If you’ve ever had to dig a hole and then fill it back in, you may have noticed a mysterious thing: often there’s a little dirt left over. This can also be seen when construction crews dig down to get at sewer pipes or electric cables. How can there be more dirt going in than there was coming out?
The amount of soil itself remains the same; what has changed is the arrangement of the individual grains.
Different factors such as rainwater and passing feet can press soil down so that the grains lie closely together with little empty space, called “pore space.” When a shovel pulls the soil up and scatters it in a big pile, the soil grains fall into a pattern closer to the way they were before being packed down.
While you can get the hole level by refilling it and leveling it off with the back of the shovel, there’s still a possibility for there to be leftover soil. That’s because it’s very hard for humans to efficiently and exactly pack the soil as it was.
Some scientists also note that the variations in moisture retention may also effect how much pore space there is. Depending on the composition of minerals, the topography of an area, and even things like decomposition (animals and/or plants), there may be differences in the amount of pore space. Depending on the type of soil, fifty percent of what you’re walking on might actually be empty space.