There have been flowering plants on this earth for a very long time. When they first made their appearance and began to thrive, humans were not the ones helping them out. There were no bottled fertilizers and plant-food pellets at that time. But there were many animals, and the interaction between the animals and the plants points the way for today's gardener. Many animals have always instinctively buried their feces.
Animals are particularly vulnerable during the process of defecation.
So in those early days, mammals would seek out plants, with their camouflaging leaves and colorful blooms, and they would dig a sizable hole right at or near the base of these plants, where they would deposit their feces. Then they would dig quite energetically, still at the base of the plant, to cover any evidence of their presence. A study of this digging behavior, and the resultant benefit to the plant, has led many leading horticulturists to develop the Mammalian Soil Exchange method of gardening. This method involves a total exchange of the soil from the middle of the underground roots to the surface with that soil from the middle of the roots to just below. In order to do this one has to very carefully remove the top half of the soil, and set it aside.