What determines the thickness of the fertile layer?

The soil is the upper loose fertile shell of the planet Earth. But in different territories, the level of soil fertility is very different. It depends on many natural and anthropogenic factors (human influence). The relief directly affects the amount of humus and nutrients in the soil. For example, on slopes, soils are washed out, while in low areas, the opposite is true. Vegetation also affects fertility. Steppe plants leave a huge amount of organic residues in the soil every year, but in the desert this cannot be due to the scarce vegetation. Man also has a significant effect on the soil cover. There are agricultural firms that drain the soil with their aggressive work. A more positive example is ecological offices for the cultivation of agricultural crops.