that you could eat on it without eating any dirt.