After the last glacier melted there was no organic matter in the soil, no earthworm, no microbes, nothing. The climate (and weather) was very unsettled. Roughly 3000 years ago, the weather stabilized to something similar to that seen today. SOM (soil organic matter) is an artificial concept created by people to try to explain what they didn't know. SOM is the sum total of a series of different soil fractions, each with different chemical and biological characteristics. The biological characteristics in soil are characterized by how long they can withstand microbial attack on their substance defined by biological half-life. Half-life is how long it will take soil microbes to destroy half of it. This number can range from minutes to thousands of years. The hierarchy is sugars to proteins to lipids to hemi-cellulose to cellulose to lignin. The time frame for degradation varies depending upon conditions - temperature, humidity/moisture, oxygen content, etc. The only way for surface organic matter to be incorporated into soil organic matter is if it is incorporated into the soil. In row crop country, that means plowing - but unfortunately plowing incorporates so much "excess" oxygen into the soil that it speeds the degradation of existing soil organic matter. by soil microbes. The net result is that plowing (tillage) results in the loss of more SOM than is gained. And no, earthworms played no role in soil development as there are no earthworms native to North America. 

One can easily grow 500 BPA corn in a hydroponics system with no soil and nor organic matter. In real life, SOM is important (but not necessarily critical) in crop production because it serves as a buffer (or bank) holding nutrients and water within the soil matrix that plants can tap for growth. SOM also serves as the medium for microbiological growth. 

The concept of sequestering carbon produced through ag crop production is a false narrative because the half-life of SOM is so short - with one exception - biochar (or charcoal). Some of the chemical structures produced by burning biomass are very resistant to microbial degradation and may have a half-life of 1000 years or more. From an agricultural perspective, biochar is unique. Properly "made" biochar has the ability to bond nutrients (both anions and cations) strongly enough that they are kept out of the ground water but not so strongly that plant roots can't pull the nutrients out to support growth. At the same time, the porous structure of biochar provides an ideal foundation for microbial growth, reduces soil bulk density to enhance water storage/flow, and much more. Biochar is also the foundation for objective carbon sequestration.

3000 years ago, the climate changes to something similar to what we know today and soil formation as we see it today in ag country started. The forests died out, "prairie" came into being and humans started managing their environment for their benefit. Unfortunately, ag scientists and farmers alike have generally forgotten history - and we pay the price in production impediments, environmental impact, etc.

Yes, the SOM needle can be moved as those fortunate enough to farm black prairie soils should acknowledge the efforts of native Americans who started active management practices 3000 years ago.