In this case the answer is fuel
If I had to guess it would be C metabolism
Soil
Soil is the upper layer of the ground comprising of a mixture of sand, silt, clay, loose rock fragments, and organic matter that can support the growth of rooted plants. The relative proportions of sand, silt and clay determine the texture and size of the pores of the soil, which affects its ability to allow air and water to penetrate/infiltrate.
